I had the same problem with my 750CFM Holley. Changed the jets to #70 (comes with #72) but, the problem persisted. Found out that it was a bad vacuum leak at the intake. If the car is getting that much air, it will want to suck down more gas. Hope this helps.
1969, X-44, Export Model, Hugger Orange, Std. Int., BC Rear(soon to be a BT 12 bolt), PDB, GMPP 350HO, M22 Supercase, SSM Lift Bars, AirGap, Holley 750, FlowMaster American Thunder Exhaust, FlowTech Headers