[lnfletcher]

Engine:
87 Roller 302 with B303 Cam, Wieland single plane intake, 9.1 TRW pistons and oversized 194/160 valves. Edelbrock 650 AVS carb. Ford duraspark dist with MSD 6AL box.

With WOT starting at 3000 rpm or below, the engine sputters until it reaches about 3000 and the clears out and runs good. On hard engine braking, it backfires through exhaust.

Running 10 deg base with max 38 deg timing with vacuum advance connected.

I have tried advancing to 12, 14 and 16, no change.
I have used stock ford duraspark dist curve and recurved for faster advance, no change.
I have gone through the carb with a rebuild kit and cleaned just in case of some trash, no change.
Plugs look good with light grey color.
Tried rejeting carb with leaner secondaries, no change.

I am at a loss, nothing seems to effect this problem. Can one of you experts much smarter than me give me an idea or direction to look at?