Holley Avenger 770 extremely rich... Help!

[80Camaro]

I took this off of my engine about 2 months ago and rebuilt the engine because of a wiped cam lobe. The engine had been running perfectly before that happened. The only difference in the engine now is the slightly larger roller cam (From XE268H to XE276HR). I just started the car and it runs OK but it's extremely rich. It backfired out of the exhaust one time before I adj. the timing. I just checked the power valve in front and I don't think it's blown. Fuel level in bowl is OK. What the hell could have changed? It has the #72 prim. jets that came in it.

I am having trouble with timing. I can't retard it down to 34 total... not even close. Any thoughts? Thanks