In my experience using these engines:
If it's new or nearly new you just have to set it right meaning needle adjustment.
If you bought it used and it has sat around for awhile on someones bench it always seems to be the pump and as before mentioned in this thread I agree that the only thing you can do is send it in to have the pump repaired/replaced by an authorized OS engines repair facility.
I steer away from buying them used because they always seem to have pump issues.
No more OS pumped engines for me.
Good Luck!