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MG MGB Technical - Backfire Through carbs at cruise


I got my 74 B on the road finally and have uncovered a problem. Car runs great - starts easily and accelerates very good. The problem begins after the car has been running for 15 minutes or so. At cruise - 40-50mph, I start getting backfiring through the carbs. If I accelerate or decelerate, the problem stops. If I cruise at a lower speed, same problem occurs. If I hold at the cruise, the problem worsens.

I have rebuilt the carbs, replaced the points, timing is right where it should be. Possible ideas I have heard:

Vacuum advance leaking
Coil going bad

I am open to any idea, just need to understand why the problem would manifest itself at cruise only.

NP p

I have a question. When you say backfiring through the carbs do you mean it pops loudly from the engine room? It sounds like a valve timing issue, possibly even a loose dizzy cable or two if your really lucky.

A vacuum advance leak would make it run very uneven, due to lack of advance but that doesnt sound like the problem.

Id start eliminating by checking the leads then the dizzy, rotor etc if it all is as it should be- its definately a valve timing issue.

Hope you solve it!


JS Ahlund

Regarding the vacuum advance, that's easy to check....
Remove the hose from the manifold, remove the cap and rotor, and suck on the hose, if the advance mechanism moves, and no air leaks , the advance is working.
With regard to a bad coil.....That will usually manifest itself under acceleration....You will get a misfire, or obvious loss of power.
If your distributor timing is correct, using a timing light, and with the vacuum on the manifold blocked off, then that is not your problem either.....
I am assuming that you have the dash pots properly filled, on your SU's.
If there was a vacuum leak anywhere on the manifold, your idle would be rough, and it would be difficult to accelerate.
I am suspecting a faulty fuel pump....It works fine, most of the time...but then, heats up and stops working momentarily...The engine starves for fuel, and suddenly the pump comes back on ....
Is it an SU pump, or aftermarket?...If SU, it may just need the screens and discs cleaned, or the points replaced.
Edward Wesson 52TD

An after thought leave no stone unturned- swap the vac advance unit for a new one even if its not old, theyre cheap it wont hurt and your engine will no doubt run better. vac line is worth swapping out too if its a few years old and showing signs of being brittle.

Without going through it methodically its like grasping at straws.

Good luck.
JS Ahlund

Check the coil leads. Mine had the lead just loose enough to get up to 3000 RPM then started to break up. Check secondary coil wire as well for failure (pending). Again, recheck points and also may have a weak condensor. Just because the condensor may be new doesn't mean it's a good unit.

Let us know how it turns out

79 MGB
gary hansen

Is the tach jumping about when this happens? If so it is an ignition problem, on the LT side including the coil, but that would normally make it backfire in the exhaust. Could also be the condenser with the same effect, but usually when they go they stay gone.

Can't really see a vacuum leak causing it under any circumstances. Valves possibly sticking when hot, although I can't see how accelerating would change it, decelerating maybe. Can you reproduce it at a standstill? If so, and you have an adjustable timing light, you can connect that to various plug leads and twiddle the knob to 'freeze' any each of the rockers/valves in turn so see if any *are* sticking.

Are you sure it is backfiring, and not just hesitating i.e. a fuel problem?
PaulH Solihull

This thread was discussed between 08/03/2013 and 09/03/2013

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