Tuning a Gasser: Keep It Simple Terry's Custom from Flying Giants

I have gotten a couple of good questions about tuning gas engines. I am by NO MEANS an expert, but I have struggled with it for years and have settled on this method. In fact. I don't think there is a better way, and this is the method "masters" have shared with me so I have always used this method for tuning my gas engines. Keep it simple. I got this off a forum years ago, on Flying Giants, "by Terry's Custom".   Works well! The hardest part is trying to figure out if the engine is rich or lean...

RICH: Too much fuel, may need to turn needle IN (Righty Tighty = IN)
LEAN: Not enough fuel, may need to turn needle OUT (Lefty Loosy = OUT)

Good stuff for gasser tuning. 

From Terry’s Custom:

Here is the process:

Read each step and follow exactly and move the sticks exactly as I indicate, there is no slow stick movement when checking the H and L needles. Since both needles work together, slowly throttling down or up will tell you very little when doing your base needle settings.

Don't omit any steps and don't do any steps out of order and IT WILL WORK, follow each step and do exactly as described: 

Basic tuning:

1 - Tune the engine when it is warm, and double check the tune after a flight.

2 - On the Walbro there is a little silver screw on the carb that is an "idle set screw". it has a spring on it and it sets how low the low throttle arm will physically go down. Take that screw OUT and deposit it in the trash can. Do not remove the black H and L screws obviously.

3 - Start the engine and run it for a good few minutes with varied throttle and some full throttle run ups to get it nice and warm.

4 - If you have not done so, adjust the servo travel for a consistent idle, does not need to be perfect as long as it is as low as it will reliably run for at least 10 seconds or so. You can not properly tune the low needle if the idle is too high! For smaller engines set the low as low as it will reliably run and you can adjust after tuning. You will NOT use a high idle setting during tuning as you cannot tell exactly where the low needle is on a high idle. A good low idle should stand still on the ground while idling. An idle up should just make the plane want to roll on the ground.

5 - Tune LOW (L) needle first. Now run the engine up to clean it out with a couple full throttle run ups and then go to to low idle. Listen to the engine.........how long does it take to start to "load up" or start to slow down in RPM? If it is rough right away you are likely way to rich. Normal starting point on 100cc engines is 1.5L and 1.5H. I have had to go as low as 1 3/8 on the low needle on the Walbro carbs. Adjustments on gas carbs are VERY small, like the width of a screwdriver blade or 1/16th of a turn or less.

A good "safe" tuning on the low needle you should be able to clean it out with some run ups, return to idle and it should have a nice steady low idle for at least 15-20 seconds before it starts to load up. If it does this it is still just a "touch" rich but will not die on you and it will run nice and smooth.

6 - How low should react. Now that you have it running, clean out the engine and let it go to idle. Let it sit for 10 seconds or so and then slam the throttle to full, not slow, nail it!! Listen to what it does. If it dies or almost dies, you are too lean. If it stumbles and works it's way up (may see smoke in exhaust) you are rich. As tuned in number 5 above it should run up quickly with very little or no noticeable stumble.

7 - Now tune the HIGH (H) needle. There are several ways to do this with RPM but the most simple way for a safe needle setting is this. With the engine warm and the low needle tuned run the engine up to full throttle for about 5 seconds. Now chop it in one quick motion to low. Listen to what it does then. - If it returns to a steady idle, you are very close. - If the idle goes way low and works it's way up to a steady idle you are too rich.- If it stays at a high idle and then slows down you are too lean.- If it dies you are either way too rich or way too lean. With the H at 1.5 (on 100cc engines) you should not be too lean.

8 - When H is set return and double check the L and then double check the H. Then it's time for a flight. Listen to the engine carefully for all of the noted symptoms while you are flying. One engine test I do in-flight is for the H needle. Get the engine nice and hot, maybe a high speed pass or something. Now do a full throttle straight up line. Listen! Engine should be smooth and steady until you can't go higher. If it starts to sag at all, land immediately and richen the H needle just a tad and try it again.

This fade can also be caused from overheating not tuning related so make sure your engine is properly baffled if you now the tuning is correct.

There you go, pretty simple and really only takes a few minutes once you get it down.