[tirod]

It's a matter of the pedal opening the throttle, the engine begins to rev up, which moves the alignment of the linkage to the throttle plate, closing it. The engine rocks back, the throttle opens up, and if it gets really nasty, the swings in rpm get more severe.

If the linkage uses rods, check to see that as the engine revs up, the throttle plates don't move more. If cable operated, the cable housing should have enough slack to prevent it affecting the plates. Once the plates don't change from the engine moving, the problem should be greatly reduced.

Also check the motor mounts for looseness - they should control most of the motion, and on some installations, it may be necessary to simply chain the motor down on one side to minimize it.