Originally Posted by Tod74
How much initial timing are you running? Try locking the distributor at 36-38 and see if it fixes it.
Vortec heads won't like that much. I do however think timing is an issue as well, When you put in even a mild performance cam it will respond best with a fair amount of initial timing. You need 20-25˚ initial, yet limiting the total to around 32˚ to start with those heads. It MIGHT take a little more, maybe 34˚ max. The more efficient the piston/combustion chamber combination the less it needs for timing. My 461 SB2 use around 29˚ total, my 427 23˚ engine around 40˚. You need to set your distributor up to limit the mechanical advance to about 10˚. Set it up without a vacuum advance hooked up, once you get it dialed in for steady loads you can add the vacuum advance hooked to manifold vacuum IF it needs more timing for large throttle changes, like going idle to WOT.