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MG MGB Technical - Backfiring when letting off pedal

Put a new exhaust on my 77B. No air leaks found. When letting off the gas I get a significant backfiring coming through the exhaust. Have ZS carb adjusted mix to rich but cannot eliminate backfiring. Any suggestions? Timing set to 10 degrees at 1500 rpm.
JCH Hibbard

Forgot to say that the gulp valve and emissions had been removed so no problem there. If I give it some choke when running it lessens the noise. Set the ZS to full rich and still can't get rid of the problem.
JCH Hibbard

Backfiring on overrun is usually an air leak in the exhaust system. Denis
Denis4

Denis, thanks and I agree but I'll be darned if I can find a leak. Used the soap trick and have checked closely. The more choke I give it the less it backfires. Can't quite get it figured out.
JCH Hibbard

Is it a big bore exhaust? I have found with e full 2" exhaust on my supercharged B I get a bit of a burble on hard overrun. You could try a leaner mixture as there may be excess unburnt fuel in the exhaust. You are only drawing on the idle on overrun so so try the idle a bit leaner, it wont effect up top much. also make sure the carb piston is closing fully. ZS has the piston diaphragm doesn't it? check for an air leak there. We only had twin SUs in OZ. Denis
Denis4

Two glass packs in line for the exhaust. No leak at the diaphragm as a matter of fact has a new diaphram. Will try to lean it out and see what happens. Thanks denis.
JCH Hibbard

Make sure the ring gasket from the exhaust header to header pipe is secured evenly and properly. If the retaining collar which secures the header pipe to the exhaust manifold is not secured evenly then an exhaust leak will ensue. Many DIY mechanics will tighten the two outside hex nuts but back off on that inside hex nut 'cause it's difficult to access. The key here is "after installing a new exhaust system" etc. You need to think it through logically. You didn't mess with the carbs so that's not in the picture.

Also, the ports on the exhaust portals could have substantial carbon build up and could cause combustion of raw fuel through the exhaust. Check the exhaust header for any spurs around the orifices of the manifold. Adjust mixture per manual and then timing and you should be home free.
R Murray

Took extra care to secure the retaing collar between manifold and pipe evenly. The more choke I give it and higher rpms reduces the backfiring?? All emissions equipment has been removed for some time gulp valve etc? What about the egr valve. Could that cause a leak on the manifold allowing air in? Can't figure this out.
JCH Hibbard

J C
I don't think giving more choke, i.e., enriching fuel/air ratio, is the reason it would back fire less. However, the fact that it reduces backfiring when rev-ing up the engine suggests that the timing is changing, as it should. More revs, more vacuum, more advancement of timing, whether by the mechanical weights on the bottom distributor plate or the vacuum module on the distri.

Something is missing on your description. When decelerating, you would be increasing exhaust manifold pressure momentarily. If the fuel/air mixture is running rich, and there's a sudden increase in manifold pressure, hence the backfire, then something is obstructing the exhaust in the header or manifold. Later MGs had a baffle plate in the header with an external spring. As the engine warmed up, the spring would expand and thus open the valve inside the exhaust header. This was part of their emissions control system. Needless to say, this was a bad design but do you have anything that resembles this configuration on your car?
R Murray

Thanks R. No nothing resembling what you described. car has the standard manifold that for the 77 year with the ZS card. Any open ports as a result of removing the emissions equipment has been sealed off with no leaks. I've double checked timing and plugs all is ok. Wondering if the choke or carb is causing the problem?
JCH Hibbard

I see that your '77 comes with a catalytic converter. If the inside of the can has corroded baffle plates or any other obstruction, that would cause back fire. Also, check your coil and ignition wires. The newer style ignition wires have a plastic carbon core to suppress static on the old AM radios. Opt out and replace them with the metal conductor spark plug wires and make sure those wires are not arcing to anything. (BTW, The old style solid metal conductor wires play havoc with police radar equipment).

Check inside your distributor cap and rotor for any signs that don't look right.

When you installed a new exhaust system, did you change out the catalytic converter as well?
R Murray

Catalytic converter went bad some time ago and was replaced with a pipe. No leaks there. Coil and wires have both been replaced with new within the last year as well.
JCH Hibbard

JC

Don't assume that just because you replaced the coil and wires last year that everything is "perfect", re-test and verify".

Start from the beginning again. What changed after you installed a new exhaust system. After troubleshooting everything directly related to the problem then expand your search. If the ZS Carb was working properly before, then it has to be eliminated as a source of the problem.

Once you have thoroughly & carefully tested everything twice, then start to think about a worn or stretched timing chain.

How many miles are on this engine? Does it run HOT? Connect a vacuum gauge while testing. Make absolutely sure you don't have an exhaust manifold leak. Did you install a new exh. manifold gasket from the head to the manifold? If you are totally convinced the problem is beyond everything you've tested and done so far, then it's probably time to consider the timing chain. (Do you have over 100k miles on the engine?) However, I have to agree with our good friend Dennis4 from Victoria, AU with the exhaust leak.

As you rev up the engine, with timing set dead on to 10 deg. BTC as you indicated, and yet it backfires as if the timing is "lagging" at higher RPMs then all indications point to a worn and stretched timing chain. Here's what you have indicated so far.
1. Exhaust System - No.
2. Emissions system - No.
3. Ignition System - No.
4. Carb & fuel System - No.

Go figure. It's the end of the line in your fault tree analysis excepting of course, the cam shaft. Then you have to answer the question: "What does that have to do with installing a new exhaust system with the resulting back fire problem?" Back to "square one" and re-check for a leak or an obstruction in the exhaust system.

JC, good hunting ol' chap.

R Murray

This is interesting. I had installed a flame thrower distributor with a flame thrower coil about a year ago from moss. I believe the flame thrower coil is a higher voltage coil than what was on there, which was the standard lucas. Today i re-installed the standard lucas coil and gave the zs carb a 1/2 turn richer and it significantly reduced the backfiring to a burble on deceleration. Engine runs cool. No problems there. There is 75,000 miles on the engine and usually runs very efficient. Maybe a step in the right direction?
JCH Hibbard

Ah, you did two things, which did what?
Paul Hunt 2010

Reduced the backfiring but still can't get it worked all the way out.
JCH Hibbard

This thread was discussed between 07/04/2010 and 13/04/2010

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