[ted]

If the vibration is just showing up at idle and is intermittent or comes and goes, then engine balance itself is likely not the culprit. If engine balance is an issue, it will show up the worst between 1700 and 2100 rpms when bringing the engine rpm's up in neutral. As this vibration is showing up at idle, then check out the accessories such as water pump, alternator, belts, pulleys, etc. Check that the belt pulleys do not have rust pitting within the grooves also.

This particular vibration could also be carburetion related in that you have a carb that's leaning out or surging on one side of the carb and causing an intermittent leanout that feels like a vibration. This would be more noticable on a dual plane intake and not as much on a single plane manifold. You might try cleaning all the air bleeds and see if that helps. A warped throttle shaft or misadjusted blades can also cause this where the throttle plates are not exactly alligned with each other. A vacuum gauge might be of help if this is the problem as you'll see the an erratic vacuum reading or one that's not rock steady.

The harmonic dampers on the Fe's are all zero balanced from the lowly 352 up to the 427 and the externally balanced 410 and 428. Although they may vary in size or weight, they are all zero balanced (SCJ included) and are basically interchangable. The 428 SCJ used a sleeved "pie cutter" counterweight behind the damper in which to compensate for a missing counterweight at the middle of the crankshaft but the damper itself is still zero balanced. The only nuance here is if the balance shop balanced the rotating assembly through the damper (a balance shop no no) and the damper is replaced later. Then you're back to an unbalanced rotating assembly again.

Ted Eaton.