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MG MGB Technical - Embarassing moment

Hello all,
Was looking for some possible reason for my 72 'B' would not start...

Was at the local MOT centre on Tuesday having driven down from home, but just before the test centre, the car started to miss slightly ....

Got to the centre and was ushered forward into the test hall .. so far so good, was asked to turn the ignition off .... but on trying to restart, no joy ... engine would turn but fire .... eventually had to be pushed backwards out of the centre (and failed the MOT!!) .... I let the car sit for a while and then it started fine but on driving home, if the revs dipped it nearly stalled, but as long as I kept the revs up, it was ok and I got home safely ... a few suggestions so far are a dodgy fuel pump .... any ideas pls ?

I would suggest ignition problems as a likely source. Sounds like possible weak spark or deteriorating spark voltage as the engine runs and gets warmer. It might be as simple as a worn or cracked distributor cap. A cracked distributor cap often acts this way. When warmed up, the crack expands expanding the distance the spark has to "jump".

Check the model number on your 72 distributor. There are several articles in the archives on distributors and most B distributors don't hold up that well. I would recommend considering a rebuilt one (some fellows here can direct you to the right source).
Rick Penland

Same thing happened with mine at the MOT today. Running fine all the way there, ran like a dog and kept stalling in the MOT centre, ran fine on the way home. Maybe they just want to give us a fright!

Good luck on the retest Jon.

G Lewis

Could be a huge number of things.

First thing to look at is the electronic tach (post 64 cars). If that starts flickering all over the place when the engine is stumbling, or drops suddenly to zero when the engine is still turning, it is an ignition LT problem. If the ignition warning light comes on as well until the engine has stopped the ignition supply through the ignition switch has failed. If the light only starts glowing when the engine has stopped then it is connections through the coil, points, distributor ground strap.

If the tach only slow drops as the engine spins down it is HT or fuel. Using an inductive pickup-type timing light on the coil lead and plug leads should tell you which. If the timing light continues to flash as the engine spins down, and the timing is still about right when on plug leads 1 and 4, then the problem is fuel. If the HT flashes get irregular or suddenly stop on the plug leads but not the coil lead it is the distributor cap and/or rotor breaking down. If the same happens on the coil lead then it is the coil.

"Most B distributors don't hold up well"? Rubbish. Most work just fine and last for years. Don't go splashing out on new kit until you have fully diagnosed the problem. Unless you have money to waste.
Paul Hunt 2

Jon - For a fuel delivery troubleshooting guide, go to:
This will help you determine if your problem is in that area. Cheers - Dave
David DuBois

At the risk of stating the obvious, check the points gap.

Back in the olden days, I used to do 300 miles a week in a mini.

When it missfired and wouldn't idle it meant that the heel had worn on the points, and there was no points gap to speak of. Adjustment was a fiddly job on a mini, so it got done when it missfired!
Martin Layton

I used to do 150 miles per *day* in my V8 for several years, switching to the roadster when it was fine and sunny. Adjustment on my Mini was far easier than on the 4-cylinder MGB, although not as easy as on the V8. I've had a set of points in both cars now for over 10k miles and not had to adjust them at all on the roadster, only had to adjust them once on the V8 after about 9k as it only has a 1 degree tolerance on dwell whereas the 4-cylinder has 5 degrees.
Paul Hunt 2

This thread was discussed between 29/09/2006 and 30/09/2006

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