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Engine Tuning 101 (General Tuning)

Engine Tuning 101 (General Tuning)


Old 06-06-2007, 10:43 AM
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Default Engine Tuning 101 (General Tuning)

Something useful for those who aren't sure about tuning, or need help tuning. This is one of those guides I always show to anyone interested in buying a mill or who already has from me.

By Steve Pond
So now you can get your engine started on a regular basis, but you’re still struggling with the fine-tuning that will score you a win at the racetrack (or bragging rights at the parking lot). Properly tuning a nitro engine can make that difference without jeopardizing its health. It takes time to learn how to really tune your engine, however. There’s a certain “feel” to how your car drives and a certain sound you’ll come to know when your engine has been ideally tuned. Other cues that you feel and hear tell you what to adjust when your engine isn’t running properly. When it’s time to tune your engine, there’s no substitute for plain old experience. Reading about engine tuning is helpful, but you need to experiment with your engine to improve your tuning skills. The good news is that the following tips will help you avoid some of the pitfalls of fine-tuning and achieve tuning proficiency more quickly.


Engine manufacturers often include base-line settings for the mixture needles, so it’s wise to start with these. If this information is not provided, then you must arrive at needle settings that will get the engine started. A universal starting point is usually about 1 turn open (counterclockwise) on the low-speed needle and somewhere in the 2- to 3-turn range on the high-speed needle. This varies among engines, but it gets you started running, and then you can make the necessary corrections. After the initial startup, follow the proper break-in procedure, then worry about performance tuning!

The proper sequence for adjusting the mixture needles is hotly debated. When you start to fine-tune the engine, it’s generally best to start with the high-speed needle, then set the low-speed. First, however, get your engine running, and keep it running before you worry about race tuning.

During break-in, the engine typically idles a long time, so it’s best to adjust the low-speed setting first so the engine runs slightly rich (loading up every 30 seconds or so). It requires an occasional “blip” of the throttle to clear out any raw fuel that has accumulated in the engine. Once break-in is finished, then get the high-speed needle in the ballpark.

Place the car on the ground and accelerate smoothly to give the engine a chance to build some heat. With the high-speed needle in the proper range, the engine should be able to rev relatively well up to full speed once it has been running for a few minutes on the track or parking lot.

A word of caution first: there’s a fine line between the perfect tune and a blown or damaged engine. Nitro-engine fuel also contains engine lubricant, so as you get close to dialing in the mixture to where there is just enough fuel to burn and deliver maximum power, you also are close to having just enough oil to keep the engine lubricated. Anyone with experience in tuning 2-stroke engines can tell you that they run best right before they seize or blow up. Our engines are a little tougher and more capable of taking some abuse than bigger 2-strokes, but there’s no sense in pushing the mixture settings so lean that you risk damage to the engine. I can’t say this too often: get the engine up to full running temperature by running the car exactly as you would on the track or parking lot. The high-speed needle setting depends on the type of driving you do. I’ll start with a racing setup.

Racers will tune the high-speed setting to get from point A to point B as quickly as possible. (Performance also depends on a proper low-speed needle setting, but for now, let’s concentrate on the high-speed setting.) The best place to race tune your engine is on the track where you run. I prefer to set the high-speed mixture so the car can leg out the track’s longest straight section in the shortest time. Estimate the time by “feel” and gut instinct, or use a stopwatch for more accuracy. The high-speed mixture should be set to maximize engine performance for that particular track configuration. A short, tight track may require a main mixture setting just a shade on the lean side to provide maximum power out of the corners. You needn’t be concerned about high-rpm performance because the track is too small for the engine to ever reach peak rpm. A long, high-speed track may require a slightly rich main-needle setting. If an engine constantly revs at the upper limits of the rpm range, the fuel mixture should be richened to ensure proper lubrication across the entire rpm range. This slightly rich setting might reduce bottom-end acceleration to a degree, but longer tracks require a slightly richer mixture setting to let the engine rev to its limits without running dry of fuel and oil.

Why is it important to tune the engine to the track? A mixture needle can only provide optimum performance within a relatively narrow rpm range. Anywhere below this hypothetical rpm range, the engine runs slightly rich; anywhere above, it gets progressively leaner. Until we have fuel injection that constantly optimizes fuel mixture throughout the rpm range, there needs to be a degree of compromise with the mixture settings. So, ideally, set the mixture to provide the most power in an rpm range that is best suited to the track on which you run.

Running in parking lots, particularly larger ones with a lot of breathing room, requires unique mixture settings. It’s a common mistake to establish mixture settings for maximum punch—as though the car will be run within the confines of a tight racetrack—and then to go out and run at wide-open throttle (WOT) in a huge parking lot for 5 minutes. This type of running is incredibly unhealthy for an engine to begin with, and compounding the problem with an excessively lean main-needle setting is a recipe for disaster. If you still insist on torturing your engine, the high-speed mixture setting needs to be as much as 1/4 turn richer than typical to provide optimum fuel for the upper rpm range. The engine will be a little softer when accelerating from a standstill, but it’s the only way to ensure there is an adequate supply of fuel and oil when running at the upper end of the rpm range. The inherent danger is that this type of running taxes the connecting rod and other engine components to their limits, but having the proper mixture setting will at least delay the inevitable.

The high-speed needle is dialed in, so now let’s properly set the low-speed needle. It’s important to set it last because it simply regulates the fuel that flows from the main needle at low throttle settings. Lean out the main needle, and you automatically lean the low-speed needle as well. For this reason, it’s wise to finish with the low-speed setting.

Again, it’s imperative for the engine to be at full operating temperature. There are many methods of testing the low-speed needle setting; one is to pinch the fuel line. When you pinch the fuel line, the engine rpm increase slightly. Keep pinching it, and the engine will eventually stall. If the engine rpm increase dramatically, it indicates that the low-speed setting may be too rich. Or, if the low-speed setting is already too lean, the engine rpm may not increase much at all, and the engine will stall rather quickly. It’s a somewhat crude method and doesn’t tell you what to expect from the engine on the track, but it will get you into the ballpark.

Another common way is the “see-how-long-it-will-idle” method. The low-speed needle adjustment affects how long the engine will idle. A too lean fuel mixture causes the engine to race and possibly stall, limiting the duration of a steady idle. A too rich low-speed-mixture setting causes the engine idle to steadily drop and eventually stall. The ideal setting allows the engine to hold a smooth, steady idle for 10 to 20 seconds (max), and then the engine rpm decrease steadily because the crankcase loads up with fuel. Why? There are no awards given for the longest-idling engine. If the engine is able to idle steadily for a longer time, then it may start to lean out and heat up during a race and make it difficult to drive the car and keep the engine running. The only flaw in this method is that it doesn’t tell you whether you have an artificially rich mixture to compensate for an idle speed that’s too high.

A common mistake is to set the idle-speed screw to keep the carburetor open too far. The low-speed needle must then be artificially rich to bring the idle down to a reasonable rpm. The symptoms are similar to a too rich low-speed-mixture setting; there’s just a delay in the loss of engine rpm. How do you avoid this? This is also something that becomes easier with experience, but just continue to reduce the idle speed and lean the mixture until you know you can’t go any further. Bottom line: adjust the idle-speed screw to suit the fuel-mixture setting, not the other way around.

The simplest and most foolproof method to properly set the low-speed mixture is, again, to do it on the track. Set the low-speed needle so your car gets the strongest launch after sitting still for about 10 seconds. The engine should be able to pull strongly off the line without hesitation. A noticeable hesitation might be the result of either a rich or a lean low-speed mixture; knowing the difference takes experience, but look for signs that help point you in the right direction. How an engine decelerates can tell you as much as how it accelerates. If the engine spools down and rpm drops uncharacteristically low, it indicates that the low-speed-mixture setting is too rich. Or, if the engine takes too long to reach a steady idle and seems to want to keep revving, that tells you the low-speed-mixture setting is too lean. It can also indicate a lean high-speed-mixture setting, but that setting should have been addressed by properly setting the high-speed mixture first.

It will take a little time to get it right. If you make small adjustments and are patient, you really can’t do anything wrong. An adjustment you make in the wrong direction is reflected in engine performance; to correct the problem, simply go the other way.

Changing to a higher percentage of nitro fuel sounds like an easy method of developing more horsepower, but it isn’t always that simple. Without getting into all the particulars of nitro fuel, I’ll just say that there is a point where you can have too much nitro. Adding up to 10 percent more nitro than is typical produces more power, but you have to know how to adjust your engine to accommodate the extra nitro. Fuel-mixture settings need to be slightly richer when nitro content is increased. Also, you may have to increase head clearance by adding an extra head shim. The extra fuel introduced into the combustion chamber increases compression by adding non-compressible matter; this also increases cylinder pressure during the combustion process, which may cause detonation. Detonation occurs when the fuel explodes instead of burning, and that can cause internal engine damage. The extra head shim will likely prevent detonation when fuel with higher nitro content is used.

A final note about fuel: fuel with a lower oil content (for manufacturers that actually disclose the amount of oil in their fuels) should be run with a richer mixture setting. This doesn’t so much relate to performance as it does to the benefit of the engine. Conversely, fuels with higher oil content have the extra lubrication that allows a leaner mixture setting with less risk of engine damage. Fuels with a lower concentration of lubricant are intended for competition use by experienced engine tuners. These fuels will make marginally more power because the lubricant that’s removed is replaced with power-producing nitro and methanol. Evaluate your tuning ability honestly before you run out to buy fuel with a lower oil content.

A glow plug’s temperature range is critical to proper performance. Small-block engines generally use warm to hot glow plugs, while big-block engines use plugs in the colder range. If you choose a plug in the wrong temperature range, you could be chasing the tune of your engine till the sun goes down. Changes of the relative temperature of the glow plug can be beneficial, however.

A combination of compression, heat and a catalytic reaction between the platinum in the glow-plug coil and the methanol in the fuel creates combustion in a nitro engine. Altering the heat range of your glow plug can alter the timing of the combustion process. Nitro engines don’t have an ignition system that can be used to advance or retard combustion timing, but a hotter plug that causes ignition a little earlier in the combustion process can have the same effect. “Advancing” the ignition timing can increase overall power output, especially at higher rpm. There are limits, however, and installing too hot a plug causes pre-ignition (detonation) and risks damaging your engine.

It’s a challenge to figure out a glow plug’s temperature range. Manufacturers don’t use a consistent and universal standard to rate the temperature ranges of their glow plugs. You will probably know the temperature of a plug relative to others within a given product line, but currently, no rating system allows comparisons among manufacturers. Here again, plain old experience with a variety of glow plugs will help you to know which are best for the effect you want.

“Reading” the glow plug is a tuning technique advanced by Ron Paris. It suggests that looking at the glow plug tells you something about how your engine is running. The element in a glow plug will turn gray in an engine that is close to the optimum fuel mixture. This method requires a new glow plug, as the element will eventually turn gray regardless of the needle settings; the length of time it takes to turn gray is the issue. Plugs that turn gray in just a tank or two of fuel (running at race pace, not diddling around) indicate a fuel mixture close to ideal—but also close to trouble. If the plug stays wet and shiny for a few tanks of fuel, you’re in the safe zone; a little rich but safe. When the plug wire gets distorted or broken, however, you’re in real trouble. It’s a sure sign that the mixture is way too lean, or that there is too much compression and the engine is detonating.


Engines are essentially air pumps. The engine takes air in, mixes it with fuel, and then the mixture is compressed and ignited. The additional pressure created by the burning fuel increases by a factor directly related to the amount of compression: increasing compression increases power output. But there are limits to the compression an engine tolerates. Too much causes the fuel mixture to combust too quickly, and that returns us to the same detonation scenario of an excessively hot glow plug.

The amount of compression is determined by the number and thickness of the shims (gaskets) between the cylinder head and the top of the piston sleeve. Well, it’s determined by many other factors, but the only one easily changed is the head clearance via head shims. More shims = less compression; less shims = more compression. Removing or replacing shims with thinner ones increases compression. Some engines have only one shim, so it isn’t advisable to run without a shim at all. Moderation is the key. Go slowly, and make small, not drastic, changes that will minimize the risk of damage to your engine. First and foremost, be sure the piston won’t hit the cylinder head if you remove a shim (or shims).

You can also change compression with glow plugs. Some manufacturers make a longer glow plug that protrudes slightly into the combustion chamber, effectively reducing the area in which the fuel mixture is compressed. This area is already small, and the little extra space occupied by a longer glow plug will raise compression. This is not the most desirable method, but it can be used on engines that have only one thin head shim. It’s unlikely that the longer plug will even come into contact with the piston, but just to be safe, check the head clearance before you install a long plug.


It’s a simple fact: for optimum performance, you must retune your nitro engine every time you run it. Anyone who assumes that the needles can be left alone once they have been set is sadly mistaken. An overnight change in weather conditions may prevent an engine from running or may put it at risk of some damage if adjustments aren’t made to the fuel-mixture settings. Ignoring an engine’s tuning needs compromises its ability to make horsepower. In response to certain changes in weather, equipment and other variables, nitro engines must be regularly retuned.

Temperature. Hot weather requires a leaner mixture setting; cold weather requires a richer setting. Most people assume the opposite because they treat the mixture needle like a thermostat. It is wrong to assume that colder weather requires a leaner setting to keep heat in the engine and vice versa. Cold air is denser than hot air. The denser, colder air packs more oxygen into the engine, so going from hot weather to cold needs a commensurate increase of fuel to balance ratio of fuel-burning oxygen and the fuel itself. The opposite is true in hotter weather. Going from cold to hot weather requires a leaner mixture setting.

Humidity. Humidity is the amount of moisture (water vapor) in the air. Moisture in the air takes up volume that would otherwise be occupied by fuel-burning oxygen. Less oxygen means less fuel is required to maintain a proper ratio of air and fuel. High humidity requires a leaner mixture setting than dry conditions.

Barometric pressure. A barometer measures the atmospheric pressure (generally listed in the local newspaper or on the local weather forecast on TV). Higher barometric pressure readings mean more air is getting into the engine, requiring a richer mixture setting to balance the air/fuel ratio.

Altitude. Altitude is an important factor that most of us ignore, yet it affects the engine’s performance possibly more than any other element. The general formula for power loss with increases in altitude is 3 percent for every 1,000 feet above sea level. If you race in Colorado at 5,000 feet instead of in California at sea level, you can expect to lose about 15 percent of the engine’s potential power output, if the engine is tuned properly.
Air is thinner at higher altitudes, which means there’s less fuel-burning oxygen than at sea level. You might sense a common theme here: less air (oxygen) means less fuel to maintain the proper air/fuel ratio. So, running at higher altitudes requires a leaner mixture setting than running at sea level.


This chart indicates the direction in which you should adjust the fuel mixture when faced with changing weather and other conditions. It assumes the engine is currently well tuned. You could face any combination of conditions listed in the chart; knowing which way to go with the mixture adjustments is half the battle.

Higher air temperature: Lean
Lower air temperature: Rich
Higher humidity: Lean
Lower humidity: Rich
Higher barometric pressure: Rich
Lower barometric pressure: Lean
Higher altitude: Lean
Lower altitude: Rich
Higher nitro content: Rich
Lower nitro content: Lean
Higher oil content: Lean
Lower oil content: Rich
Hotter glow plug: Rich
Colder glow plug: Lean


Anyone who has been around 2-strokes knows that the exhaust system plays a major role in engine performance. Pipes and how they affect performance is a complete article in itself. I don’t want to get into the science of tuned pipes here, I’ll simply suggest that volume (assuming the pipe doesn’t stray too far from convention) determines where the pipe will go to make the best power. Smaller pipes with lower overall volume make the best top-end power, while the fatter, longer pipes with greater volume provide the best bottom-end punch. Selecting the proper tuned pipe can have a very noticeable impact on your application.

Headers can be modified by almost any enthusiast. The length of the header is important to squeezing more power out of your engine. Longer headers deliver better bottom-end power, while shorter headers make better top end. Shorten a header by cutting it with a hacksaw or a Dremel tool. Cut it in 1/8-inch increments, and measure the performance to determine whether any improvement has occurred. Continue cutting until performance levels off. If you need to add back on to the length of the header because you’ve cut too much and performance is suffering, simply increase the gap between the header and pipe, but don’t expose more than 1/4 inch of coupler. If the header is too short by greater than 1/4 inch, just get a new one.

I’ve spent lots of time trying to chase away a nasty bog in the engine as it came off the line or out of a corner, only to find later that the problem was the clutch. Some clutches are built properly at the factory, but in my experience, most engage too early, which hobbles the engine coming out of every corner. Tuning the clutch to engage at the proper rpm puts more power to the ground than most could imagine.

You’ll notice that I have not once mentioned a temperature gauge. The worst thing you can do is to tune an engine to run at a specific temperature. Engine temperature is affected by a number of factors, only one of which is fuel mixture. Weather and many other factors play a role in engine temperature, so tuning to run at the same temperature every time shortchanges the engine’s potential to make power. Yes, most engines run in the 200- to 300-degree range, so checking that the engine stays within this range is valuable to a certain extent. As a result of different weather conditions and other variables, however, the same engine—when tuned for peak power output—can vary as much as 50 degrees. A temp gauge is a reference tool that you should use only to build a data bank of tuning information. A temp gauge should not be used as a tuning tool. Don’t tune an engine to run at 230 degrees all the time. Prevailing conditions may require a mixture setting that causes the engine to make maximum power while running at 270 degrees. You’ll never know that if you always target the same temperature.

Engine tuning is not a black art; it just takes time to learn the particulars so you can maximize your power plant's performance. Take the time to read and learn, and you will avoid the mistakes most of us made in learning the ropes. I thought I had it licked 10 years ago, but I’m still learning. We never really stop learning; sometimes, we just get too smart for our own good. Experiment a little with some of the tips, and you’ll find there’s more power to be made with less effort than you thought. Some of the modifications involve a bit of risk; just take it slowly, and use your “noodle.” It’s hard to make a mistake you can’t correct.
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Old 06-06-2007, 01:05 PM
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Good info!!
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Old 06-06-2007, 01:56 PM
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lots of people need to understand the whole temp gauge thing. Personally, I think its one of the many important parts of the article to read and understand.
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Old 06-07-2007, 06:51 AM
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what break in method do you prefer

Last edited by pitdog; 06-07-2007 at 06:52 AM. Reason: spelling mistake
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Old 06-08-2007, 05:18 AM
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Default STS Dealer locator

Originally Posted by Kemorc View Post
lots of people need to understand the whole temp gauge thing. Personally, I think its one of the many important parts of the article to read and understand.
Hey Kemorc, I have a couple of questions about STS engines:

1. What is their website?
2. I am new to onroad touring, does STS have an engine comparable to JLR Red Dot?
3. Where can I find a dealer listing?

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Old 06-08-2007, 08:33 AM
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Bud there is no comparison between the STS and the JLR.
JLR is a VERY good engine
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Old 06-08-2007, 11:26 AM
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Originally Posted by va_connoisseur View Post
Hey Kemorc, I have a couple of questions about STS engines:

1. What is their website?
2. I am new to onroad touring, does STS have an engine comparable to JLR Red Dot?
3. Where can I find a dealer listing?

my temporary site for now is www.maxpages.com/kemorc, I'm still adding engines as I go along. The D3R engine is a fine engine for the price, but I have a feeling you would be better suited with the STS D12X. Its a modified engine straight out of the factory.

I've never ran the onroad .12 engines that STS has, but i do know that STS's .12 offroad engines are a little too powerful when strapped into a stadium truck.

PITDOG, to answer your question... I follow the rules of heat cycling.
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Old 06-11-2007, 05:19 AM
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Thanks for the site link. I do not see any information on the .12 series engines. I will see what research I can find. I am looking for a "starter" engine, something that is not too expensive but will be competitive in the sportsman/novice class. The JLR Red Dot is a competitive engine but for a newbie, it looks almost too fast.

You have a PM
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Old 06-11-2007, 10:06 AM
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see i didn't check out your reply before I saw the PM, so i figured it was spam.

If you are looking for a starter engine, I've got 4 versions for you. D12E (economy .12), D12EX (economy turbo head .12) , D3R, and the factory modified D12X.

D12E is $100.00
D12EX is $110.00
D3R is $135.00
D12X is $215.00

Last edited by Kemorc; 06-11-2007 at 10:36 AM.
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Old 06-11-2007, 10:37 AM
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No problem. Not spamming you. I work for a company that specializes in small business web sites. Say your site, figured I would extend a hand. No worries.

To give you an idea, the company I work for is the power behind this site (http://www.brooklynhobbies.com/).
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Old 07-03-2007, 01:29 PM
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I'm good with what I got now, the server is up and running, its at www.kemorc.com Its pretty much ad free as can be.

I keep it as simple as possible to keep loading times to a minimum
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Old 07-09-2007, 07:54 AM
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Nitro Engine Tips
Basic nitro engine information, by Ron Paris

There is no ideal temperature for any engine. There are many variables that affect it such as ambient temperature, fuel type and nitro content, altitude & barometric pressure, pipe/manifold type and setting, clutch setting, glow-plug, gear ratio, available traction, how hard you’re driving, on road or off road, etc. All of the following conditions must be met before any temperature can be considered correct (assuming the engine is in good shape):
1 - There must be a visible trail of smoke when accelerating from every corner.
2 - The idle must be stable.
3 - The glow plug wire should stay somewhat shiny, and the coil should stay round (un-distorted).
4 - The performance must be good.

More is not indicative of being better. If more ports were the only solution to more power, all engines would have maximum number of ports. Simply put, there is only so much room for ports in any given displacement engine. Some engines with twice the ports can have less total port area. This is not good or bad - it is just different. Every thing is a combination of many variables. Some of the most powerful engines I have built were three ports! We top qualified the world championships in Holland with a three port engine wile many of the competitors were using 5, 6, 7 and even 8 port engines and we had the fastest top speeds displayed on the board. It is true in most cases that within any brand of engine their power ratings go up with the amount of ports, but this is due to more than just the quantity of ports. Generally there are many other components in the engine that have more refined or modified parts than those with less ports, complementing the combination. You can only compare the quantity of ports between engines of the same manufacture. They all have their own combinations. In other words, brand A’s 17 port may or may not run equal to brand B’s 29 port.

There is no industry standard for hp or rpm testing like the full size automotive SAE (Society of Automotive Engineers) and there are so many things that can dramatically effect readings such as nitro, altitude tested at, with or without a tuned pipe, absorption unit differences (or prop size, brand and manufacturing tolerances) etc. You can not under any circumstance compare hp or rpm ratings between manufactures. You can however use these ratings to compare relatively between different engines in a manufacturer’s own product line.

New racers often ask where to set the needles on their carb. There is no such thing as a definitive universal setting for any engine. Every application will have its own unique requirements. Even two identical set ups can (and most likely will) have at least slightly different settings.
Some racers make the mistake of setting the idle speed opening to wide (high idle) and set the bottom end too rich. This will give a false normal idle speed even though the speed is set to high because it “loads up” the engine with excess fuel causing the idle to be lower than set. The end result is a very unstable idling engine that surges and may cut out as full throttle is applied because the over rich bottom end can disguise a too lean top end setting!
It is possible too set the idle screw adjustment in to far, but yet the idle speed is not high! Even though the air regulation (carb barrel or slide) may be set to a position that would normally equal a vary fast idle, the idle is low because the low speed mixture adjustment is set so rich that the engine loads up with excess fuel, and the engine goes into what’s commonly called a four cycle idle. One tell-tale sign of this is if after revving up the completely warmed up engine, it tends to idle fast for a few seconds, then drops to a lower idle speed. In other words, it’s something like da..da..da..da..da..da..da..da then it drops to da….da….da….da. If you start leaning the bottom end a little at a time (then repeat the revving up and idle test) and it takes longer before the idle drops, you’re going in the right direction. Eventually as you keep leaning the bottom, the idle will stay too high. Now it is time to lower the idle to where it belongs by re-adjusting the idle screw. Now that you have the idle set correctly, the top end may be too lean! Keep in mind the fuel does not directly enter the cylinder area like a 4 stroke engine; it enters the crankcase area first, then is transferred or pumped up to the cylinder area by the piston movement. Simply put, the crank case volume can hold much more capacity than the cylinder, so it takes some time to burn off the residual fuel. If the bottom end is too rich, the engine will be supplied by this residual fuel briefly, and depending on the demand you may be actually be experiencing a lean condition on the top end that can range from:
1 - Seems to run well, but engine life is short
2 - Seems to run well, but car continues to get hotter the longer you run to the point of overheat!
3 - Seems to run ok on the bottom, but sputters starves or strains to gain rpm
4 - Seems to run ok on the bottom, but when you give full throttle it cuts out or stalls
An overly rich top end can act the same as 3 & 4, but excessive smoke and oil are usually present with a distinct blubbering sound.
There is no reason for an engine to continually get hotter unless the tune is wrong (classic #2 symptom) or there is a mechanical problem causing more load or drag on the engine as the run continues. An exception is if the weather or track conditions change dramatically - for example, a light drizzle starts and the off road track goes from a very dry loose to high traction condition, or during a race a rapid weather front happens, causing something like a ten degree change in temperature.
It is very important to fully warm up your engine, clutch and chassis before making final adjustments. The chassis in most applications also works like a heat sink to the engine so it is important to fully saturate the chassis!!! I like to start the engine at least three or four minutes before the qualifier to get some heat in the engine. If you don’t yet have your radio, you can operate the throttle by hand. Remember though, it will still take at least two to three minutes of hard running on the track to fully saturate the chassis!
Carb Needles
There is a series of restrictions to control fuel flow at different throttle/air flow positions called needles These are the five basic parts of the carb to concern yourself with.
The slide or barrel regulates the amount of air that can enter the engine. It is controlled by the servo. It simply blocks off the airflow to the engine, proportional to how far it is open or closed.
The idle/air speed screw sets the absolute minimum air the barrel/slide can control to maintain idle speed. It simply is an adjustment screw that comes in contact with the side at the nearly closed/idle position.
The high speed needle regulates maximum fuel flow allowed to enter the engine at any throttle position. It is simply a tapered needle that screws into the fuel flow orifice (an adjustable restriction). This maximum fuel flow ideally is adjusted to the correct mixture ratio for the surrounding conditions when the throttle is wide open.
The low speed needle regulates fuel to engine at idle. The low speed adjustment simply restricts the flow at idle speed. If you look down the bore of the carb you will usually see a long tapered needle. When the carb barrel/slide is closed the larger part or diameter portion of the long tapered needle is inserted into the spray bar, this is what’s adjusted when you turn the low speed/minimum adjustment. It literally moves either the tapered needle or the spray bar farther in or out, changing the restriction independent of the barrel/side position. This leans (more restriction) or richens (less restriction) the flow from the spray bar at idle. On some carbs the spray bar is moved and on others the needle assembly is moved. Both have the same effect.
The mid range needle regulates fuel to the engine after idle and before full fuel position. As you open and close the carb the tapered needle (mentioned above in item 4) enters into a small tube this is called the spray bar/jet. This spray bar is where ALL the fuel enters the airflow stream regulated by the high speed, mid range and idle/minimum adjustments! Normally at somewhere between ½ and ¾ throttle open position the needle is completely out of the spray bar, This is what is called full fuel position or FFP. At this point 100% of the mixture is controlled by the high speed needle. On many carbs the low speed and midrange are not independently adjustable, so the mid range is a factor of the needle taper and is engineered by the factory.
If there are adjustments on both the slide and the carb body, one is an adjustable mid range, and the other the low speed!!! Check with the engine manufacture before attempting to adjust these types of carbs! It is very important not to use the mid screw to adjust the low end by mistake; it is very easy to get the carb way out of sync, and the gains are VERY small and mostly limited to minute midrange drivability/economy changes that only the most sophisticated driver will recognize. If you do this wrong, you might end up with a carb that is so screwed up only an expert can get it back in tune!

When a new plug wire just goes slightly gray after a 5 or 10 minute hard run it means your very close to an optimal horsepower tune, but be careful - the next step is TOO LEAN!
If the wire and surrounding bottom of plug is wet, with like new shiny wire, you’re on the rich side of optimum power.
If the wire and surrounding bottom of the plug is starting to dry and the wire starting to gray, you’re very close to optimum power.
If the wire and surrounding bottom of plug is dry, and the wire is totally gray but not distorted, you’re at optimum power.
If the wire and surrounding bottom of the plug is dry, and the wire is distorted, you’re slightly lean.
If the wire and surrounding bottom of plug is dry, the wire is broken and distorted or burnt up, your engine is extremely lean and there’s the possibility of engine damage.
You can only “Read” your plug in a nearly new state (wire like new and shiny). A gray plug can still operate well. After it has totally gone gray, performance can start to fall off. To test, just put in a new plug. If there is no difference in performance, save the gray one or put it back in. If your engine does not feel or run right, try a new plug before making major tune changes.

A longer manifold means more bottom end torque (at the expense of some rpm). A shorter manifold means more top end rpm (at the expense of some torque). You can go to far in either direction, causing the engine to run erratic or hot! You must keep it in the “sweet spot”. There is no set length for every engine or pipe. Experiment with the length to suit your driving style or track needs.

If the engine is out of control and at high RPM never use your hand on the flywheel! Some racers flip the car over and stick the toe of their shoe on the flywheel, but the engine can break before it’s stopped. A better way is to hold the car down hard so the wheels can’t spin (this may waste the clutch shoes but it is cheaper than an engine). Yank the body if you can’t get to the air cleaner, then yank the air cleaner and put your finger on the carb intake. Do not just push your finger down on the foam air cleaner, which will suck all the dirt into the engine! Holding a rag on the exhaust works too if you happen to have one handy, but, be sure there are several layers of rag it will take more than a few seconds to choke it and it will be very hot!

Always shake the fuel before using it. Remember that water absorption is the #1 reason for bad fuel. The methanol in the fuel is hygroscopic, meaning it tries to absorb water.
1 - Never leave the bottle open any longer than it takes to fill your fuel bulb.
2 - Never leave the FUEL BULB uncapped any longer than it takes to fill the car.
3 - Always rinse out you fuel bulb at the start of each day with a little fresh fuel.
4 - Never draw fuel back into your bulb or put it back in the jug.
5 - Keep your fuel out of direct sunlight and off the ground.
6 - Store your fuel in a stable temperature.
7 - Squeeze the extra air out of the bottle before tightening the lid.

If your engine was tuned correctly, to start with it will not harm it running out of fuel. If just before running out of fuel the car was tuned slightly rich, it will speed up for a brief moment just before it dies as you pass through the “ideal performance” mixture stage. If it was tuned for ideal performance, it will fall off in power just before it dies as it passes through the too lean stage. If it was tuned too lean to start with, it will bog down noticeably and may blow the plug as it pass though the “way to lean you just screwed up stage”. This can cause some damage to the piston and sleeve if the element wire breaks and falls in to the engine. However, that brief lean moment should not hurt the bearings, bushings, rod, crank etc. if you’re using high quality fuel.

If you switch brands it would not hurt to let the engine idle a few minutes before you go out on the track. Run one half or one tank of fuel with the engine tuned on the rich side. Check your glow plug, and if it looks good, fine tune and go for it. The reason for checking the plug is that different fuels have different oil additives and detergent packages. This may wash away some of the built up oil deposits and platelets or bonding agents (as the oil companies call them) and foul the plug. After a few runs your new fuel will leave deposits of there own. With some brands you can see the residue, because of its blue or red color. With other brands you can’t see the residue because they’re a light yellow or amber color but it's still there. It is normal to have these build-ups and they cause no harm.

It is always a good idea to use some after run oil. Marvel Mystery oil works great and can be had at any automotive or hardware store. Other substitutes would be automatic Trans fluid, 3-in-1 oil or mineral oil from the drug store! Never use any oils that contain silicones! To use after run oil, let the tank run completely empty at idle until it runs out of fuel (try re-starting the engine several times). Next put a few drops of after-run oil down the carb (hold it open) and a few drops down the glow plug hole. Leave the plug out and put a rag over the top of the engine. Spin the engine over for five seconds on the starter box. Put a few more drops down the carb, re-install the glow pug and spin the engine over five more seconds. You’re done!

First, follow the after run oil instructions above! Remove the engine from the car. You can then wash it off but be careful not to wash dirt into the front bearing (even if it has a rubber sealed type). Brush it off behind the flywheel then wrap a strip of cloth around the crank, “wedging” it between the front of the engine and back of the flywheel. Plug the carb and exhaust (be sure there is a glow plug installed) then squirt it off with methanol or denatured alcohol. Don’t use isopropyl alcohol as it is 30% water. Do not use brake cleaner or motor spray. If compressed air is available, blow of off the engine; if not, shake the engine vigorously and blow it off until dry with a hair dryer, being careful not to get it hot, just warm to the touch. Remove the strip of cloth and let the engine come to ambient temp (so there is no condensation formed inside) and put the engine in a zip lock bag or re-install in your car.
Removing the carb should not be a problem and is something that is commonly done by racers during cleaning and maintenance. If the carb seems tight compleatly remove the pinch bar assembly (screw and to clap bars) by totally removing the screw. You can then thread in the screw from the opposite side to allow you to "Pull" out the bar half that is threaded. Then you can tap out the other side bar with a small pin or punch. Drip a bit of oil down where the assembly contacts the carb before twisting out the carb.

The most common cause is "Hydraulic lock", meaning too much fuel is getting in the engine when it is not running. Liquids do not compress. Don’t force your engine to turn, or you may damage it. You can remove the glow-plug, then crank the engine over to get rid of the excess fuel. To prevent this, always empty the fuel thank when working on or storing the car. Also, always keep the carb higher than the fuel tank when it is not running. You can use a forceps (surgical clamp) to pinch off the line between runs, or store the car standing on end, nose down.

A common cause of dirt ingestion in off-road racing is through the exhaust. You crash or cartwheel the car, the exhaust stinger gets stuck into the dirt, and the engine stalls. There is always residue fuel and oil in the pipe, and it mixes with the dirt. Then you carry the car, and are not careful how you hold the car, which allows the dirt/fuel/oil to run back into the exhaust port!!!! A second cause is that the pipe has a reverse pulse action (it’s part of what make a tuned pipe work) and when the engine is re-started, the dirt in pipe ends up in engine!!
On-road racers often suffer from another variation of this problem. You’re at a race, and you blow up your old trusty engine. You bolt in your spare or even brand new back up engine. DID you remember to wash the debris out of your pipe from the exploded engine before mounting and starting your new engine? Not doing so has led to many brand new engines ruined just spinning them over on the starter box the first time!

A longer line "can" lean out the midrange when the throttle is applied rapidly! Keep in mind pressure is self equalizing (the tank and pipe pressure will be the same). The only consideration is if the pressure feed line is restrictive, (i.e. long length or small diameter) it can delay the time it takes the tank to reach maximum pipe pressure. Pressure dictates fuel flow at any given needle setting; therefore a leaner midrange can result! There is no ideal length; this is something you can experiment with. In most cases 8" to 14" is common. This is something I would not concern my self with until you have completely mastered the art of tuning the standard low and high-end needles. Line length has a very small effect by comparison and should be considered "fine" tuning.

Many engines are damaged every year by "locking devices" By the way, the thread pitch on the crank is “national extra fine” and is a special aircraft pitch, which is normally cut on the lathe! A safe way to lock the engine is to hold the engine upside down (with glow plug installed), fill the cylinder through the exhaust port with after run oil (do not use silicone oils) and let it "hydraulic lock" This evenly distributes the load over the entire surface of the piston. When done simply pull the plug and drain the excess oil!!! No harm done and the cost is pennies.
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Old 07-09-2007, 07:57 AM
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Basic Engine Tuning
by Mike Myers

Tuning your engine is one of the first things you need to learn, but which takes a long time to get good at.

The following article is based on information I’ve gotten from Art Carbonell, Rick Davis, Lou Przybyla, and Ron Paris. Much of it is written around a paper that Ron included with gas engines that he sold. I’ve tried to simplify things so that the article will make sense both to someone who’s never worked with gas engines, as well as people who do this all the time..
If you’re first starting out with a new motor, you have to thoroughly clean it to remove any manufacturing debris. When that’s done, you need to seal the areas that otherwise might develop air leaks. The last step, is to tune your engine.

The Carburetor
If you’ve got a new carburetor, you need to clean it out, removing any manufacturing debris. Don’t assume that this was done for you at the factory. You also need to eliminate potential air leaks - if you’ve got air leaks anyplace in the fuel supply system (fuel line, carburetor, etc.) your engine won’t run properly and you won’t be able to tune it. Check for burrs and chips in the slide and throat of the carb, then loosen the hex fitting that holds the fuel inlet nipple, and position it in such a way that your fuel line from the gas tank is as short and straight as possible. Retighten the hex fitting rather snugly, then apply some automotive silicone seal around the fitting gaskets to prevent air leaks. Before installing the carb on the engine, place a small amount of silicone around the original "o" ring seal on the carb to prevent air leaks there. Remember that the silicone should be fully cured before attempting to start the engine, because the vapors from curing silicone are very corrosive, and will ruin your glow plug. Glue the bellows (boot) to the carburetor slide with some of the thick, gap filling super glue. Don’t overdo it. The large end of the bellows that goes over the carb casting should have a small tie-wrap around it to prevent air leaks. (Newer engines have many of these things set perfectly by the factory. If you're working on an older engine, they are still important areas to check, before you use the engine.)

Air Filters.
The air filter is one of the most critical components of your racecar. A good quality, well maintained air filtration system will dramatically increase the life expectancy of your engine. The smallest amounts of dirt or dust will act as sandpaper between the piston and sleeve, and at RPM up to 40,000, it will only take seconds to ruin the critical piston fit of your engine, resulting in loss of compression and performance. Air filters come in two main styles, paper and foam. If you’re using a foam filter, you need to saturate it with air filter oil, then squeeze out the excess. Clean (or replace) foam filters frequently. They are far less costly than a new or rebuilt engine. Don’t wash paper filters; clean them by blowing them out gently, from inside out, then tap the open (air inlet side) of the air filter sharply 4 or 5 times on a clean piece of white paper. If dirt still comes out after blowing, replace the element. Hold the element up to a light bulb and inspect for tears or holes. If in doubt, replace the filter. Use a tie-wrap on the rubber boot to the carb, and another on the boot to the element.

Types of Fuel
Many engines are ruined each year by using the incorrect fuel. Use a fuel designed for R/C cars. Airplane fuels will not protect your engine. Keep in mind that the airplane’s propeller also keeps its engine much cooler than in a car. Some experts used to recommend adding additional oil to any fuel that you’re not absolutely certain is mixed properly for r/c cars. Add 2 ounces to a gallon of pre-mixed fuel. R/C fuel is available in different percentages of nitro. The higher the nitro percentage, the more power you get out of the engine, but also the more wear and tear you’ll get. Most of the people I’ve talked to recommend 20% nitro as a good mixture for general all-around use. Higher nitro percentages are common, but make sure your engine has adequate "head clearance" if you're going to use high nitro percentages. Make certain that you store your fuel in well-sealed containers, kept in a cool area. Remember to NOT suck the fuel back out of the fuel tank and return it to your fuel bottle when you're done racing. Fuel in the fuel tank is contaminated by the exhaust gasses that get into the tank to pressurize it. At the end of the day, that fuel should be disposed ot.

Engine Break-In
"Break-in" is easier than it used to be, because most modern engine parts are precision finished to the proper specifications. All that has to be done is to burnish-in the piston and sleeve fit. Ron Paris didn't recommend long, over-rich break-in periods. You should get the engine up to operating temperature as soon as possible. Ron tuned new engines so they went out on the track using a slightly rich fuel mixture (not quite a 4-cycle sound). He ran1 tank of fuel, non stop, then let the engine cool. He tuned the second tank of fuel for race conditions and that’s all there is to it. Art Carbonell recommends running several tanks of fuel through an engine, starting with an overly rich mixture, and using a little more oil in the fuel mixture. Ron's latest advice on engine break in were to start the engine, and let the car run at idle for five minutes, then shut it off and let it sit for five minutes, turning the flywheel such that it was "free", and not "locked up" at TDC (the very top). Repeat this process three times, then go out and drive smoothly, gradually leaning out the engine for proper performance.

Race Tuning Tips
The hardest part of 1/8 scale racing is fine-tuning the engine. You need to learn this on your own. Spend some time, even an entire practice day, developing this skill. Always remember, it never hurts to have the fuel mixture too rich, but never, NEVER let the mixture get too lean.
Ron’s tuning procedure is as follows:
1 - set the engine’s idle speed up slightly higher than normal
2 - start tuning your engine with the settings too rich
3 - always tune top end first
4 - tune bottom end next
5 - reset idle speed

Most people tune their engine by measuring the engine’s temperature, and making the required adjustments to make the temperature correct. There are two ways to measure the engine’s temperature:
a) Temperature gage method: Get the engine up to running temperature, bring the car into the pits and immediately take a temperature reading. Place the temperature gage directly over the engine, pointed at the glow plug, and take a reading. It’s easier to do if you have a second person to take the readings for you.
b) Spit method: Get the engine up to running temperature, then pull in as quickly as possible and put some spit on the engine’s cylinder head (quickly, so as to not burn your finger). The saliva should just slowly boil off (2 to 3 seconds). It should NOT dance around as if it were on a hot griddle, nor should it lay there and steam. In short, if your spittle will sizzle, it's too hot.

If the engine is too rich, the engine temperature will be colder than desired (and vice versa). If the engine is too lean, the engine temperature will be hotter than desired. Go out on the track with the top end rich. If you’ve got the mixture set right, there will be heavy smoke from the engine on the straightaway. Run four or five full laps to get the engine up to running temperature before touching the carb. Bring the car in, and take a temperature reading. Start leaning out the top end by turning the adjusting screw only 1/12 turn at a time (picture a clock’s 12 even spaces). Take your time doing the adjusting - don’t be in a hurry. It may take a while. Your goal will be to get the car to just "punch clean" when you come onto the straightaway, which is what it will do when the mixture is set properly. For 1/8 scale cars, you’ll have the correct mixture when the temperature is about 200 degrees. The 1/10 scale cars run a little hotter, maybe around 250 degrees.

Once you think you have the top end set, run 3 or 4 laps, then stop the car close to you on the track and let it idle for 5 seconds, then "push off". It should have slightly loaded up, but still accelerate quickly. If the engine died before the 5 seconds, check the following:
If the engine seemed to load up and slowly stop, it was probably too rich.
If the engine’s idle speed increased before it stopped, it was probably too lean.

Always run a few laps before testing bottom end and idle. Ron likes his car to idle clean for 5 seconds, but by 6 or 7 seconds his engine loads up when he punches off (heavy smoke with a slight stumble). At this point you may have to re-adjust the engine’s idle speed. If the idle speed is too high, the clutch will not release completely and you will lose "snap" off the corners. If the idle speed is too low, the engine may stall on the starting line, or at the end of the straightaway when you let off the throttle.

Things to Remember:
1 - Never try to tune a cold engine!
2 - Adjust top end first
3 - Always tune from rich to lean. If in doubt, richen it up first.

Your glow plug wire should stay bright like chrome. If it turns dull, or gray, the engine was probably too lean on top or bottom, or both. Note: wire may also distort or be burned up if it’s too lean. If glow wire is still shiny like new, but distorted, you may have to add a 0.004" shim or use a lower percentage of nitro.
Over 99% of all engine complaints are usually related to the tuning of the engine or clutch.

The clutch setting will have a significant effect on engine performance. If the clutch is set too heavy, the engine will bog and there will be very little exhaust smoke (which is very easy to confuse with a lean bottom end). If the clutch is set too light, this will let the engine buzz, (too much rpm) and will over-heat the motor. It will start sagging off the corners (which is easy to confuse with too rich bottom end). It may take 5 or 10 laps before you notice this. Dirt or sand imbedded in your clutch shoes will also make your engine hot. Clean the engine’s clutch shoes by scraping them with a sharp knife.

Adjusting the linkage:
Another cause of "boggy" engine performance is the carb and brake linkage. Make sure that you are getting full throttle, and that the brakes on the car are fully released before the carb starts to open at all. Also, check to see that your brakes do not start to come back on at full throttle (the brake linkage going over center may cause this).

Care and Maintenance
Proper engine maintenance is very important, yet relatively easy. Never leave fuel in the engine when you put your car away. Fuel is "hydroscopic", meaning it absorbs moisture from the atmosphere, and this will cause rust to form inside your engine. If you run regularly (3 or 4 times a month), and use good fuel, remove any leftover fuel in the car’s fuel tank, then let the car run until it runs out of fuel. If you don’t run that often, or if you use lower quality fuel or synthetic oils, remove the glow plug and air cleaner, put a few drops of Marvel Mystery Oil (available at most auto parts stores) down the carb and cylinder head, spin the engine for 2 or 3 seconds with your starter, and then replace the glow plug and air cleaner.
A good practice is to drain your fuel tank immediately after running, even between rounds. If you should work on your car with fuel in it, and raise the fuel tank higher than the engine, excess fuel will bleed into the engine. Fuel is liquid, and liquids do not compress. If you try to turn your engine over while it has fuel inside it that has leaked into the engine, you may bend the connecting rod, or shatter the rod bushings. If this happens, the engine will fail soon afterwards. When you’re ready to start your car, it's a good idea to first loosen the glow plug and crank the engine over a few times. If there is any excess fuel that leaked into the engine, this way it can safely escape through the glow plug hole, instead of damaging your engine. A tip Lou Przybyla showed me is that when he isn’t working on his car between rounds, he stands it up on end leaning against his work-table, nose end down. This way any excess fuel will run down into the gas tank, not into the engine.

Connecting Rods:
Connecting rods are the most vulnerable part of the engine and should be considered a "wear item" along with your tires, clutch, glow plugs, etc. If a rod fails, it can easily ruin the piston, sleeve, crank, or even the engine case. Ron recommends changing rods on a regular basis. There is no set time limit, but here are some tips. If you have over-heated the motor badly, you can expect the rod to fail soon afterwards. The heat-treat will be ruined and the rod starts to stretch and will eventually snap. Over revving is also a common source of connecting rod failure. This can be caused by losing your radio signal, resulting in the car running away with the engine revving up, with no load. Two-speed cars missing shifts will fatigue the connecting rod very quickly. It may fail on the spot, or soon afterwards. Replacing rods before they fail is cheap preventative maintenance, and will save you money.
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Old 07-09-2007, 11:16 AM
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ok... i think that about clears out a lot of engine questions

I'm gonna throw this one in here... Sealing and the PINCH Test

Airtight is Right

In order for an engine to idle, transition and tune consistently it must be airtight- airtight meaning that no air can enter the engine except through the carburetor intake. Surprisingly, even brand new out-of-the-box high-end engines can have air leaks and until these leaks are found and eliminated, the engine won't run consistently and cannot be precisely tuned. The good news is, several years ago I figured out an easy way to check for and fix air leaks. You'll need the following:

• A small glass of soapy water
• A small brush
• About a foot of fuel tubing

Step 1. Be sure that the high-speed needle is open at least two turns out and the throttle is at fully open.
Step 2. Install one end of the fuel tubing on the carburetor nipple. You're going to be blowing into the other end of the tubing to pressurize the engine crankcase.
Step 3. Place your thumb over the carburetor intake to prevent air from escaping, then blow into the fuel tubing while brushing soapy water around the base of the carburetor, on both sides of the pinch bolt, around the fuel fittings, at the high and low speed needle valves, around the front bearing and even around the back plate.

If any bubbling occurs you've got an air leak. But don't be alarmed. Fixing the leak is easy and your engine will run remarkably better and more consistently when you've eliminated these leaks.

The Fix

Brushing soapy water around all possible air leak points while pressurizing the case will easily identify air leaks.

The most common air leaks seem to occur around the pinch bolt. Applying a dab of RTV (or silicon) over both exposed sides of the pinch bolt will fix this type of leak. RTV is also great for sealing any leaks around the base of the carburetor. Simply remove the carburetor and run a small amount of RTV around the base of the carb where the leak occurred. Reinstall the carburetor and allow the RTV to ooze out between the crankcase and the carb body. For leaks around the high-speed needle valve fittings, usually retightening the high-speed needle assembly will take care of these. Leaks directly at the high speed or low speed needle usually means there's a damaged O-ring that must be replaced. Many racers don't realize that the front bearing can leak air. That's why nearly every high-end engine uses a rubber sealed front ball bearing to not only keep out dirt, but also air. If your front bearing leaks air, it's time for a bearing replacement.

This simple leak check can easily be done even when the engine is installed in a car. After you've done this test on several engines you'll be surprised of two things: first, how often unwanted air leaks occur and second, how much better your engine runs, idles and tunes when it's airtight. To run right it must be airtight!

Airtight Fuel system
The Fuel system must also be airtight. If the fuel tank, tank lid, fuel lines or filter has a leak then the same kind of tuning issues as described above will occur. While you can check the fuel system by using the same method of brushing soapy water onto the suspected offending area while blowing into the tank, I generally find it better to simply remove the tank from the car and submerse it in a sink of water while blowing into the vent line and plugging the fuel line. If bubbles are present you've got a leak.

An engine at full throttle produces about 6 to 8 psi in the vent line and conversely in the tank. By blowing really hard you're also able to produce about 6 to 8 psi for a couple of seconds. If you're able to blow hard enough to cause the tank lid to open slightly and leak, it's time to replace the tank lid spring or bend more preload in the spring. The entire system (fuel tank, fuel filters, fuel lines, tank fittings, etc.) must be airtight to achieve consistent repeatable performance. I always that my fuel system is airtight when it's new but also recheck it every time I rebuild a car after a race. It easy insurance plus soapy water is a good way to thoroughly clean the tank. Just be sure it's totally dry before using it by flushing out any water left in the tank with fuel before installing it in you car.

Tuning in a Pinch
The pinch test.

Many racers find it difficult to properly tune the low speed needle. We've developed a method that makes it easy, even for beginners, to adjust the low speed needle to within 5% of peak performance and it's done right on the bench. We call it the pinch test and here's how it works.

Step 1. Start your engine and warm it up for about 30 seconds by revving the engine from idle to mid-throttle while holding the wheels off the ground.

Step 2. Allow the engine to idle for 10 seconds. If the low speed needle tuning is so far off that it won't idle, simply bump the idle position higher until the engine will reliably run at a higher idle.

Step 3. With the engine at idle, pinch and hold the fuel line near the carburetor, cutting off the fuel flow and carefully listen to the engine rpm.

If the low speed needle is set correctly, the engine should increase rpm only slightly and then die.

If the engine increases several hundred rpm before dying, then the low speed needle is too rich and needs to be turned in.

If the rpm doesn't increase at all and the engine simply dies, the low speed needle is set too lean and should be richened or turned out.

After doing this a few times you'll get the hang of it, and tuning the low speed needle will become easy. The only way to get that last 5% performance on the low speed tune is through careful track testing. By evaluating the engine as it accelerates from a dead stop or out of slow corners during actual race conditions is the only way to get those last few percent of power from your engine. Variables like the clutch setting, gearing, traction, atmospheric conditions and even the car's weight will have an effect on this last 5% of tuning performance, and the optimum setting can only be reached by careful on-track tuning.

Fine Tuning in a Pinch
On high traction tracks instant throttle response and acceleration are the keys to being competitive. To get that last 5% of performance thru tuning, many top drivers and engine tuners use the punch test. Here's how it works.

With the high speed needle properly set and the low speed needle set per the pinch test above, run several laps at race pace to bring the engine up to temperature. Now stop the car at the beginning of the main straight and allow it to idle while you count to 15. At the count of 15, hit the throttle instantly full punch while carefully listening to the engine and note the rate of acceleration. If you did a good job with the pinch test above and the high-speed needle is properly set as described below, the car should rocket off the line with a powerful, crisp consistent engine sound. The trick here is to tune the low speed needle for even greater acceleration and an even crisper throttle response. Do this punch test several times until your very familiar with the acceleration rate (it helps to judge the amount of wheel spin) and the sound of the engine as it accelerates to full throttle. Now lean the low speed needle 1 hour (or 1/12th of a turn) and do the punch test again. Is the engine's acceleration rate and the crisp consistent sound better than in the previous test? Remember judging the wheel spin (the amount the wheels slip during heavy acceleration) is a good way to compare. If it's better or there's no change, lean the low speed needle 1 hour more and try the punch test again. At some point the engine will lean bog (hesitate) off the line during acceleration indicating that you've gone too far.

When you finally find this point through repeated punch tests then leaning the low speed needle one hour each time, richen the low speed needle 2 hours. This will be your engines peak setting giving maximum acceleration and throttle response for the specific conditions of the day. Just remember if you change anything, like gear ratios, clutch settings, exhaust systems, fuel, glow plugs and even changing atmospheric conditions like temperature and barometric will cause this optimum setting to change. Repeat the punch test often and especially just before a heat race to ensure maximum performance during the existing conditions.

The Dreaded Full Tank Lean Bog

If you've done much gas racing, you've probably encountered the dreaded lean bog that occurs right after a pit stop with a full tank of fuel. Right after the tank is filled, for about the next 3 or 4 laps the engine sags (lean bogs) when accelerating off of tight right-hand corners. On most tracks this happens when accelerating onto the straight. Seemingly, the lean bog magically goes away after a few laps but sure enough, after the next pit stop, it frustratingly returns-slowing lap times for several laps once again.
The key to eliminating full tank lean bogs is pressure tube position or adding a pressure reservoir as shown.

To solve this frustrating issue, it's important to understand what's causing it.
Here's the scoop. As the throttle is released entering a turn, the pressure in the tank is now greater that the reduced pressure in the pipe. When the tank is full, some of the fuel sloshes up into the pressure line. The centrifugal forces encountered in a right hand turn further pulls the fuel into the pressure line, causing reduced pressure in the tank. When the throttle is cracked open to accelerate, the engine goes lean because of this reduced tank pressure and hence the dreaded lean bog occurs. The reason it only lasts for a couple of laps is first, as the fuel level goes down in the tank it's less likely to slosh as much into the pressure line and second, as the fuel level goes down there is a larger open space in the tank that's pressurized, maintaining adequate pressure long enough to prevent the low pressure lean bog problem. (If your exhaust system is mounted on the left side, as with some of the latest monster trucks, then this phenomenon will occur in left hand turns.)

The Fix

Keep the fuel out of the pressure line! That's sometimes easier said than done. We've had the most success by rerouting the pressure line so that as it come off the pressure nipple, the tubing goes to the right side of the fuel tank, then makes a loop on the right side of the tank. This prevents the centrifugal force of a right hand turn from filling the pressure tube. We've also had good success by adding one of the big reservoirs (they look like a big aluminum fuel filter and are available from Mugen or OFNA) in the pressure line. Many manufacturers, like Mugen and Serpent, recognize this problem and are making special pressure pickups and tank lids with large open volumes in the lid itself. You may have noted that many top drivers' cars have several loops of fuel tubing wrapped in their cars and wondered what was the purpose. Now you know. The key to fixing this issue is to experiment with pressure tube routing and, if necessary, installing a pressure reservoir in the pressure line.

Vapor Lock

Compressed CO2 sprayed directly onto the carburetor is a great way to prevent vapor lock during long main flame-outs.

If you've ever had your engine flame out after a reasonably long run and then found it nearly impossible to re-start, you may have experienced vapor lock. Vapor lock occurs most often in .12- and .15-size engines, especially when they're enclosed in a body with a 1/10-scale gas sedan or stadium truck. Vapor lock normally occurs after the engine is shut off. Heat from the head and combustion chamber transfers through the crankcase, heating the carburetor to the point that the fuel that reaches the carburetor through the fuel line boils off. This makes it impossible to start the engine because it's starved for fuel. The only way to get the engine to run is to cool the carburetor below the fuel's boiling point. If you're sport driving, you can simply wait 5 minutes for the carb to cool on it's own and then restart. If you're in the middle of an important long main, here are some creative ways racers have (including at the world championship level) cooled their carburetors preventing vapor lock:

1. A bottle of compressed CO2 (or air like those that can be purchased to operate airbrushes) with a nozzle attached, allowing cold CO2 to be directed over the carburetor even when the body is in place. We've seen several top-level race teams use this CO2 method anytime the engine flames out to prevent the possibility of a vapor lock.

2. By pulling the air filter and simply dumping fuel from the fuel bottle over the carburetor, usually bringing the temperature back to a tolerable level within a minute. Just be sure to avoid getting fuel on the brakes, radio equipment or tires or you could be in for a bad race day!
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Old 07-09-2007, 11:35 AM
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