Help! Carburetor is backfiring!

[pjmoreland]

Maybe combustion in one cylinder is leaking through the burnt valve, out through the exhaust manifold, and back into the other cylinder through the other burnt valve. It's a long shot, but I suppose it could be possible.

[pjmoreland]

The way I see it, the only way anything can pop back up into the carb is if somehow ignited fuel is making it past an intake valve. Both of the intake valves in these two pictures look ok on the outside. I just don't understand how a worn cam lobe could cause the carb popping if the valves are being allowed to close all the way. Does that make sense?

I'm thinking that exhaust pulses from the other cylinders are leaking back through the burnt valves while the cylinders with the burnt valves are on their intake stroke with the intake valve open, allowing the exhaust to shoot up into the carburetor. Is that possible?

[biglou55]

If the exhaust valves are not opening all the way and not staying open for the full duration of the cam cycle whatever is trapped in there has to go someplace. out the intake valve while it is opening on it.s cycle. hence back up through the carb as popping.(bad cam lobes)antifreeze will crack valves like that(head gasket) change both heads or have them rebuilt. one head is off intake is off dizzy is out your.e three quarters of the way there. no better time that way everything will be equal. check for bent pushrods caused by backfiring.I would also replace the lifters while i was at it. just my honest opinion

[pjmoreland]

I think I will try to get a single rebuilt head from the local Parts America auto parts shop as long as they can get me a head with the same specs as the one I have. They want $159 for a head. Thanks a lot guys for the help. I'll let you know how it turns out.

[pjmoreland]

Hey, at least I didn't get stranded somewhere!

The pushrods all look straight. They all have a shiny section on them where they were rubbing on the heads. The length of the shiny section looks the same length on all of them. This makes me think that the pushrods are all moving the same amount which may be an indicator that the cam is ok. Also I cranked the engine over and watched the rockers moving. Just by eye, they all look to be moving the same amount and for the same duration. Of course it is impossible to tell for sure just by looking at them moving.

If the exhaust valves aren't staying open long enough, I suppose that would explain a blown head gasket if exhaust is being trapped in the cylinder and has nowhere to go until the intake valve opens. I'd probably be wise to pull my cam shaft out.

I'll see if I can find a good pair of rebuilt heads. It's probably the safest thing to do.

[pjmoreland]

Just purchased these heads on eBay. Hope they work out ok. They have smaller intake valves, but that's probably good for my tired old engine.

http://cgi.ebay.com/ebaymotors/ws/eB...RK%3AMEWA%3AIT

[mr402]

I agree with many of the comments on here. Check your cam. Those valves didn't crack for no reason. Small blocks are notorius for wearing the lobes flat. Also take a good look at your rockers. Make certain they aren't wearing through at the ball and pushrod surfaces. If the exhaust valve isn't opening or is only opening a little that will cause the engine to backfire. No sense having to tear your engine down twice.

I had an exhaust rocker split on one of my 402s. Engine was backfiring so bad I couldn't get it to run. Finally figured out the problem and removed the intake rocker to drive it home on 7. As an aside note, if anyone does this you still need to be careful. If the lifter pops out of the hole you could loose oil pressure.

[pjmoreland]

Any recommendations on what cam shaft to buy? I don't want anything radical, just something that will hold up well.

[pjmoreland]

Continued here:

http://67-72chevytrucks.com/vboard/s...10#post1224610