My OS 91 got cranky and while it would idle perfectly I could not advance the throttle beyond 50 - 60%. It would get rough and die

Look in the venturie with the throttle wide open and look to see if at least 50 % of the cut out in the spray bar is visible

Adjust the mixture screw so that at least !/2 of the slot is exposed, set the high needle to 3 - 4 turns out for a starting position ands go from there

This may not be the problem, but it sounded so similiar to my experience that I had to put in my 2 cents

Roger Aka. GIFLYRC