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MG MGB Technical - 73 stalling when hot

Please if anyone can guide me: I have a 73 mgb gt with very low miles, that suddenly began to stall on me when hot as one comes to a stop. Foot to the floor and cranking will not restart it only pushing off on a hill or letting cool down will let it restart. Obviously must be flooding out some how, symptoms are: starts fine cold and runs great, heats up (after about four miles of driving, choke completely off, as one slows down, a bit of popping in the exhaust pipe and rpms drop more radically than they should, and if the engine is not revved up high, the car will then stall and not restart at all unless pushed off down a hill with one's foot to the floor, or letting the car cool down. The carbs have had new kits installed about a year ago points and condenser dwell and timing are good. Must figure that there is some problem with the carbs though. Anyone have a suggestion for me before I take the carbs apart again? Thank you in advance. Feel free to e mail me personally.
george hussey

George, I did had exactly the same experience with my 76 mgb. after a lot playing around with the carbs and ignition I cured the problem with replacing the coil.
First I had it replaced with a new standard coil but this must have been a ''monday morning'' coil because this one lasted only a couple of months. I replaced that one with the Lucas sports coil, now its OK.
J Halma

Did u pull a plug and check for spark when stalled?...That would reveal much...i.e. ignition vs. fuel/carb

Had the same problem in my '74 as you described; twice actually. First time the ignition coil went bad after less than 1 year, replaced it and the car started. Second time one float was leaking and the engine flooded. As indicated above look for poor spark or no spark first because this is easy to do. If the engine was flooding, you should see gas dripping out from the bottom of your carbon cannister when you have the ingnition on but not cranking the engine.

Have you confirmed it has spark when it won't restart? At the correct timing? I can understand a coil failing to allow a hot restart after standing a few minutes, less so for it to stall when coming to a halt at a junction or similar. Running rich seems more likely. If you have a SU pump (the sort that clicks rapidly when you first turn on the ignition then slows down or stops) turn on the ignition but don't start the engine and listen to the pump. Once it has filled the float chambers it shouldn't click more than once every 30 secs. If it clicks more frequently it could be a float valve leaking which will give a rich mixture. Remove the vent pipes from the float chambers, you will see a leak here much quicker than at the cannister, and if it is only a small leak it could still take some time to show even here. If that's OK it could still be float valves leaking but due to vibration when the engine is running. To check for that put a catch-bottle under pipes from the float chambers and see if there is anything in it. In either case either the valve could be faulty, or the float contains fuel and is partially sunk. Fuel in some floats can be difficult to spot. OTOH if you have an after-market pump these often put out to much pressure, which overwhelms even good float valves, and you have to fit a pressure reducer.
Paul Hunt

Have just experienced something similar. Followed Haynes recommendation to check for a spark when separating points with ignition on. Found no spark, so investigated further and ended up lightly cleaning points' faces with fine glass paper and now all ok.

Question is what caused the contamination? Burning deposits? Not sure, but drove the car yesterday and no sign of hesitation, jerks, popping exhaust at all. Cheap fix!

1970 BGT.

PS If anyone has a tip for easily getting the condensor and earth wire screw back into the distributor plate, would be grateful!

E S Bowden,

Perhaps a tool like these to hold the screw would help:

One's for slotted head, the other for Philips.

Wayne Pearson

Thanks Wayne. Found similar on a UK web site that should do.



The points screw at least should have the tip plain and the rest threaded, perhaps in the ratio of 1/4 to 3/4. That helps a lot.

Could have been oil contamination that turned hard and sticky under the action of the spark, but then I've found mine liberally coated in greease or oil with no problem. Another possible cause that I have experienced is that pulling a strip of paper through the closed points to clean them of said oil from feeler gauges, if you pull the edge of the paper through, and particularly if it is a torn edge, will leave fibres between the points that will insulate them. To that end I pull the paper through but open the points before the edge reaches them.
Paul Hunt

What's glass paper? I've seen it referenced to fuel pumps, control boxes, etc. I'm pretty sure it is NOT emery cloth.
Robert McCoy

Perhaps I should have said sandpaper - (I am not an ex-school woodwork teacher but I do remember him);

Sadly no emery to hand, and at 11.30 pm the "glass paper" was going to have to do ...... she didn't complain!


It was glasspaper not sandpaper according to my woodwork teacher when I was at school, which was a loooong time ago. Sandpaper is even older. Either is easier to say than 'aluminium-oxide' paper ... especially as Americans can't even say 'aluminium' ...

Emery cloth is different again, as is wet-and-dry. Both tend to retain their grit better than glass/sandpaper, which is why the are better when used on mechanical parts.
Paul Hunt

Thank you all for the input. The fact that I can push the car down the hill and it kicks off and as long as I keep my foot in it it runs, I thought eliminated the spark problem.............not so???
george hussey

E S Bowden,
Another handy way to start screws in awkward places is to use an extendable magnet (like an aerial with a magnet on the tip).
These will hold screws and small bolts firmly enough to start the thread and avoid the possibility of dropping them.
I keep one in my pocket whenever working on the cars, as they are also invaluable for retrieving dropped spanners, sockets etc. !

David Overington

"The fact that I can push the car down the hill and it kicks off and as long as I keep my foot in it it runs, I thought eliminated the spark problem.............not so??? "

Doesn't that also eliminate fuel? And every other cause of not running? OTOH maybe cranking is lowering the voltage, so it *will* only restart with a push, which could well be a spark/ignition problem.

You will have to do a proper diagnosis when it won't start with cranking and *confirm* you have sparks at each plug, not just guess. Then check fuel either lack of or flooding and so on.
Paul Hunt

This thread was discussed between 19/09/2008 and 28/09/2008

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