Ok, so I took on a project car, that might have been a bit more than I am qualified for. I have worked on a lot of modern cars and trucks but I have never so much as even driven an older, carbureted car! About 6 months ago I picked up a '79 Camaro with the stock 305 engine for $1000. aside from needing a whole new interior, I thought the car was in decent shape. I may have been wrong. the car ran and was drivable when I got it but it idled rough and uneven, and would sometimes shut off when putting it into gear (automatic trans). I decided to rebuild the carb. it is an Edelbrock 1406. I got the kit, and took the carb apart, followed the instructions to a "T", cleaned everything out and removed a tremendous, unbelievable amount of unknown crap from the larger passages in the carb. the gunk had a very rubbery, latex sort of texture and color, very strange, did not seem like "old" fuel to me. put it all back together. afterwards the car was easier to start and the idle adjustment screw and idle mixture screws became much more effective and I was able to get the idle tuned just right. It still shut off when going into gear unless I hit the gas right away.
unrelated, the valve covers were leaking oil fast enough for me to watch it happening so I decided to replace the gaskets. A very simple and quick thing to do, but when I started the car the next day after letting it set, it was backfiring at all RPM's from idle up to WOT. while in gear, if it backfires, it near shuts off. After many more adjustments, I now have it so that it starts easily, and idles well at a steady 800 RPM. at any other throttle position it will backfire, and the higher the RPM the worse it is, and it is twice as bad if the car is in gear. I'm not sure if the A/F mix is too lean, too rich, or if it is even a carburetor problem. I replaced all vacuum lines as some were cracked hoping that was the issue but no luck. no carb adjustments seem to have any effect but the problem is not as pronounced when the engine is cold and gets much worse as it heats up.
I should clarify that it is backfiring through the exhaust only, so im not 100% sure that "backfire" is even the right term for this but the sound rivals that of a shotgun. doesn't help that the exhaust stops right under the drivers seat.

Just today, I replaced to spark plugs and wires with zero effect on the problem. This was not an issue prior to replacing the valve cover gaskets so I don't think it would be a timing issue, nor do I have a way to test the timing.

could I have messed something up under the valve covers that could cause the backfire/afterfire?

whoever had the car before me added on a lot of stuff that I have found out about as I go. what I know so far is, carb, performance intake manifold and cam shaft, rockers, and exhaust headers, are all aftermarket. as best I can tell everything else on the car is factory.

any help would be much appreciated! thank you in advance for any advice on what to try next!

 

it looks like you want some help so im going to go ahead and move this to a tech section. maybe do another intro. lol, welcome to the forum.

 

sure you got the valve covers on the right side and the right way so the rockers are not hitting them. check and make sure the plug wires are in the right places, maybe pull the distributor cap and see if anything is going apart in there.

 

I don't see how you could have messed up anything under the valve covers, unless you were wailing on the rocker arms with a hammer! And the valve covers are interchangeable, there isn't a designated left or right cover for rocker clearance, just as the heads could go on either side of the engine as well. So don't concern yourself there.
Check to make sure none of the plug wires got knocked loose, which could have happened when messing with the valve covers. Double check to make sure none of the plug wires are crossed. It would help a lot for you to pick up a repair manual. The left bank of cylinders (drivers side) are # 1-3-5-7 from front to back. Right bank 2-4-6-8 front to back. Firing order 1-8-4-3-6-5-7-2. So, trace the #1 plug wire (front drivers side) back to the distributor cap. Mark it if you need to. The rotor inside the cap spins clockwise, so the wire to the right of the #1 wire on the cap (going clockwise) would be the #8 wire (rear passenger side cylinder). Then follow all the way around the cap to check the rest.
You say it runs a little better when the engine is cold. When the engine is hot, remove the air cleaner and make sure the choke butterfly is completely straight up and down. If the choke isn't opening, that can be your problem.
Also, crap in the carb most likely means crap in the gas tank and/or fuel filter. Find the fuel filter, remove and replace it. While the filter is in your hand, rap the inlet side on a clean paper towel, and see what comes out. You may still have a crappy fuel problem.
If they don't look like they've been changed in a while, replace the spark plug wires. You could have a bad one. Also make sure none of the plug wires are touching the headers, or anything else nearby that could cause them to jump spark to ground. Sticking your head under the hood at night while the engine running can help you to spot any fireworks being given off by a bad "leaky" plug wire.

 

Backfires out the exhaust can often mean inconsistent ignition, so the cylinders and exhaust fill with gas/air mix and then when it catches they ignite. Need to check not only the firing order, but also each plug wire. You can check plug wires by pulling them one at a time (with the engine off) then put a screwdriver in the end, and start the engine. Put the screwdriver close to an exhaust manifold to see if the spark jumps to ground. Repeat for each plug wire.
I didn't notice if you had changed the cap and rotor, but that's probably a good idea. Check the electrodes on the cap inside and out when you replace it for signs of corrosion. If you see any on the outside terminals, then be sure to clean or replace any plug wires that were on those terminals, as they are probably corroded too!
Next put a timing light on the engine and check the timing. Sounds like it might be off a bit.

 

Ok. Thank you everyone for the feedback. I finally got time this weekend to work on this. The problem was 2 fold. First the spark plug wire were crossed. Second, both the smaller wires coming out of the bottom of the distributer cap were frayed in several places and sometimes touching. No more backfiring now but the car still turns off if I put it in gear without holding down the gas. It's not as bad once the car gets got but it will still cut off if sitting at a light too long even after warming up. Also, now all of a sudden I don't think the car is shifting into top gear. It drives very well but just stays at a high RPM. I can only go up to about 35-40 MPH before the engine is revved too high. My gut tells me the problem isn't the transmission but I'm not sure if something I've adjusted has any effect on the shifting. Any ideas?

 

The idle thing might be too low an idle in neutral, timing, vacuum leak, or the trans/torque converter. The trans shifting issue might be an adjustment. If it's TH350, then you need to see if the vacuum modulator line is properly connected to the engine and trans, with no cracks or breaks. Also need to check that the TV cable that connects to the carb from the trans is not too tight, or it might be causing it to hold in low gear.

 

Ok, so after sitting a week, now the problem was worse this morning. it would not shift at all, staying in 1st gear. I did check the TV cable and adjusted it as it did seem to tight in the idle position. after this I test drove the car and found that it will now shift gears just fine 1-2-3, if I do it manually. it will not go into 4th but if I start in 1st and move it into 2nd manually, it shifts smoothly. then I move the selector to "D" and it shifts into 3rd smoothly. it will downshift correctly on its own as I slow down. coincidentally, it no longer stalls out while sitting still in gear. how can I find out exactly what transmission this car has? also, so I can inspect the vacuum lines, where do they connect?

 

Your car originally came with a TH350, 3 speed automatic. That would probably explain why it won't go into 4th gear!
Check all your vacuum hoses for any breaks or disconnections. There is a vacuum line from the intake manifold to the modulator on the trans, which is what applies pressure to the throttle valve and controls shifts. The 700R4 (4 speed) has a TV cable that does this, the 350 doesn't work that way. Yours is a "detent cable", or what's referred to as a kickdown cable to down-shift the trans when you plant the gas pedal to the floor. Normal up-shifts will work without it, but in your case (being too tight), would want to keep the trans in a lower gear. You got that problem solved by loosening up the cable more to where it wants to be.
Here's how to tell one Chevy trans from another: Chevrolet Chevy Transmission Identification and Decoding