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.
Clutches:
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.