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MG MGA - misfiring problem

Dear All,

My 1960 Mark I has been misbehaving recently. Always occurs when the engine gets too hot in traffic. I always pull over before then to cool down but before I do the engine starts misfiring as if only 3 cylinders are firing. Once the engine has cooled a bit the car misfires until I get up to a decent speed.

Someone has suggested the cause is petrol evaporating in the carbs but it hasn't always done this.

Any suggestions to the cause gratefully received.

Safety Fast!

Jason
Jason Ogelman

Jason
Do you have a heat shield in place or could perhaps the fan belt be slipping?
I am sure the experts will be along soon with some other suggestions
Graham M V

Thanks for your excellent suggetions Graham, a heat shield is installed and the fan belt seems fine.

Regards,

Jason (novice)
Jason Ogelman

What sort of temperature are we talking about Jason?
Lindsay Sampford

Jason, I think there was a similar thread on this recently and a faulty coil was suspected. If you have a spare it might be worth a substitution.......Mike
m.j. moore

Mike, I was thinking along those lines, wouldn't think it was vapour lock with the weather we've had lately. Mine will get up to 230 and still be running OK.
Lindsay Sampford

Thanks Lindsay, very close to 212.

Jason
Jason Ogelman

Hi Jason,

My money would be an ignition problem. Try substituting the coil first, Then look at the distributor cap and leads.

The usual symptom is a misfire, sets in when the engine is hot, initially when the throttle is wide open and the engine is working hard. Gradually it gets worse until the problem you mention.

Don't forget you need an un-ballasted 12v coil.

Regards

Paddy
P Reardon

Normal fast driving temperature on my old bus! Go with Mike's idea first, it's quick and simple. Another thought, if it's only missing one cylinder, is a faulty plug, cap or lead. Next suspect would be the dreaded rotor arm, I hate those things for what one did to me!
Lindsay Sampford

Jason, if you've got CB points, check the gap and make sure the cam-follower is free on its spindle, it should get a little drop of oil from time to time.
Lindsay Sampford

One further thought before I go to bed! If your mixture is too rich, your plugs could be fouling whilst on tickover for long periods, and the clear once you get up to speed again. Have a look at the colour of your plugs, no, not the white bit on the outside, the bit inside! If the are black on the insulator nose, that could be your trouble, The plug nose should be a light tan colour and the end of the threads a dry, powdery grey. Different fuels can give all sorts of colours (I've had red sometimes!) but if your plugs are black and sooty, your mixture is too rich. There, you've got a few things to look at now!
Lindsay Sampford

many many thanks Lindsay. I removed a plug an think you're correct. The threads were sooty. The engine exhaust has also been spluttering as you come off the revs when driving hard. (I have a fast road cam.) Now off to the workshop manual to see how to adjust the mixture.

Much appreciated.

KR

Jason
Jason Ogelman

Jason, what colour is the plug nose, that's the important bit.
Lindsay Sampford

this is what one of the plugs looks like Lindsay. Look ok to you?

thanks again,

Jason


Jason Ogelman

That looks good Jason, if they ALL look like that, it is not a mixture problem.
Lindsay Sampford

Jason

I normally vote with those going for electrical problems. However, on this occasion I just wonder if it is the fuel boiling off as you describe.

My car normally runs cool at around 160 to 165. On one very hot day this summer I got caught up in a monster queue on the M25 and suffered almost identical symptoms. Once I got going all was fine again within about 3 miles and I have had no repetition.

I would be interested to know what your normal running temperature is and can you supply any recent history on engine condition, cooling system and anything else you may have changed or fiddled with?

Steve
Steve Gyles

Steve, you are such a "cool" dude it's unreal! Are you running on liquid nitrogen?
But seriously, my car gets frighteningly hot sometimes (230F), but still doesn't miss as much as a beat, even when in a traffic queue. I have checked my temp gauge at boiling point, and it is spot-on. The only time I get "vapour lock" is when I stop the engine when it is very hot (bonnet almost too hot to touch) and then re-start before it has had time to cool down (i.e. after less than an hour). It will start but then splutter for two or three minutes; a couple of hundred yards and everything is back to normal.
Lindsay Sampford

Lindsay

Have you not just described what Jason suffered, give or take the way one describes symptoms?

I prefer neat oxygen or an enriched oxygen/air mix. I used it for 25 years when flying. Good after a heavy night night!

Steve
Steve Gyles

My first guess would be a spark plug lead or distributor cap. I doubt if a coil would cause just one cylinder to misfire unless the park was quite weak. If the cap and leads have not been changed in a while I would replace them with new ones, and if you have CB points, you might as well change them, the condensor, and reset the ign. timing. I suspect one plug begins to carbon up in the heat due to a weak spark, and when driving at speed again, it takes a few minutes to clear the carbon off the plug and begin firing again. I wouldn't touch the carbs at this time. I've been driving MGBs and MGAs since the early 1970s and I have never known a carb to adjust itself. You first always want to get the ignition system working up to snuff, replace what you need to, and adjust the valve tappet clearances. After you do that, almost always what seemed like a carb problem is gone.
Good Luck.

Ralph
Ralph

Steve, yes, but mine is OK until I stop the engine for a while. Jason's appears to do it when the engine is still running but the car is stationary. I rather thought it might be down to the plugs sooting up at tickover due to an over rich mixture, but having seen what the plug looks like (and presuming they are all running as clean as that) I feel we may be back to something non-fuel related getting too hot and breaking down, as Mike said earlier, the coil could be the prime suspect.
Lindsay Sampford

Lindsay. Ok, but mine did exactly as Jason's did. Engine still running, stationary on a very hot day. It ran rough and idle dropped. It took those 3 miles to clear and that was that - back to 165 degrees and no problems previous or since.

My car has electronic ignition, Sports coil etc. Runs absolutely great except on that one occasion. That's why I just wonder if his problem was the fuel boiling off.

Steve
Steve Gyles

Jason - I note that you are running a fast road cam - could it be that you you are using the wrong plug type - doesn't it need a "hot" one rather than the standard one. It likes to run fast and not sitting in traffic so maybe one of them is fouling - just a thought - good luck and welcome the wonderful world of MGA - cheers Cam
Cam Cunningham

Dear all,

I am overwhelmed by the response to my problem. Thank you all.

All plugs are the same so I can rule out a rich mixture.

In terms of normal running temperature this is about 190ish and nothing has been fiddled with recently.

I have young kids and don't do many long runs throughout the year, to get the running temperature right up there, apart from a quick jaunt around the country or a local car show or down to Tescos/ice-cream trip and am rarely in traffic. The real test is the lads annual trip to France for Le Mans 24 Heures.

I have just replaced the ignition coil and what with the "hot" weather behind us it perhaps won't be until next Summer in that I really get a chance to test it again.

In hot weather, the car has behaved exactly as Lindsay has experienced before when stopping the engine when it is very hot and then re-starting before it has had time to cool down and then spluttering for a while before returning back to normal. It does sound like vapour lock but I'll pack a spare set of leads, distributor cap and points too for the future.

Thanks again,

KR

Jason
Jason Ogelman

This thread was discussed between 24/08/2010 and 30/08/2010

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