Small Engines (Lawn Mowers, etc.): 2-cycle engine tuning, mcculloch chainsaw, adjustment screw

Question
Marc, I have a McCulloch Chainsaw and a Stihl trimmer which came to me out of tune. Do you know the procedure for tuning these engines? I do not have the manuals as they were bought used. The chainsaw has plastic caps that cannot be removed without damaging the screws. The trimmer will run but bogs out when I apply throttle.
                               thanks
                               Larry B.


Answer
The starting point I like to use for the mixture screws is 1 1/2 turns out from lightly seated (low & high speed). The low speed is closest to the cylinder and the high speed is right beside it. The idle adjustment screw is not in the same proximity of the mixture screws. I like to set the high speed first. Hold the throttle wide open and begin turning the high speed screw in until the engine revs as high as it will go (this is too lean and don't do it for too long), then back it out 1/4 of turn (mixture will be sufficiently rich to provide power at high RPM). With the engine idling (you may have to adjust the idle screw slightly so it will idle somewhat normally), open the throttle rapidly. There should be no hesitation. Turn the low speed mixture in 1/8 of a turn and open the throttle rapidly. If there is still no hesitation, turn the low speed in another 1/8 of a turn. Keep doing this until you find that it hesitates, then back it out 1/4 of a turn. This will be sufficiently rich to provide power off-idle. Then you can fine tune your idle. The idle RPM is not critical except that it should be low enough to keep the chain or trim head from turning while idling. Hope this helps.