Step one...... Retest with a timing light that has no dial back type circuits in it.

Ran into this years ago with a motor on the dyno using a dial back light that had never exhibited the issue before.
Tried a non-dial back light...... Problem gone.


68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads