In trying to tune gas engines for general sport flying, I have been trying to find anywhere a definite instruction rather than the old 'back off a couple of 100 rpm's', as to how much should the top end be reduced from it's max rpm before flying to guard against lean running.

I have heard some folks say, reduce by 100 rpm, others say 2 or 300, and another faction advocates to richen the high speed needle a 1/4 turn.

Anybody willing to take the plunge to be more specific on this, or is the variance of such that one can't be specific as each make and size of engine requires a different setting.

Best thing to do is if you think your close, put it in the air and run it full throttle for a minute or two. Pull back to idle and land immediately and shut off engine. ie. no taxing around. Let it cool down and pull the plug. If the center electrode is tan, you're just right. Black means too rich, and white is too lean. Adjust high needle 1/12th turn one way or the other depending on if it's black or white and retest. Don't forget if you adjust the low, this will effect the high speed mixture as well.

karolh, I think it is simply not that critical. If you are to the point that you need every last rpm that an engine will turn then you may need a larger engine.

I richen my engines by about 200 rpm from peak which is usually about 1/8 turn and have never had a problem with lean runs. The spark plug also runs a nice tan color.

I don't think you will find any information more precise than this, only different opinions.

Ken

Set the high speed needle so the engine pulls best in a long vertical upline. If it sags or slows down, you need to richen the needle.

You say "gas" engines. Do you mean gasoline or glow fuel? For a gasoline engine, they can be run at a full lean setting. In fact, once set, they seldom need adjusting. Glow fuel engines are another breed. Set the needle for a full lean setting on the ground. Briefly pinch the supply fuel line. If the engine starts to die immediately, it's too lean. If it runs a while, then dies, it's too rich. If it speeds up a little, then dies, it's about right. Fly the plane. Pull a tight loop. If the engine sags at the top, it's still too lean. If you see a lot of exhaust residue, it's too rich.

Dr.1

Sounds like a recipe for a burned up gasser .
I am guessing you don't fly aerobatics .

Dr.1, where did you get this information? Seems to me like a sure fire way to cook a gasser.

Ken