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MG MGB Technical - Exhaust popping

My 1975 BGT was making a popping noise from the exhaust while going downhill on the overrun. Not done it before. Can anyone give me some ideas as to why this happens.


Rod
R E Merrall

Odds on an exhaust leak somewhere, maybe from a joint or rust hole. Best bet is look for sign of blowing which'll be a black sooty stain somewhere along its length. If you've still got original manifold it might be one of the manifold to downpipe rings blowing. If you see sooty mark , while car running put hand near (not on - will be hot!) stain. You may well feel hot exhaust blowing.

Somebody well come along with a better suggestion, but when I've had similar symptoms, it's been invariably this.
P A Allen

Yep
Usually an exhaust leak and usually towards the front rather than the rear
Could also be retarded ign timing but I'd check for an exhaust leak first up
willy
William Revit

Thanks guys i'll check that. The only other change is I used suoermarket fuel last time albeit super unleaded.

Rod
R E Merrall

Mine has always done it slightly, on longer steeper downhills, but I've never bothered with it. A leak hasn't been picked up on the MOT.

American spec cars do it much more noticeably if the gulp valve is faulty, due to their air-injection system.

Look at it this way - when it's popping, it's resulting in a cleaner exhaust :o)
paulh4

Possibly --could be idle mixture ??? But if it's just started all of a sudden ,probably not --it might be that fuel
Good excuse to drive till it's gone and try some other
Was it ethanol blended fuel you bought or real stuff
They tend to run a bit on the lean side with the blended fuel which COULD be your problem--

willy
William Revit

Willy

I usually use Shell V power but last time I had to use supermarket fuel; maybe that's the problem.

Rod
R E Merrall

Rod-
Interestingly, the most common cause of backfiring in an MGB is the simplest to diagnose and fix: a leak within 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 cooler fresh air through the leaky joint within the exhaust system. The engine, due to engine braking, is still at a relatively high engine speed. Because of the high level of vacuum resulting from the closed throttle discs, the vacuum advance control capsule of the distributor causes the timing of the ignition spark to be too late for the combustion of the fuel-air charge to have enough time to be completed. Unburnt fuel then condenses inside of the exhaust system due to the induction of the cooler air and mixes with it, creating a condition rife with the potential for combustion. When a pulse of hot exhaust gases hits it- Bang! This problem can be aggravated by either a too-lean (weak) or a too-rich fuel-air mixture that will result in the production of increased amounts in unburned fuel. The introduction of the leaking air in between the pulses of exhaust gases also interferes with scavenging of exhaust gases from the cylinders. In order to find out if this leakage is the origin of your particular problem, mix up a thick solution of water and liquid dishwashing detergent. Not the powder kind that you put in the dishwasher, the other liquid kind that your wife uses to cut grease with when she washes stuff in the sink. With the exhaust system cold, squirt the mixture onto the joints of the exhaust system (do not forget the joint at the bottom of the exhaust manifold), and then fire up the engine and look for bubbles forming. If you see bubbles forming, then you have found the leak.
Stephen Strange

The U.S. version emission controlled A and B engines pop at high altitude. The popping at 5,000 feet above sea level indicates proper fuel mixture. The condition only occurs if the air pump belt is connected.

Both the 1971 midget (since new) and the 1970 B have exactly the same pop at altitude.

I have noticed that when the mixture is too rich, the popping is reduced. At sea level, the popping is not an issue.

I would lean out the mixture to see if that reduces the condition.
Glenn Mallory

This thread was discussed between 10/10/2016 and 24/10/2016

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