my yb100 just wont seem to go anywhere in 4th it either stays the same speed or like most of the time drop i change from 3rd to 4th at about 40-45mph bought 1 smaller sprocket no difference

i sorry but it would seem you are losing power maybe worn bore or needs new rings,pipe blocked head coked up there is many reasons on old two stroke bikes to loose power. chris

or brakes dragging