Author Topic: How to Tune Gas Engines  (Read 667 times)

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Offline Chuck Baker

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How to Tune Gas Engines
« on: May 31, 2018, 10:59:38 PM »
Thanks to Vern Doty for finding these (2) articles on tuning gas engines:


How to Tune a Gas R/C Engine with a Walbro Carburetor

It is important to keep in mind that the low (L) speed needle is always active, and that the high (H) speed needle is only active above 1/4 throttle when there is sufficient low pressure in the venturi for the high-speed needle to start drawing fuel.

Here is the drill to start from scratch:

Low Speed Idle mixture:
Open both needles L~1.5 to 2.0 turns, H ~2 turns. This will ensure a rich setting. Close the choke valve and switch on ignition. Flip prop until the engine pops or runs for a short time then Open the choke valve. Using a slightly high idle throttle setting, start the engine and let it warm up a bit. Adjust the low speed idle needle (closest to the engine) for best rpm and then open it 1/4 to 1/2 turn. This will be a safe rich setting to start tuning, and from here the high-speed needle can be set.

High Speed mixture:
With the idle needle set about right, slowly apply full throttle. If the engine stays too rich, close the high needle a bit so the rich condition is cured, but no more than that. This is the basic high-speed needle setting, at which the engine will run without damage due to lean mixtures. Keep this setting while running in the engine. Let the engine run for about five minutes, then the spark plug can be checked. It should have a desert sand tan, or slightly darker. Not black, because that is an indication of an overly rich mixture, nor pale-white, because then the engine is too lean. This causes damage!

The next step is to get the mid-range just right. This can only be done after the engine is run in. If done before that, settings will change, and the process has to be repeated.

So far, you have been running the engine with a rich idle, and quite rich main needle settings. This causes four-stroking in the mid-range. Let the engine warm up, and apply full throttle, until the rpm has stabilized. Now slowly reduce throttle, until the engine starts to four stroke. Cure that condition by leaning the idle needle. Go back to full throttle, and adjust for max rpm, then open the needle 1/16 turn extra, and throttle back again. This time, the four stroking transition will be at a lower throttle setting. Repeat the above steps, until the engine runs well at all, but slightly high idle. check for crisp throttle response. If the engine lags, then open the idle needle until the condition is cured. Let the engine idle for a prolonged time and apply throttle. All should be well now, and throttle should be accepted quickly and clean, with maybe a very slight initial four-stroking to clean the lungs.

It is a fact of life, that two stroke engines do not fire every stroke when the rpm approach idle speeds. That means, that below half throttle, the engine may break into an uneven pace, so do not worry too much if the above-mentioned method does not provide clean running all the way down to idle. If after these adjustments the engine becomes harder to start, the idle needle is too lean, and if easy starting is needed, the needle should be opened up again and the high needle readjusted (leaned). Clean mid-range will suffer though. Sometime you can, but most of the time you can’t have the cake and eat it.

To recap: Because at full throttle both needles contribute to the mixture, you can run the engine with lean idle and richer main needle for good mid-range but starting will suffer. You can also run the engine with rich idle, and leaned out main needle, but mid-range will suffer and burble. A larger prop will run cleaner at mid-range. Too large a prop will make a good tuning very hard to do, and it will be almost impossible to get full throttle just right


Xipp engine tuning method (Salsa)

Petrol (Gas) Engine Tuning ***

Carbs aren't too difficult to tune up if you know what you're doing. First of all, you need to know how the carb works and how the settings interact with each other. About 95% of all the gas airplanes I've seen at the field are somewhat out of tune. How can I tell this? Simple, at some point the engines "four cycle" in flight. Two Cycle engines are not supposed to "four cycle" PERIOD. This is caused by a rich mixture that is forcing the sparkplug to intermittently miss making it sound like a four stroke. This is not good. HOWEVER the good news is; gasoline two stroke engines are very tolerant of rich settings (most of the time) and will run fine. You'll just consume a little more gasoline than necessary, and create a little more oil mess on your plane. You may eventually foul your spark plug as well. So why do so many people leave their engines tuned like this? Simple answer, the engine will start much easier when it's cold AND there's little or no warm up time needed prior to flying. Those are pretty good reasons! But the fact is... the engine is not running like it's supposed to.

(1) The low end needle on a Walbro carb is ALWAYS the one closest to the engine, the high end needle is the closest one to the intake/choke.

(2) There is no fuel adjustment for idle fuel, only air feed set by the idle stop or servo.

(3) Both low end AND high end needles feed the top end fuel supply.

Let's tune up a Walbro!

Set the low end & high end needles to about 1 to 1 1/2 turns each. Choke the carb or prime it, until the carb is wet. Fire up the engine and let it warm up. Let's set the top end first since it's the easier of the two. Go to full throttle. Adjust the top end needle for peak RPM. Leave it wide open for about a minute to see if it changes any. Should the engine go lean, open the low end needle slightly, if this doesn't work... you will have to adjust the needle valve inside the carb.( I will explain this later) If the top end runs OK, then slowly pull the throttle down until the engine begins to "four cycle" hold the throttle there. Adjust the low end needle until the "four cycling" stops. Now lower the throttle more until it "four cycles" again, and adjust the low end again. Keep doing this until you reach full idle. Now, from full idle begin to throttle up until the engine starts to bog or hesitate. Open up the top end needle just enough to eliminate the bog or hesitation.

When this is done right, you will be able to set the throttle in any position and it won't four cycle, plus you will be able to transition from idle to full power without any hesitation at all.

-Xipp, Member