What causes backfiring?

[Ibby]

I'm thinking I need to give my 304 a tuneup. It's running a bit rough, it likes to backfire and pop whenever I slow down or let off the gas. If I don't 2 pedal it it likes to stall at idle. What causes this? I've got a 304 with a mild RV cam, hi flow water pump, stock everything else and 2100 carb. The carb had a rebuild kit go in it just after Christmas time by a fellow who knows his way around one (I don't). He also adjusted the 2 screws at the base of the carb. I'm told you turn them all the way in, back them out 2.5 turns and start tuning from there. He had to turn them 9 turns to get them all the way in so I'm thinking they might have backed off again?

[ol' school power]

I'll guess when you say when you let off means it's from the tailpipes.
Exhaust leaks are the most common culprit letting air get sucked in and ignite the unburned fuel.

[tufcj]

Has it still got the AIR system? If the diverter valve sticks open, it will give you the same symptoms. The diverter is the silver tube with 3 hoses and a vacuum just behind the distributor.

The screws at the base of the carb are idle mixture. if if idled OK 9 turns out, you may need to open them back up to get it to idle.

It could also be a very sloppy timing chain. How many miles, and has it ever been replaced? Mid-late 70s engines had a nylon upper gear that would lose teeth with age.

Bob
tufcj

[Ibby]

Quick answers, thanks guys. Could be some exhaust problems. I've gone through a couple of header gaskets on the driver's side, the exhaust port closest to the firewall likes to burn through the gasket for some reason. It's hard to get the last bolt there tightened up a lot of the time.

The AIR system is gone, no EGR stuff on the engine at all. I never looked close at the distributor though, so I'll take another look at it tomorrow.

Oddly, I have another timing chain handy. I hope that's not the problem as the engine is about a year and a half old, but it's the same one I had the oil issue with in February (ran it dry due to the relief piston sticking). Could timing be a problem? I noticed it's much further advanced than the old 304 was with a stock cam in it.

Is there a good book around that details this engine? I've found some good bits and pieces on the Web but I'd love to get some sort of detailed factory service manual.