I have a 1969 Camaro with a 230 inline six original motor with a Saginaw M15 3 speed that was converted to a floor shift. The mileage is at least around 150,000 if not way more, and the only 4 things ever replaced on it were the carburetor in the 80s, the starter, coil, plugs, and plug wires last year. Obviously, the oil has been changed several times, and I am currently running with premium gas, led substitute, and a high mileage/high viscosity oil to compensate for the extreme amounts of oil that I burn and possibly leak. As soon as you go for, well, what resembles open throttle as I've never actually dared to try it, it backfires through the intake manifold consistently with the firing of the cylinders. This also happens at around 3,500 RPM and higher, I typically shift between 2-3 grand consistently to avoid further damaging the motor. I've tried several things to fix the issue, including setting the proper initial timing to the factory setting of 4 degrees advanced, and I've adjusted my Rochester Monojet's idle speed and mixture. It actually runs really good besides this issue; however, I'm not sure how much longer I can drive it without completely destroying it. I'm assuming this is a valve train issue, I suspect a burnt exhaust valve or bad seats. I did a dollar test on the exhaust pipe, and I'm pretty sure it does suck it in every now and then. I have no idea if any of the rings are bad, and I have not performed a compression test as of right now. I'm debating adding engine restore and/or a valve sealer just out of curiosity as I've never been able to clearly pinpoint this issue even with the research and various opinions I've gotten over some time. Any thoughts?
(I have a video of it backfiring but the file is too big.)
Backfiring when lift off the throttle sounds like a typical lean carb?
This could be jets or worn butterflies.
Timing issues? You could try to increase the timing a bit more.
@mascot15
It's when I punch it, not lift off (If I have that correct.) Also, I used a vacuum gauge when tuning, and the needle was rocking back and forth, which I hear is a sign of valve related issues. While I have the timing set to 5 degrees advanced right now, everything ignition-wise is fine. Cap, rotor, plugs, wires, etc. The carb was our first suspicion, but I can't find a replacement that's the correct Monojet type.
Hmm Have you checked the fuel pump pressure? I recon when it gets old, it can actually increase the pressure significantly which can lead to several issues.
Maybe there is some vacuum leak somewhere ? Like the manifold or around the bottom of the carb?
If you cant find a replacement carb, why not rebuild it? I'm sure there are rebuild kits?
I'm no expert in those older cars, hope someone with more experience like Scotty can give some input.
@mascot15
No, I haven't. I did check for intake and carb gasket vacuum leaks using WD40, but the idle never changed. At one point I noticed moisture coming out from the carb gasket awhile ago, which I'm assuming is either the backfiring blowing the gasket out from the inside, or it is an actual vacuum leak I missed. Here's a link to it running: https://drive.google.com/file/d/1r-YRcln8-xcx1neMK0Gy7clCJh2Li8_m/view?usp=sharing