Does anyone know what would cause my timing mark to be bouncing around? The truck is 77 crew cab dually with a 69 396 motor with a HEI distributor

Hi Tim,

There are two things that can cause this.

1) - the mechanical advance mechanism needs some attention in the HEI.

2) - the timing chain is shot! Better take care of this, otherwise it will let go and can cause somes fairly nasty damage.

Also, the outer ring of the harmonic balancer may be loose - causing the mark to "move". This would not appear as erratic movement however, more slowly, the mark would move around the dampner.

Also the most simple the plug wire may be bad. Rock the dampner back and forth and see if you have any slop in the chain.