I just finished swapping some pro toplines on in place of aluminum L98's. It will idle fine but as soon as you goose the throttle it will backfire out the exhaust. I have a Holley 4160 on it. The timing is set to 18 degrees btdc which is what I had with the old heads. Suggestions? I have the car at work and would like to get home some time tonight hehe.

 

chalk up another bonehead mistake for me... i mixed up two ignition wires at the cap. much better now

 

after reading both your post, especially the second one i'd say you've got a couple of plug wires crossed.