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MG MGB Technical - cough and spluter

hi there and happy new year.
a bit of a strange one. i have a 1976 mgb gt which only on 3 occasitions after 150 mile no stop drives the next morning the car start but sound rough and then when trying to move the car she cought and splutters for a while then back fires then no problem. checked plugs, leads and dissy no problem there and carbs seem fine but i'm no expert any ideas. the 3 incident were about 3-4 week apart from each other.
thanks john
jr williamson

John. Insufficient information here to make any form of judgment. Hence, difficult to provide any real guidance.

First thing we need to know is how often do you drive the car? You mention three trips of 150 miles which might indicate that the problem is related to running the car for a trip of that length. But, if that is the only thing the car is used for, it cannot be said to be a definite factor. Are you driving the car regularly, with a number of short trips, after which there are no problems? If so, we can assume that the length of the journey or the length of time the engine is being operated is a factor.

What are the ambient air temperatures where the car is stored and when it has been started and runs "poorly"? These are old cars with carbs. They do take a while to warm up, do not run particularly well while doing so, and this was something that people knew about and lived with when such cars were the common standard. Driving my wife's "new" Mini or my daughter's Miata (MX-5) when first started from cold is a completely different experience than driving one of my older cars in the cold weather. If I had little experience in older cars, and most of my experience was with modern cars, I would believe there was a problem with the MG rather than understand that it is how older cars operated.

Your car should have the SU HIF-4 carbs installed on it. These had some form of temperature compensation device as a part of the design, a bi-metallic strip which was supposed to help enrichen the mixture when the carb body is cold. This, I believe, operated along side the choke mechanism used to get the engine started in cold weather. This may be a factor in what you define as a problem, but I have very little experience with that model carb. Most of my experience is with the H and HS series carbs.

Does the engine run properly when up to operating temperature? Answers to these questions would help us to diagnose possible reasons for you.

Les
Les Bengtson

the car is used daily (150-200mile per week)and kept on the drive. temp was different each time from about plus 15 to minus 5. had alot of experience with old car in the past,the symptom are not just rough running but the car litrallhy won't idle and has very little power almost as if the diaphragm are gone on the carbs but symptom stop after about 5-10 minutes car normally at running temp after 2-3 minutes. on all three occasions the symptoms stoped after a large back fire if that helps.
jr williamson

It is quite possible that one of your carb pistons is sticking, giving you a very rich mixture. The backfire may be releasing the piston, allowing the engine to run as normal. If you have the capacity, remove each suction chamber and piston then clean with carb cleaner and reassemble topping up the pistons with the proper weight damper oil. Keep us posted as to your progress. Ray
RAY

UK cars all had SUs which didn't have a diaphragm, is this an American import with single Zenith? If you have some other non-standard carb that will make useful diagnosis even more difficult. If this only happens when warming up from cold it sounds like insufficient choke as much as anything.

With the misfiring is the tach flicking up and down i.e. more than just the change in engine revs would imply? If so that would be caused by intermittent connections in the ignition LT.

But so infrequent, and being kept outside, it could be condensation on the HT or in the carbs, or even carb icing, and is going to be difficult to diagnose as you won't know whether anything you are trying had had an effect, or it simply isn't going to occur on that occasion.

150 miles per week if spread over several days and more than one journey per day isn't very many miles per journey and the car almost certainly isn't getting up to full temperature, try a longer journey and see what it's like after that.

It's not really the weather to spend much time working on a car especially outside at the moment but with intermittent problems like this the first steps should be to check valve clearances, plug gaps and condition, points condition and gap/dwell, HT lead, cap, rotor condition, timing including centrifugal and vacuum advances, and only when everything is correct with those check the carb mixture, and air flow balance if twin SUs.
Paul Hunt 2

John

If you follow Ray's advice and clean the carb pistons, do so one at a time. They are matched to their domes and should be kept together.

FWIW

Larry
72 BGT
Larry Hallanger

"They are matched to their domes and should be kept together"

At least they were originally. They could have been jumbled up in the past, or had replacement components, which can easily effect this. If there are rub-marks inside the cover they need attention (either swapping over and trying again) or relieving. But no rub marks can equally be a problem if the gap is too big which will lead to insufficient vacuum above the piston and too low a position for a given throttle opening. The only way to check that is remove the cover and piston, block off any air holes, invert them, and see how long the cover takes to drop off the piston. Should be 3-5 secs for 1 1/4" and smaller, 5-7 secs for 1 1/2" and 1 3/4", 7-10 secs for 2".
Paul Hunt 2

Try checking the Brown lead down under at the starter solenoid.
Daniel Wong

"Try checking the Brown lead down under at the starter solenoid."

If the lights are flickering on and off.
Paul Hunt 2

This thread was discussed between 02/01/2008 and 05/01/2008

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