I fired my engine for the first time since putting on the aluminum heads; I have a backfire through the drivers side exhaust about every 3 seconds at any throttle position. Engine also does not idle down as well as before. No changes other than new heads. Same problem cold or hot. Fuel pressure 5 pounds at carb. Timing seems not to affect the problem.

?? float too high, bad valves/seals in carb,(carb is 3 years old, about 2 hours of run time in my driveway), ?? 5-7 induced spark (thought this would be an intake backfire), spark plug misfire, exhaust valve sticking open?

3 years ago new carb ran smooth right out of the box, no mods to floats or jets.

Chev 454, 9.5:1, Holley 750, Msd 6 box, Msd distributor, Comp cam 235 duration at 0.050, hydraulic lifters, 2" headers.

Jim