QUOTE:
It does not rev up, engine speed only increases as accelerator pedal
is pressed down, then suddenly it jerks.
Jack it up so the rear wheels are off the ground, and block it. Remove the seat. Push down on the accelerator with a piece of wood or something to get the engine running, and see if the clutches are opening and closing SMOOTHLY upon acceleration.
I suspect your engine is reving up, and when the engine RPMs get to a certain level, the drive clutch all of a sudden closes...Clutch Could be in need of a good overhaul, or replacement as the rollers and/or cams get worn and stick.