After break in (following the manufactures recommendations) I use my tac to find the highest rpm that the engine will turn with the prop that is mounted. On my OS 91 I hit 11600. As I go leaner the engine drops off so I quickly come back to the highest point then richen the high end a until I lose a couple hundred RPM (about 11400). Now i am sure that I am not to rich and it allows for engine unloading when in the air. If the plane is small enough, I hold the plane nose high when the engine is running and have some one run the throttle up to max just to be sure it does not go lean. Thats it.. high speed is set right. Now low end is another story.

Once the engine is broken in and the high speed set where I want it, I set the engine to its lowest steady idle.. lets say it is 3000 RPm. I pinch the fuel line near the carb and listen to the engine.. If it starts picking up rpm it is too rich on the low end, I turn the engine off and I turn the idle mixture screw in a 1/16" I then start the engine run it back up to full throttle, then back down to the lowest steady idle. It should be getting lower.. say 2800 RPM now. Keep repeating this untill when the fuel line is pinched the engine increase very little or holds steady for a second before fall off and dying. If the engine is broken in, the idle should be around 2000-2200 RPM and steady with no signs of dying. Some engines will hold much lower idles, and the bigger the engine, typically the lower the idle due to the heavy prop acting as a flywheel.

If you do the pinch test and the engine dies immediately you are too lean.. richen up and try again. When set correctly the transistion from an idle to high RPM should be steady and almost as fast as you can move the transmitter stick. If it is too lean it will falter or die, if too rich it will lag behind the stick movement and will be rough. You will learn to recognise the difference quickly.

Once you have a good steady idle you go back to the high setting and reset according to my first paragraph as it will like be affected by the idle mixture if you had to change it very much.

It is good idea to change the glow plug after it has been broken as new engine has oils and metal shavings that will adhere to the platinum wire. While the engine may start and run ok.. it can make the idle hard to set and the plug usually fails early in any case.

Once the idle mixture is set you seldom have to mess with it, but weather etc can affect the high speed need so it should be checked prior to each days flying, just takes a second, run it up to where it falls off, back off to highest rmp then richen a few hundred RPM more and your good to go. I never have to play with the high speed more than once in a given day unless I start early in the morning when it is cold and the weather changes significantly later in the day