this is a common problem or symptom on these. what happens is the needle a jet are worn in the closed position. if your needle wasnt worn it would give you proper mixture throughout the power range. most people just find a happy medium, but if you know someone who know how to rebuld them you could have them rebuilt. the throttle shafts are probably worn and the throttle stops are not working, usually i would say $650 for somebody who know what theyre doing.the throttle shaft bushings will have to be made on a lathe, they are no longer available.