I assume its caused by bad timing or a vacuum leak, but what is the physical process that causes it to happen? I know my spark plug isn't firing on the intake stroke because it only does it under medium throttle. I just changed the intake gasket to fix an oil leak. God I hope I don't have to change it again.
Hi Tom, check the distributor, make sure its set properly. Its possible you are a tooth off. Slight popping through the intake is possible. You could make up for it mostly with the timing, but you should verify the TDC and the distributor on #1.
If this happens while the engine is running and not starting, a badly burnt intake valve or very weak valve springs can cause this. If you are using 30+ yr old valve springs this may be the problem. After I married my wife I detuned my ss-454 Chevelle (took off the rectangular port heads, cam and 850 holley, torque converter) and tried to do a inexpensive head swap using weak old valve springs. Within a few weeks, after the engine warmed up I would get a slight but constant "popping" back through the carb. I knew it was the springs, replaced them and that solved the issue. Ps. My wife graduated from college driving that SS. Note: Hopped up the car did 103 mph in the quarter, took all that "good stuff" off and it Idled like a sewing machine and still did a 103 mph in the quarter.
Sticky centrifugal weights in your distributor are also a likely culprit, pop the cap off and clean and lube them (dry film lube works well). Old pionts or a heat damaged HEI system can do the same.
