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MG MGB Technical - Backfiring, lumpy, poor idle and very unhappy

Hi guys,

I fixed my oil leak - turned out it was the rubber gasket on the oil filter bracket, nice and easy once I'd realised where the leak was coming from.

However, I've now got other problems. The engine is lumpy and takes a while to start. Once it is running, it is lumpy as hell, backfires, revs drop to zero and it either dies or pulls itself back into life. It isles between 1000 and 2500, whatever it feels like. When I give it some revs, it will happily rev to 5,500 (i don't like taking it over that level), but as the revs stabilise or if I back off the throttle, it backfires, dumps fuel out of the carbs and the revs die to zero.

Setup is a 2099cc, HR270 can, twin HS6 carbs, lumenition optronic ignition, 43D(non-vacuum)

To help solve this, I have:

-Fitted new Lucas coil.
-Fitted new HT leads.
-Fitted new spark plugs.
-Fitted new Lucas distributor cap.
-Checked sparks (lots, on all cylinders).
-Checked fuel filter (clean).
-Drained fuel tank, filled with new fuel.
-Checked and cleaned carb floats (clean).
-Tightened all ignition connections on ignition switch.
-Tweaked the idle, but can't get it stable.

So far, nothing seems to have made a difference.

Any thoughts?

Huge thanks in advance,

Grant :-)

G Hudson

Hey Grant,

It sounds as though you've gone through all the basics to try and eliminate the problem.

I've experienced similar symptoms in my '77 B and found two things that can produce the symptoms you describe:

1. Vacuum leak
2. Leak at the PVC connection.

Hope this helps.

JR Jim) Ross

In addition to a vacuum leak, check out your distributor rotor. Many on the market today are high in carbon and can cause your ignition system to seek the closest path to ground.Look for a red rotor at your auto parts store. These are specially made out of low carbon plastic. RAY
rjm RAY

If the tachometer drops to Zero before it dies it points to an ignition problem. BUT it backfires, dumps fuel out of the carbs and the revs die to zero? Is this "dumping" out of the overflows or back through the venturi?
One of the checks you should do is a compression test. A sticking or bent inlet valve, or a "picked up" hard seat (i.e., unleaded) will give the symptoms you describe.
Allan Reeling

If you can reproduce one or more of these things with your head under the bonnet monitor the timing with a timing light on each plug lead and the coil lead, watching the flashes as well as pointing it at the crank pulley, and see if the flashes get erratic or the timing (when on 1 and 4) is jumping around.

If it's backfiring in the exhaust then that is a sympton of intermittent sparks, with the next firing igniting unburnt fuel in the exhaust.

As Allen says if the tach suddenly drops to zero when the engine is still spinning then there is a break in the ignition LT circuit through the coil and ignition system. What year is this? If a rubber bumper where have you picked up the power for the ignition module from?

That's one of the main things to follow, the other is the 'dumps fuel out of the carbs'. If it's backfiring through the intake then you could expect some fuel to be blown back, but I wouldn't expect it otherwise, and if from the vent/overflow then it could be that the float valves are leaking or being overwhelmed by pump pressure and there is excess fuel coming up the jet and flooding the engine.

What fuel pump do you have? If SU then with the ignition on but the engine stopped that shouldn't click more than once every 30 secs, and in practice it could be several minutes.

You say you have a 43D distributor, is any vacuum advance port on either inlet manifold or carb sealed?

Idling all over the place could be due to any of the above, or carb linkage issues, but not worth looking at those until you have eliminated the other things.
Paul Hunt

Hi guys,


Jim, I'll check for leaks. Don't have a PCV fitted as it just has a filter for crank case venting.

Ray, it's a new Lucas rotor arm, and all is clean inside the dizzy cap.

Allan, the fuel is dumping out of the overflow. How do I do a compression test?

Paul, I'll check the the ports on the carbs and manifold tonight for leaks. It's backfiring in the exhaust. The pump I'm using is an aftermarket electronic one, possible it might be too powerful, i'll put a gauge on it and see what pressure it's throwing out. The engine is a 18V block using an electronic tacho, I'll check the wiring on it and get back to you.

Thanks so far guys, you've given me a few more things to try out,

Grant :-)

G Hudson

In your checks listed above you don't mention setting valve clearances. Tight or no clearance will certainly cause backfiring and rough running .
If backfiring into the inlet manifold............could this pressurise the float chamber?................ Possible I suppose, especially with the air filters in place.
If backfiring into the exhaust it could be a valve problem or timing problem, either ignition or cam!
A leaking, bent or sticking valve allows mixture to be pushed into the manifolds on the compression stroke, then ignited.
A compression test will exclude, or otherwise, this problem. You need to beg, borrow or hire a compression tester.

"The pump I'm using is an aftermarket electronic one".
Is this one of the alloy bodied, blue cap, German pumps, the Black plastic MOPAR, or a Facet type? A Facet usually has to be used in conjunction with a pressure regulator. However if the carb valves are being challenged I would expect to be obvious with ignition on but engine static.
Allan Reeling

Your fuel pressure should not exceed 3.5 psi. Any more and the float needles will be overcome with fuel. The fact that your new rotor is labeled Lucas is of little concern. The company name was sold off a long time ago and the label means very little today. Quality rotors can be obtained from the Distributor Doctor, in the UK, for a small amount of money. RAY
rjm RAY

"If backfiring into the inlet manifold............could this pressurise the float chamber?................ Possible I suppose, especially with the air filters in place."

A good thought. Although with the throttle released the carb pistons will be down, which will greatly restrict any air flow from the carbs to the air cleaners, hence trapping any pressure from a backfire. In that case the pressure would be passing over the top of the jet, being squeezed between piston and bridge, so I can imagine some at least would escape down the jet and into the float chamber.

OTOH it would only be a jet and jet pipes worth, and the North American anti-runon system for example does work by sucking fuel out of the jet by applying a vacuum to the carb vents. However when I fitted a version of that to my car, even with the original vent pipes still open to atmosphere, until I restricted the amount of vacuum applied it did suck fuel out of the vents and deposited it into the inlet manifold, and the engine continued to run albeit badly!
Paul Hunt

If the throttle shafts are worn, it is impossible to properly tune the S.Us. There will be vacuum leaks at the shaft bearing points and if very worn, a (usually the front) throttle plate will sometimes stick open and result in an accelerated idle. Check by spraying carb cleaner at the throttle shaft bearing points to see if idle drops. If it does, there is a leak. Also try moving the shaft to see if the throttle can be further closed.

My B had the problem you described and it has been tracked to extreme throttle shaft (and carburetor body) wear.

It is possible that the backfire and poor running are related to lean mixture since tuning with vacuum leaks becomes arbitrary.
Glenn Mallory

This thread was discussed between 02/08/2015 and 26/08/2015

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