help me

[jmt.7]

there is a fault with the Waukesha GSI F 3521 motor, the motor present high and low RPM, the oscillation is between 10- 150 RPM. We have already checked the plug (bujias), magneto, the ignition system, governor motor, carburettor and the wastegate valve.
the normal oscillation is between 0-5 rpm.
how can i fix this problem?
if anyone can help me i will appreciate, thanks.