need help any ideas

[1studiac]

Ok, I plugged up all the unecessary vacume ports and got it running better. The dizzy did'nt have any vacume after it warmed up. Tightened up an exhaust bolt but still had what sounded like a lifter/ exhaust leak (kinda sound the same) I found out that when they pulled the engine they put a dent in the pass side valve cover (on the back) just deep enough to make contact with the rocker, so I'll be fixing that tomorrow. I will also do a timing check. Now for the problem it runs good except if you hammer the gas as fast as you can then it back fires out the carb, not a big back fire just kind of a poomp. I have adjusted the mixture screws all the way from 1/2 a turn out to 1&1/2 turns out but it still does it, btw, new plugs, rotor and cap. If the timing looks good which I have a feeling it is what do I look for next?????

[PlasticBoob]

I'm fairly new to the whole engine thing, but you might want to hook up a vacuum gauge to a manifold vacuum port on the carburetor and adjust the mixture until you get the highest reading. I thought I read that if it's lean it will backfire through the carb.

[mandoneer]

your vacuum is advancin the spark too fast or the dent is holding the intake valve open and that is why you are getting the backfire and just what plastic said hook up a vacuum gauge and adjust the timing to max and retard a little (you will see a drop in the vacuum if ya advance too far ) if it pings on a road test that retard it a little more