Hey guys, novice here. So I have acquired a cheap 83 305 e4me. I drove it home alright (about an hour) then started to clean it up. The first thing I did was seafoam in fuel tank and plugs because it ran kinda rough. since then I haven't been able to get it to run properly. Carb has been rebuilt. Redid the idle adjustment screw because the previous owner cut out one side and adjusted a single screw way out. Checked the timing, set to 6 BTDC. Adjusted lean limiter screw to get back flaps to start opening. When the vacuum flaps do open they have fuel on them when they close. I am getting a backfire through the carb and the exhaust. There is a leak before the cat, or in it, I'm not 100%. I'm curious if temporarily cutting off the cat would have a decent chance of fixing the issue?

Thanks for any help

Probably not.

Backfire into the intake is caused by incorrect spark plug wire order, lean mixture or leaking intake valve, or possibly head gasket blown between 2 cyls. Backfiring into the exhaust is usually from an ignition miss (raw air/fuel passing through the engine).

The catalytic converter doesn't "cause" backfires.

OK, I keep coming across it when I try and search for my issues. So plug wires are correct, definitely could be a valve issue given the age of the engine. the engine seems to run fine up to about 2500 rpm then starts to sound like its hitting a limiter then the backfiring starts. It's not always through the intake but it is always in the tailpipe around the cat area with the exhaust leak at the higher rpm.