Fly-wheel loose on crank shaft (old models with keyed-on wheel).
Fly-wheel out of balance.
Worn or broken timing-gear teeth.
Cylinder loose at its base.
Timing gears loose on shaft.
Engine loose from frame.
Piston too small for cylinder, causing side slap.
Poor push-rod adjustment—gap too great.
Exhaust or inlet valve sticking in guide.
Spark plug touching valve.