Timing retards on its own...

[redliner]

You'll find that this can happen with electronic ignition systems, especially magnetic triggered systems.

Recently noticed this on a big block that had a 7AL-2 and an MSD distributor. We figured swapping to a crank trigger would solve it. It did not. The timing still dropped back, and still varied slightly.

What I ended up doing was repairing and rerouting the magnetic trigger cable. After that, the timing did not move.

I suggest using a shielded cable, routing it carefully to keep it away from anything that might upset the signal, and even grounding the shield on both ends.

Thanks!

[In-Tech]

It's a magnetic trail related issue, Gm and ford distributors have it as well as msd's, the only way you won't see it is if you use a hall sensor. The good efi boxes allow you to correct/setup for spark delivery lag time, msd knows about it and the majority of their newer boxes corrected for it for a while, not sure if they still do, it's a bigger problem on a distributor due to reluctor spacing than it is on crank trigger, gap changes the value as well.