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MG MGB Technical - cant fix mgb misfire

hi,i'm hoping someone out there has suffered the same problem i am having with my 76 mgb. After a few minutes of driving when i reach about 40mph it starts to misfire and slow right down as if its going to stall,but if you ease off the gas i can just keep it going but it runs very rough and will not accelerate.All the usual items have been checked or replaced,plugs,cap,points,condenser,with the fuel line disconnected at the carb it will pump 2 pints in 1 minute ruling out a faulty pump,timing has been checked,i'm at a dead end please help.
dd edwards


Do you have a spare coil you can try? I know if I say this enough times, one of these days I'll be right!

C R Huff

Do you have a vented fuel cap?
Dave O'Neill 2

I'd be inclined to try another coil. Mike
J.M. Doust

i've driven it with the fuel cap removed, didn't make any difference. I'll try a new coil and report back. Also my mechanic mate advised changing the fuel flexis on the pump,although they aren't leaking they're old and could be sucking in air.??
dd edwards

As well as the coil, I would check the needle valve in the float chambers. I could just be that it is over-fuelling because one of the valves is not shutting off the fuel supply properly. It might run reasonably when the engine is cold but as soon as it needs no choke it is running seriously rich.

But, as they say, most fuel problems are electrical. Don't trust condensers. New ones seem to fail with monotonous regularity - try changing it again when you do the coil.

Just a thought - I had a similar problem with my MGC.

M D Howard

My money is on the condenser too! You don't need to take the old one out, you can just connect it across the points (distributor lead and earth) with a spare piece of wire.


If it's the coil I'd expect to see the tach jumping about. If that is steady then it is more likely to be condenser, HT or fuel, the first and last give very similar effects. If it's a new-ish condenser than it probably *is* the condenser (I've known of a couple that lasted just a couple dozen miles), and as Mike says just clipping another from the distributor (or coil -ve) lead to earth will soon prove that, and won't have any detectable effect if the internal condenser is good. Of course you have to be sure your test condenser is good ...
PaulH Solihull

I will put money on this being a faulty rotor arm. They are rubbish these days & lose their reistance when they get hot leading to a very weak spark.
G Britnell

I've got my money on the condensor
or as a longshot - blocked exhaust
cheers Willy
William Revit

I had the same problem which was eventually solved with a carburettor rebuild!
Pat Gregory

I had a similar issue. Over 2700 RPM the engine would miss and backfire. It was the main lead from the coil to the dizzy. It worked its way out of its "seat" causing the spark to jump from the coil to the wire until the spark could no longer make the jump.

Free and simple fix. Also check the consensor (as previously posted) and gap.


79 MGB
gary hansen

Surprised the UK contributers arn't agreeing with me on my rotor arm suggestion. I've run B's since 1993 & also a classic boat with a ford 100e sidevalve engine. All have suffered symptoms like the OP's & been cured with a new rotor arm - as was my mate's GT6 at the classic Lemans event last year. We couldnt get a new rotor arm but a bit of insulation tape between it and the dizzy shaft got him back home. UK made repro arms use plastic with a high carbon content & this can start conducting the HT voltage when it gets hot.
G Britnell

Now look, with all these suggestions but no clear diagnosis, should we be running a book? Who's going to work out the odds? This could be a good game Mike
J.M. Doust

Lol Mike - have you ever noticed how people get scared off the more suggestions that are made?????

I'm a tad narked as mine isn't backfiring anymore and so the birds are no longer scared of it and are using it as target practise .....!

For the record - it was poor points which earthed out on the inner side of the distributor - and not the same as this one - so that's not another idea!!!!

I'm with JM (Mike) on this one... Too many guesses and not enough science! Shame on you Rachmacb... lol

dd, most of the guesses have merit but to check on a scatter gun or change on a blanket approach is likely to result in a lot of expense without fully understanding what is going on.

Ideally we need to narrow the field to get to the real cause. This means more testing/checking and feedback...

As you've done the usual service items (except the rotor arm!) I would start with the other checks which cost nothing... check the carb needle valves are doing there job eg not stuck as MD suggests and are closing properly when the chamber is full eg not overflowing. BUT don't assume they are not overflowing if there is no fuel coming out of the overflow pipe... I had a HS2 which caused a very rich mixture by pumping the fuel up the jet in to the inlet manifold! Check by removing the dashpots and making sure the fuel level is constant in the jet. I would question your mechanic's comments on the fuel lines if you are getting two pints per min as this is about what would be expected.
Another area to check is to ensure there is no extraneous air getting in to the inlet manifold. I would check the servo and hoses and the anti run on valve if you have one.

If we can eliminate fueling issues life gets a little more complicated...

I'll be in Newcastle soonish, if you want a hand drop me an email

good luck

M McAndrew

'76 are you on a 9v coil and ballast resister wire? If you're not sure check the voltage at the coil + with ignition on. Battery voltage or less? Also beware of the wrong coil being fitted; i.e. 9V on a non ballasted won't last very long and could be breaking down, and will get very hot. 12V on a ballasted circuit will give weak secondary voltage at the plugs when running but will start fine.
Check the internal resistance of the coil, i.e. between the + and -. If you are not ballasted and the coil is therefore 12 volts, the reading should be 3.4 ohms, or very close. 9volt coil will read 1.5 ohms.
Any variants bin the coil.
When things go wrong, when they started off right, it can also indicate heat problems. Check the carb heat shield.You could be vapourising fuel in the float chambers.
Run the car up to temp,air filters off,switch off then take the dampers out. Using both hands lift the pistons up, they should move with little resistance, drop at the same speed and "clunk" when they hit the bridge. Jamming pistons when things warm up is usually because the dashpot(s)are not seated properly on the carb body...........usually muck or a burr.
Have you got a fuel filter in line, check it! Also old tanks are usually mucky things and the debris gets pulled to the outlet, starving the engine after a few miles.
If the condensor has gone the points will be "blued", i.e. they've been heated by the arc.
In 40 odd years of messing with my, and other peoples cars I've only had one rotor arm failure.............Ford Anglia!!
Let me know what you find.
Allan Reeling

wow where do i start.the mgb will have to wait till next week,my father in laws ww2 willys jeep is misbehaving now and is due at a show at the weekend (blyth norhumberland beach if anyone is interested)so need to sort that first,and funnily enough it has the same problem,bloody cars!!!
dd edwards

getting back to the mg,i'll start from the begining.the car was owned by my wifes uncle who bought it nearly new,then as time went on he used it less but always had it mot'd,in later years the car basically went from his garage to the mot station and back,until 3 years ago when he gave,yes GAVE it to me,i basically taxed it insured it and drove it home from leeds,during the drive home it wouldn't go over 45mph unless it was downhill and it was backfiring and missfiring all the way i gave it a full service which included a rotor arm,cap,etc didn't make any difference.when the mot ran out i took it off the road to build an extension to my house...and now we're up to date,i want it back on the road but obviously still have the same problem i left it with 3 yrs a bit more rust of the time this car has stood still may alter your views as to what the cause may be,but i'll redo the simple things first and see what happens.
dd edwards

dd, doesn't change much.... Does it rev freely through the gears (I'm assuming not!)? As it's not done many miles it might be worth cleaning the jets just in case they have some old petrol gum partially blocking them.
I don't expect it to be this but have you checked the timing?

Can't get the Blyth this week but should be in Toon before the end of June if you need a hand...


M McAndrew

"9v coil and ballast resister wire"

Actually 6v coil and ballast, and it depends on whether the points are open or not as to what voltage you see on the coil +ve with the ignition on and the engine stopped, and the engine running.

Points open you will see 12v, points closed you will see 6v. Engine running you will see about 9v on an analogue voltmeter at least, which effectively is the average of the two and where the oft-repeated statement that rubber bumper cars have a 9v coil comes from. Cranking you will see full battery voltage i.e. same as chrome bumper (non-ballasted) cars, but that will be 10v or lower of course.
PaulH Solihull

This thread was discussed between 16/05/2011 and 26/05/2011

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