I have no idea what it could be, i have a 1973 dodge charger, 318 904 and it has a holley 4bbl vac secondarys, rpm air gap intake, and a orange box. I messed with the timing didnt help, revs just fine in park up to 5500. In drive ill stand on it and ittle pull perty good up to 4200 and like hits a wall and starts popping and sounds horrible, please help. Thankyou..