I recently bought a 68 Firebird convertible for my wife. The car has a 69 400ci engine with a new Edelbrock AVS2 carb. New plugs, new MSD plug wires, new fuel filter, new alternator, new battery, engine in very good shape however once the car is warmed up it doesn’t want to go over 50 or 55 mph. Before it is warm it is strong at/above 60. When it hesitates and I try to get it to 60 it even backfires every now and again. At 40/45 or slower the car runs like a top. Idles great too. Just getting above 50/55 once warm is the challenge. I am banging my head against the wall trying to find the issue. Thoughts? Thanks in advance for your input.
