Small Engines (Lawn Mowers, etc.): String trimmer engine runs at low RPM, string trimmer, open throttle

Question
Hi Marc,

Thanks for helping.  I have a Husquvarna string trimmer, 2 stroke.  Starts easy, idles fine, BUT won't wind up to high RPM.  Engine has little power.  Always ran fine, then one day, it just won't run fast.  Removed air cleaner, no change, adjusted the 3 carburator screws, no change.  Shaft and head seem to be OK.  Engine just won't go to high RPM. If you open throttle the engine continues to run but low rpm and rough.  What do you think is wrong?  Thank you Jim  

Answer
It sounds like you need to adjust the mixture screws.  The two screws side-by-side are the mixture screws.  The one closest to the cylinder is the low speed mixture.  Start both at 1 1/2 turns out from lightly seated.  First you adjust the high speed mixture by holding the throttle wide-open and turn the screw in until max RPM is reached, then open it 1/4 turn...this should be sufficiently rich to provide power at WOT.  The low speed only needs to be opened just enough so the engine will rev up quickly from an idle with no stumble.  If adjusting the mixture screws doesn't correct the problem, write back with what you found when adjusting them.  Hope this helps.