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MG MGB Technical - Running Rough

I've a 78 BGT & have experienced my first breakdown in the 3 years I've owned it ...

After running fine for the first couple of miles she started becoming a little hesitant with a slight misfire. Shortly after, the misfire got worse , with a distinct loss of power , eventually not being able to pull the skin off a rice pudding !

After switching off & restarting it felt like it was only firing on 2 cylinders & could barly drag herself along.

When the RAC arrived an hour later she fired up although still running rough. On removing the plugs one at time it was discovered that No2 plug made no difference to engine note and revs, although a tester placed between HT lead & plug showed a very stong spark. Plug was changed as a matter of course , original was sooty & the bore smelt of petrol.

A compression test was made on all 4 cylinders & showed 150psi across all.

After changing the plug,points & rotor arm the car seemed slighty freer revving & drove home fine , until i reached home 3-4 miles away when it started to misfire again just as I pulled up .

It has been suggested that it may be a distributor problem, if so then I will probably fit a 123 electonic unit , but before I shell out £228 can anyone advise me regarding this problem ?

Many thanks in advance & sorry for the rambling ...lol

Steve
S Brown 1

I would suspect the fuel pump as it will display those symptoms. Have you changed the fuel cap. It will create this problem if it does not allow air to get into the fuel tank. I am not familiar with the UK version so you may not have the sealed cap as we do here in the USA.
Sandy Sanders
Sandy Sanders

Sandy

I havnen't changed the cap yet , good point. Mine is vented I believe, but I'll check for vaccuum caused by blockage in the cap

Steve
S Brown 1

What kind of carburetor do you have?
John English

Before splurging on the 123 distributor, I would suggest replacing the least cared for parts of the ignition system. The cap and plug wires are frequently overlooked when diagnosing ignition problems. These tend to be quite old, but don't show outward signs of degredation. Ray
RAY

On your side that is likely with HIF carbs, and they are prone to floats leaking and sinking. If the front carb has that trouble, it would act as it does. The really bad running starts when a plug fouls, but it will get worse if it's a float, and will commence flooding visibly.
FRM
FR Millmore

Many Thanks for your replies guys , yep I'll replace all the consumables on the ignition sysrem first , see what happens then check the fuel side

Cheers
Steve
S Brown 1

I'd take a closer look at the HIF's...sure sounds like a carb issue...
pete

It does sound like too much fuel, especially seeing as the plug change briefly got it running again. When it's misbehaving I'd have a good look and smell of plugs 1&2 again, comparing them with the other pair. Could be the float, inlet needle valve (look for petrol pouring from the carb overflow) or the choke O-rings, assuming SU HIFs. All are easy enough to put right with a bit of reading of your Haynes.
Steve Postins

A friend of mine had very similar symptoms a couple of weeks ago. He was driving happily down the road, when all of a sudden his '72B acts like it's running on two cylinders. Fortunately, he was close to home and was able to limp back to his garage. He checked compression, electronic ignition, float bowls, fuel pump, etc. - the usual checks and all was fine. What did he eventually find? While approaching his car from the front, with the hood up, he noticed that the front end plug on his intake manifold was missing - he found it laying inside the engine bay. Strange, but it does (did) happen. How? - I haven't a clue.
Steve Buchina

If the problem is only on one cylinder, any electrical problem would have to be in the plug, the lead or the distributor cap.

A carb problem would likely affect two cylinders.

The fuel tank is vented with a couple of pipes on my 1970 GT, not the petrol cap, and in case this would affect all of the cylinders.

Was the plug change an hour after the problem (ie did you do it immediately or when the RAC man came?). If an immediate plug change works then the problem is with the plug fouling up or failing in some other way. What plugs are you using? Some people recommend the NGK BP7EV as an 'upgrade' but I find that it fouls in traffic because it runs too cold and the electrode is tiny.

What about a sticking valve? That might only happen when the engine was hot?

Be careful about randomly changing parts in response to well intentioned suggestions on these BBs!

Neil
Neil Lock

Many thanks to you all for you advice .

Changed the dizzy cap, leads etc & now running fine - so much for the RAC expertise !

Steve
S Brown 1

This thread was discussed between 21/03/2007 and 25/03/2007

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