bouncing timing mark

If you are running a MSD ignition box or any other type of multiple spark box your timing will fluctuate at idle because the pickup of the timing light might pick up any of the sparks at any givin time. The only way I know of to cure it is to figure out your total mechanical advance and when it is all in and set your timing light to the total degrees of timing you want then rev the engine up to past the all in point and then check your timing. It should be rock solid on zero at that point.

Jack