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MG MGB Technical - Backfiring??

i have a '78 mgb and when ever i go to shift or let off the gas pedal it makes a really loud 'popping' noise. the engine has a zinith-stromberg carb. i really have no idea what to do about this problem. any help would be great.
thanks
daniel
Daniel

First look for loose exhaust head pipe connection at the manifold, or other leaks in the front end of the exhaust system.
Andrew Blackley

It's called overrun and generally hurts nothing. Caused by a rich condition under vaccum conditions in which unburnt gases are sucked back into the engine and ignited by exhaust valves or the header. Low restriction exhausts accelerate the problem. Not much you can do. My heavily modified Datsun Roadster does it all the time and no harm has been caused (it's a great item to keep cars off my ass in traffic) and it's loud, very loud. My Miata does it too but much less. Emissions Engineers install a dampener in the butterfly (to keep the butterfly open) to eliminate this problem and of course to lower emissions. BTW, backfire is an explosion through the carb caused, generally, by a lean condition. As soon as I get my 67 B running I'd like to see if this is present.
M Landskroner

Have a similar problem with my '77. After removing the Cat and the rosonator under the car it pops a little between shifts if I let the revs fall off. Adjusting the mixture has had little effect. Not harmful but annoying.
jim mindy

Daniel,

MGs (both Bs and midgets)only starting doing this after the air-pump emmissions equipment was added for the '75 model year and beyond - yet another reason why many people prefer the ealier cars.

What happens (as M from New York said) is that when you let off to either change gear or just decelerate, the airpump continues to inject fresh air into the exhaust ports of the engine, and since there is still unburned fuel in any engine that exists when the exhaust valves open, the fresh air from the air-pump and "gulp valve" on the intake near the carb/manifold promptly ignites the unburned fuel remaining, which by this time in the cycle has made it past the manifold and down towards the catalytic converter/close to the first muffler in the exhaust system.

I had a '77 B on which this condition got so bad that the first muffler actually exploded like a Jiffy Pop bag and needed to be replaced. Of course prior to paying to replace it (with a nice glass pack system),
the air pump and ancillaries "fell off" one weekend -
and "poof" better throttle response, no backfiring, and a measurable increase in BHP. Nicer sound too!

Cheers,

Tom


MG Tom

Gentlemen,
The air pump does indeed inject fresh air into the exhaust stream, so it could contribute to the backfiring. The root cause though it the excess fuel in the exhaust stream, which the gulp valve is intended to minimise. The gulp valve opens the intake manifold to the atmosphere on overrun, creating in effect a huge vacuum leak which leans the mixture.

Also, the air pump and gulp valve were fitted at least as early as 1969...

HTH!
Rob
Rob Edwards

Daniel-
The most common cause of backfiring in an MGB is the simplest to diagnose and fix: a leak in the exhaust system. As the pressure wave of a pulse of exhaust gases passes through the exhaust system, it leaves a partial vacuum behind it, sucking in fresh air through the leaky joint in the exhaust system. The unburnt fuel condenses in the exhaust system due to the induction of the cooler air and mixes with it. When a pulse of hot exhaust gases hit it- Bang! This problem can be aggravated by a too-lean or too-rich fuel/air mixture which results in the production of increased amounts in unburned fuel. To find out if this is the origin of your particular problem, mix up a thick solution of water and dishwashing detergent. Not the kind you put in the dishwasher, the other kind that your wife uses when she washes stuff in the sink. You know, the liquid stuff that she uses to cut greaese with. With the system cold, squirt it on the joints of the exhaust system (don't forget the joint at the bottom of the header), then fire up the engine and look for bubbles. If you see bubbles, you've found the leak. If tightening up the clamps doesn't cure the problem, your friendly local auto parts store or muffler shop can supply you with some exhaust system putty to take up the gaps in the connections that result from poorly-matched exhaust system tubing diameters.
Steve S.

Hey everyone thanks for your suggestions....i found out that it is the gulp valve thats giving me problems.

_Daniel_
Daniel

For those cars without a gulp valve I'd like to reinforce the suggestion about checking the exhaust for leaks. My 1980 LE roadster was backfiring badly after a rebuild. I discovered that even though the manifold was torqued to the correct setting in the manual, it was blowing at one end. I've now replaced the brass nuts with steel and tightened it up significantly more than before....problem solved.
ph barton

This thread was discussed between 21/04/2003 and 25/04/2003

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