When first assembling your EZ style unit, the clutch will need to be adjusted a few times as the drive belt breaks in. As the machine ages, it may seem to be loosing power, this is an indication that a clutch adjustment is needed. When replacing the clutch cable be aware the cable routing is between the machine and the belt.
Clutch adjustment procedure for 35/40 EZ, Key EZ, ThunderVolt, 60/65 EZ and SuperTow 1-4 & SuperTow Pro.
Clutch Adjustment Guide