Help me out here... my wr426f doesn't run smooth. on 1/4 throttle the engine starts to stutter. If i stay in this revelationrange it keeps stuttering. When in open the throttle wider, it stops to stutter and runs well. We've tried everything to solve the problem: new sparkplugs, cleaning the carb, needle adjust, jetting adjust, cleaning all electrial conections. Nothing seems to help. Does this sound fammiliar to anybody?