MGB 1976 mgb backfire

[jiml]

i have done a complete engine rebuild, installed a pertronixs unit,set timing and fiddled with ZS carb. engine ran good for a short time until it developed a slight backfire out the exhaust when the gas pedal is released. any suggestions?

 

[DrEntropy]

If all your emissions equipment is in place, the usual suspects: The "gulp" valve first. It's a can-like lookin' thingie in the air pump plumbing. There's a check valve at the air rail and an EGR valve on the exhaust manifold. Those need testing too.

Nittany Lions!!! WooHoo!!!

 

[jiml]

thanks i will check the gulp valve and egr valve. this may help fire up the nittany lions this saturday.

 

[Tom_Fisher]

Dr. Entropy is right, Jim. I would suspect the gulp valve. From what I've read they are of a very poor design and prone to failure.

Don't know what state you're in and, consequently, have no way of knowing if you can legally remove the emissions stuff. But if you can, do yourself a big favor and do so. I removed my emissions stuff (air pump, injectors, and gulp valve) about 3 years ago and the backfiring I had been experiencing all went completely away.

If you'll look at many MGB engines that show up periodically on these forums, you will note that the emissions stuff on most has been removed. Let's face it - MG only came up with that stuff (emissions control system)
in order to comply with U.S. regulations at the time. I doubt the MG people really put a lot of engineering know-how behind this effort.

 

[twigworker]

Gulp valves do indeed go bad but most of the time the "failure" is in the little vacuum sensing line running from the manifold to the side of the valve. One or both of the rubber hose connectors can crack and leak vacuum resulting in no function of the valve. Check that first and at the same time make sure that the metal stub pipe sticking out of the side of the valve is secure in its boss. If it is loose you can either drive it back in with a hammer whack or use something like JB weld to hold it in place. Check all of this stuff before you go buying a new valve.

Usually though a non-functioning gulp valve will result in a BIG backfire in the exhaust. The above is easy to check quickly, but before you buy a new valve go back through your tune and make double sure that all is set correctly.

Jack

 

[jiml]

after receiving several suggestion about a faulty emissions system (gulp valve, egr valve) causing an engine to backfire,i quickly removed the air pump belt and successfully ran the engine without a backfire.