I think this is a subject that comes up periodically, especially among helicopter pilots. This is because nitro engines on airplanes are much easier to adjust since the needle valves can be adjusted on the ground. However, in helicopters we have to make an initial adjustment to the needle valves and then see how the engine performs in-flight. This in itself presents two major problems.
The first problem is to determine if the engine is running too rich, or to lean, in flight. During this initial phase of engine adjustment I try to run the engine sas rich as possible to ensure adequate cooling and lubrication. On landing I check the engine head temperature with an infra-red heat gun, looking for a break in temperature of 150 175°. Once the engine is broken and I’m looking for 175 to 200° for normal operation.
The second problem is how much to adjust each needle valve to achieve the desired results. My first concern is the main needle valve since, as its name implies, it controls the fuel entering the engine. I therefore am not concerned with the idle or mid range needle valve until the main needle valve is adjusted. Once the main needle valve is in the proper operating range I then checked the idle and adjusted its needle valve as needed. To do this I pinch the fuel line and listen to the engine’s response. If the engine dies quickly at idle that’s an indication the mixture is too lean. If the engine increases in RPM for more than a few seconds, that’s an indication the idle mixture is too rich. As for the mid range needle valve I leave it at factory settings unless there is a need to change it, as indicated by the performance of the engine in flight.
This, as most things we do in helicopters, is strictly a technique and open to change and improvements at any time. Also, notice I said nothing about the sound of the engine or the amount of smoke the engine produces in flight. I think each could be very misleading in making final adjustments to engine mixture. If you use a different technique I would like to hear about it and see if I can incorporate some new ideas into the way I adjust my nitro engines.