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MG MGB Technical - machine gun back firing at 3500 rpm

Took my 1972 BGT for a run out yesterday and almost every time I rev'd beyond 3500 rpm I got a horrible machine gun back firing noise. She also seemed down on power and didn't want to rev to anywhere near the red line.

Carbs were recently balanced, dash pots topped up etc.

Any ideas/pointers would be appreciated.
S Longstaff

The best place to start would be to go back over any recent changes you may have made prior to the backfiring starting. Is it backfire or afterfire?

Backfiring (ignition in the induction system) means there is a very lean condition. Can be caused by bad mixture settings, poor fuel supply, air leaks and bad ignition timing etc.

Afterfiring (ignition in the exhaust system) means an over rich condition with unburnt gases. This can be caused by intermittent ignition (and/or spark plug) failure or a very (over) rich mixture.

John
J Tait

I recently had a similar problem with a B that I'd just put a new electronic distributor into. It turned out that I wasn't getting a full 12V to the coil and it wasn't operating the electronic side of thing in the dizzy.

Went back to points and all sorted (even with 10.7V feed). Now I need to look at where I'm getting the power drop.

In your case S, I would look at points and condensor.

Mark.
M T Boldry

Is the tach jumping about with the backfiring or steady?

Does sound like ignition. Maybe the centrifugal advance moving the rotor away from the cap contacts, or maybe the points plate earth wire making intermittent contact. This latter would would cause the tach to jump about, the former not. Could also be a bad condenser, another case of not affecting the tach.
Paul Hunt 2010

Replace the condensor and see if that fixes the problem. This is a cheap solution before you spend some real money on other issues. Your problem sounds like the same issue I have been having with my MGB as I have had horrible luck with condensors on my MGB and always carry a spare. Condensor quality is not what it used to be since electronic ignition took over. One other tip I was given is to lightly sand the outside of the condensor case to make sure that there is a good contact to ground from the case. Seems that some manufacturers are putting on a coat of shellac to make the cases look nice and this is just thick enough to interrupt the ground path.

Best of luck;

Ron
Ronald Kluwe

Thanks for the response so far. The car is fitted with a luminition electronic ignition so condenser change is not an option. Its also has a new dizzy (D45 with early adv curve) fitted. The timing of which was set up using PB's recommended degree's at 3500 rpm.

Could it be the case that I've got it too far advanced or retarded? I don't have any pinking.

Tack doesn't jump about.
S Longstaff

Assuming that you are describing afterfire, then you may have suspect spark plugs and/or high tension leads. Worth changing them because you can always keep them as spares if replacing the current items has no effect.
J Tait

Timing wouldn't normally cause this, unless as I mentioned the phasing is off which is a known problem with some aftermarket triggers. Can you reproduce it at a standstill? If so clip a timing light onto the coil lead and each plug lead in turn and see if you can spot any irregularity creeping in with the backfire. Won't be easy at 3500 rpm I'll admit, especially on the coil lead. Strobe timing is normally done at 600-1500 rpm. What figure did you use for 3500?
Paul Hunt 2010

S-
Interestingly, 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 cooler fresh air through the leaky joint in the exhaust system. Unburnt fuel then condenses in 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 a too-lean or too-rich fuel / air mixture that will result 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 liquid 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 thick liquid stuff that she uses to cut grease with. With the system cold, squirt it on the joints of the exhaust system (do not forget the joint at the bottom of the exhaust manifold), then fire up the engine and look for bubbles. If you see bubbles, then you have found the leak. If tightening up the clamps does not 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. A set of SuperTrapp T-bolt style exhaust clamps will give perfect 360O sealing in order to eliminate the leakage completely. They also have the advantage of being manufactured from durable stainless steel so that they will not corrode and are complete with Nyloc nuts. These are available from Summit Racing in both 1¾“ (Part # SUP-094-1750) and 2” (Part # SUP-094-2000) through their website at http://store.summitracing.com/ . It should also be noted that the juncture of the exhaust manifold and header pipe is the most common location of such a leak. Because the interior of the bottom of the exhaust manifold has a conical sealing surface, the exhaust manifold gasket should be installed with the conically-shaped end facing upwards into the conical recess of the exhaust manifold. The gasket is squeezed between the exhaust manifold and the exhaust pipe flange in order to achieve its seal. Note that the headpipe has a flange welded on it to allow the triangular flange to compress the gasket (doughnut) when the machine bolts are tightened. If you use an old headpipe when you do a rebuild, then you will be running the risk that the welded-on flanges have significant rust damage and that they will eventually fail to the point that they will pull down through the triangular bolt-up flanges. If you have an indication of an exhaust leak that you cannot find, some backfiring while decelerating down a hill, and maybe even a loud squeaking noise on rough roads, then you might want to check and see if your headpipe is starting to migrate downward toward the pavement.
Stephen Strange

I had a similar issue with my 79 MGB last year. It turned out the lead from the coil to the distributer was working its way out from the coil causing the spark to jump from the coil to the lead. This resulted in a backfire/misfire above 2500 RPM.

Check and clean all your lead connections to assure they are elelctrically tight. And yes if you have conventional points, a weak condensor will create your symptoms as well.. The condensor will start to falter after it warms up to operating temps.

cheers

Gary
79mgb
gary hansen

This thread was discussed between 24/05/2010 and 30/05/2010

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