I chased a tuning problem for months before finally figuring out that my base timing was off - I set it at 10BTDC, then tightened down the hold down and called it good. Problem is the dizzy was very tight in the manifold (both new), and it was not firmly seated so when I tightened it down it pulled down another 3/16" or so and pulled my timing 12 degrees retarded. I was running -2BTDC thinking I was at 10BTDC. Had to lift the distributor, shift one tooth, and re-time - my clearance/adjustment room is veeeeery tight!! Has to be on the right tooth or no room to make it right on the light.