I have a weird misfire/backfire problem. Its a 70 chevelle, 350, 4bbl, HEI system. what happens is that it lopes like there is a misfire then on mild acceleration it will backfire once or twice out the drivers side pipes. The pipes don't cross and you can hear the pop only out the drivers side pipes. You can also hear the pop in the drivers side engine bay when it backfires at idle. It only backfires maybe 2 times every couple of minutes. The rest of the time it just seems lopy like it is misfiring. I changed the plugs, and they looked decent, some ash deposits but nothing too far from normal. The other thing is that this started to happen all of sudden yesterday. I accelerated kind of hard and heard a pop and then it popped real bad for a little while until I got home, and since then it has been getting better. When it first started any acceleration would cause a lot of backfiring but now it only happens every once in a while. It usually backfires out the pipes, but once in awhile it will be through the carb, usually at like cruising rpms, and you can feel the carb lose vacuum pressure, and the cowl dowl pops open a little indicating some lost pressure in the carb.
Any ideas? Anything to check first?
ok thanks

The same thing happend to me one night. Check your header gaskets. You might have blown one. Also make sure your firing order is correct. All the wires are one the right plugs. And if its not any of those as the last thing to check for would be a flat cam lobe.

Well it turned out not to be any of those things, thankfully it was much easier. A couple of the metal prongs on the distributor cap broke off the spark was only getting there everyonce in awhile. I changed the cap and wires and the problem went away. I still can't figure out why those things would break off?