[Coloradocobra]

Mike,

Just remembered a story from an old balance shop guy. He had balanced an engine but when the owner assembled it, it had high vibration. They traced it down to a metal shaving that had fallen between the flywheel and the crankshaft flange causing the flywheel to have excessive axial runout. You might want to set up a dial indicator and check axial runout at about 4 or 6 locations on the flywheel. Just make sure you keep the crank loaded forward against the thrust bearing each time you rotate the crank. This will also identify a poor surfacing job. The axial runout creates a high couple unbalance force that is difficult to trace because when you put it on a balancing machine it may check out OK.

Unbalance force increases as a function of the square of the speed (e.g., force at 4000 rpm is 4 times the force at 2000). If the vibration is "gone" at 5000, it points more towards a resonance or a "grounding" of the engine to the frame or body.

Just a few more things to check in your quest for smoother operation,

Gary