I have just replaced a clutch motor on Brother B755-mkIII with a servo. The make has been over-labelled but model is WR561-1 and has awful instructions translated from Chinese. If I set the top speed to 1000 RPM or more, then the speed will vary with the treadle depression but with a lower top speed the treadle acts like an on/off switch, delivering the chosen top speed with no variation with treadle travel. The accelerator control is a magnetic wiper moving near to a 3-lead electronic sensor. Can anyone suggest how I can get variance at lower speeds?