RE: Funky chicken MVVS 58
With capped ignitions I used to fix the cap to the plug with a small hose clamp so it could not move.
A rubber mounted engine is harder on the ignition and carb components, so that could well be the culprit.
Do take care to tie all leads to the engine, and have a free loop to absorb the movement towards the frame fixed components. Also take care that the cable entries into the ignition potting are fixed, so the cable cores do not break at that point.