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Engine Problems

Old 10-05-2005, 06:40 AM
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Default Engine Problems

I have installed a Picco .27 in my LST. I am not the best engine tuner, but since I got the LST I have gotten much better. Anyway, trying to tune this thing has me totally confused. He is what it is doing. It acts as if it is getting no throttle, especially after take-off, so I am assuming the high needle setting. Low-end feels good, but if I give it about 1/4 throttle and hold it constant it seems to cut out just like when at full throttle. Feels like I am letting off the throttle. I first thought the batteries were bad, so I actually installed 2 receiver packs, and I did a radio check by having my friend control the radio as I walked the truck around the parking lot. So I think I can rule out radio interferance.

This thing is also a PITA to start. It will usually start the first time of the day within 3 pulls, then it takes forever to get it to start, even if allowed to cool. I found that if I remove the fuel line from the carb, then try to start it until it actually starts, then replace the fuel line, it will at least try to start within 3 pulls?

I am confused as I was thinking this was a high-speed problem, so I would richen it up all the way up to a full turn out and I get the same problems, then I would lean it out a full turn, I get the same results. During all this, the temp always stays in the 150-205 range? Even when leaning it out it hardle hit 200. This is with the body on the truck.

What is weird is that if I hold it full throttle, it just cuts out, it does not bog down or want to stall, just as if I let off the throttle. I can hold it full throttle for 5-10 seconds and it never wants to die just a mix between full throttle and no throttle.

Thanks for your help and time
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Old 10-05-2005, 07:00 AM
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adjust your idle needle it may not be opening the carb that much or try this--------MAXIMIZE YOUR ENGINE'S POWER POTENTIAL
So now you can get your engine started on a regular basis, but youíre 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. Thereís a certain ďfeelĒ to how your car drives and a certain sound youíll 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 isnít running properly. When itís time to tune your engine, thereís 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 itís 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, itís 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 itís 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 ďblipĒ 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: thereís 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 thereís no sense in pushing the mixture settings so lean that you risk damage to the engine. I canít 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. Iíll 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, letís 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 trackís longest straight section in the shortest time. Estimate the time by ďfeelĒ 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 neednít 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. Itís a common mistake to establish mixture settings for maximum punchóas though the car will be run within the confines of a tight racetrackóand 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 itís 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 letís properly set the low-speed needle. Itís 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, itís wise to finish with the low-speed setting.

Again, itís 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. Itís a somewhat crude method and doesnít tell you what to expect from the engine on the track, but it will get you into the ballpark.

Another common way is the ďsee-how-long-it-will-idleĒ 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 doesnít tell you whether you have an artificially rich mixture to compensate for an idle speed thatís 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; thereís 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 canít 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 canít 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 isnít always that simple. Without getting into all the particulars of nitro fuel, Iíll 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 doesnít 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 thatís 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 plugís 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 donít 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. ďAdvancingĒ 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.

Itís a challenge to figure out a glow plugís temperature range. Manufacturers donít 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.

ďReadingĒ 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 idealóbut also close to trouble. If the plug stays wet and shiny for a few tanks of fuel, youíre in the safe zone; a little rich but safe. When the plug wire gets distorted or broken, however, youíre in real trouble. Itís 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, itís 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 isnít 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 wonít 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. Itís 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.


Itís 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 arenít made to the fuel-mixture settings. Ignoring an engineís 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 engineís 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 engineís potential power output, if the engine is tuned properly.
Air is thinner at higher altitudes, which means thereís 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 donít want to get into the science of tuned pipes here, Iíll simply suggest that volume (assuming the pipe doesnít 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 youíve cut too much and performance is suffering, simply increase the gap between the header and pipe, but donít 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.

Iíve 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.

Youíll 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 engineís 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 engineówhen tuned for peak power outputócan 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. Donít 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. Youíll 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 Iím 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 youíll find thereís 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 ďnoodle.Ē Itís hard to make a mistake you canít correct.

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:
ē A small glass of soapy water
ē A small brush
ē 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 Punch
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!

Written By: John Adams
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Old 10-05-2005, 07:06 AM
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plus this and you should be ok just a little reading http://www.maxxtraxxusa.com/Page.cfm?InfoID=3607
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Old 10-05-2005, 07:20 PM
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Wow!! Reading overload.

Reset your carb to the factory settings.

Pull your air filter off. Set the idle adjusting screw to allow
the slide in the carb to be open about 2mm.

Once the truck is running tune the high speed needle first
by getting the engine to run at a point where at top speed
it still has a bit of visible smoke trail.

Once the HSN is set, adjust the LSN (Low Speed Needle)
so that the performance on the bottom is to your liking.

Good Luck.
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Old 10-07-2005, 06:40 AM
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Thanks for the help.

Turns out this is not even engine related. The throttle servo is possesed or something

Had a friend watch the throttle as it was sitting on a curb and I was 50 yards away. I held the throttle at a constant 1/4 throttle for 30 seconds, then at 1/2 throttle. He said the throttle was twiching so bad that it would completely cose at times.

Not sure if it is a channel issue, receiver, or throttle servo.
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