I sure could use some ideas here....My friend has a 70 396. All of a sudden it started backfiring through the passenger side muffler. I've tried just about everything that I can think of, but can't get rid of it. The car has electronic ignition, (Mallory I think),Holley 780, headers, mild cam.
I've got it narrowed down to number 4 cylinder. If I disconnect the plug wire, the backfiring stops, but the motor obviously runs bad. If I connect the wire, the motor still runs bad, but it backfires through the exhaust. I checked the valve train components for the exhaust valve and everything seems fine, checked to make sure the valve was closing all the way. Compression is on par with the other cylinders.
I've also checked a bunch of the obvious things such as firing order, it's got new plug wires, cap and rotor are fairly new and look fine.
I'm at a loss and don't know what else I can check. It obviously is firing while the exhaust valve is still open, (or leaking), but I just don't see how. I hooked up my compressor to the cylinder and filled it with air while the valves were closed and don't get any air coming through the exhaust, so I don't think the valve itself is bad.
I'm stuck and any help would be greatly appreciated.

Thanks a lot,
Matt

It is possible that the exhaust manifold or header is introducing air into the system, leaky exhaust manifolds that are warped or have a bad head to header gasget let air into the system which combined with unburned gas also in the system make for a combustable mixture resulting in a bacckfire when that cylinder fires. Maybe.

Check for a broken valve spring

Look carefully at the underside of the dist. cap for "tracking" the electron flow is always to the path of least restriction. if you have a lousy wire (check the ohm value) the spark may jump around... if you have good comp. it would be more electrical than mechanical.

John

[This message has been edited by 68ragtop (edited 05-22-2001).]