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MG MGB Technical - Stalls and will not restart when hot

1972 British registered unmodified roadster with luminition - stalls after half an hour's running and won't restart for an hour. The only clue is the lack of spark.

The owner has changed backed to points with a new coil but to no avail.

I would like to check that my list of possible causes is complete.

If anyone has a moment to add to the list (two words may be all that is needed) we would be grateful.

1. The new coil
2. The HT leads
3. The capacitor
4. The starter relay
5. Shorting fan switch
6. Shorting ballast resistor wire

Thanks for looking

Roger
RMW

Roger-
As far as I know, 72 does not have electric fan or ballast resistor wire. If it cranks then start relay is OK. Question is, when it won't start, is there full power at coil white? Check with voltmeter. If so, problem is in distributor/coil. If not, I'd be looking at Ign switch, and brown power in, white out. When it fails, do Ign controlled accessories (wipers, turn ind, etc) work? Have found a lot of bad bullet connectors sleeves lately, especially at the white leads of Ign switch and other heavily loaded items. Evidently they have a life of 30 years, even if they have just been sitting around. There's a lot to be said for wiring in an Ign relay as later cars - takes the load off the switch and much of the wiring.
FRM
FR Millmore

I agree and if you are sure the problem is a lack of sparks would look to determine LT or HT as the cause.

HT could be diz cap, rotor arm, plug caps breaking down.

LT could be a poor resistive connection which is heating up and going open. Clean all LT connections inc the fuses and holders.

Check the LT link between diz terminal and points and the one from the points plate to earth.

Check for correct coil, if it is not a ballast type it should show 3R primary. Then put the meter in series and check for 4A with the points closed.

Rich

FRM, Rich - good advice VMT.

I have never seen the car but will be giving the owner a hand either by e-mail or on the telephone. Hopefully I will be able to publish any info that is useful to others - here.

Good wishes

Roger
RMW

could it be overheating of the fuel before it gets to the carbs? (vapour lock?). I experienced exactly the same (albeit in a kit-car running the b-engine), the engine got to temperature and after a long 5mph crawl and then a quick run up a long hill started to jerk and eventually halt. What had happened was the fuel was boiling off and the pump wasn't allowing enough fuel through to the carbs. After 40mins/1 hour everything had cooled sufficently to re-start the engine and drive her home.

I'd be inclinded therefore to check the fuel pump and surrounding area to make sure that isn't an issue (the fix is to fit an electric fuel pump which pushes the fuel at a constant rate apparently).
Stefan Carlton 1

My first guess was the coil, but I see you've replaced that - presumably with the correct six-volt type. Check the output voltage from the ballast resistor: cold, and hot-post-failure. You haven't mentioned what happens when a hot start is attempted. Does the starter turn over normally? If it turns very hard or slowly, it could even be the starter itself, although that doesn't explain the stalling. There may even be two separate problems here.

But, next time it fails, you might try a jumper wire directly from the brown wires on the fuse box to the white wire on the coil. If the car starts and runs normally, you know the failure is somewhere in the white wire circuit. If it starts but dies after a few seconds, replace the double bullet connector on the white wire circuit. That connector is where the hot white wire from the ignition switch splits off - one white to the fuel pump and another to the coil. Double bullet connectors have about ten times the failure rate of single ones.

FWIW,
Allen
Allen Bachelder

I repeat: according to all my references and experience, ballasted systems DO NOT happen until 75, so anything before this should have a 12V coil. Unless someone has modified it, in which case there will probably be a discrete resistor rather than the in-harness resistance wire, and there will be other non factory wires if modified correctly.
FRM
FR Millmore

Fletcher,

I stand corrected on that. And of course I should have known it - my '73 runs a single white wire directly to the coil.

Given a replaced coil and interchange of the luminition for points, I still stand by my second paragraph.

Allen
Allen

Stefan /Allen / FRM - kind of you to show interest VMT.

The owner did not get home to his car last weekend so I am no wiser - he has had a professional on the problem who is stumped and has given up - says there is no current on the coil when the car has stopped.

Stefan - your thoughts and experience on fuel starvation are in the back of my mind.

FRM - i have never seen the wee beastie but my son says it has been well cared for. If...if it is back on Luminition, I believe it needs a 12 volt coil - I have suggested the owner check that with the Club which sells the electronic spark kit.

Allen - I agree with your analysis but a bullet connector failure would hardly be so consistent which is why I come back to something which could be affected by heat 1. The coil 2. The starter relay 3. What do you think about a breakdown in the starter solenoid ?

Thanks again

Roger




RMW

Roger,

You're right, a bullet connector would hardly behave so consistantly regarding heat - unless it's internally generated heat from a weak connection maybe? My first guess, like yours, was the coil, until I read that he had already replaced that. And now the pro says there's no current "on" the coil when the car fails. (I'm presuming this means no current TO the coil). So I'm thinking in terms of heat generated within the electrical system by connections tantamount to total failure, although I would be inclined to think such failures would be a little more random, and permanent after a few random failures.

But if there's no current to the coil at time of failure, it should be pretty easy to trace the failure upstream. Is there power to the bullet connector? The fuel pump? The white lead at the ignition switch? The brown lead at the ignition switch? There are a very finite number of failure points. If we're talking about total electrical failure here, how about the brown leads at the solenoid? Again, not prone to such a predictable failure pattern.

A starter failure had occurred to me earlier as such failures frequently are temperature-related. But neither that nor a solenoid failure or a starter relay failure would explain the stalling itself.

Maybe he just needs the smoke recharged!

Allen

Allen

It sounds like the coil is still bad, or HT/LT problems still exist. Maybe a short in the replacement points or bad ground strap in the distributor? Bad luminition, and all of the above?

Is it possible that a short in the alternator is causing the battery to overheat? If the charging system dies, it could restart after cooling? I know - not likely, but its worth checking everything, right?

Are the plugs fouling with fuel when it dies? Is the fuel line hot to the touch? Is the heat shield in place (with insulation) behind the carbs?
Jeff Schlemmer

Hi Roger,

<< - he has had a professional on the problem who is stumped and has given up - says there is no current on the coil when the car has stopped. >>

This is a definite statement so I'm with Allen here, get your friend to make up a 2ft wire with a lucar on each end. When it next stops pull the white wire from the coil and one of the purple wires from the fusebox (or use a spare tag alongside the purple if there is one). Replace the pulled connections with each end of his new jumper wire, to supply fused 12V from fusebox to coil, and see if that get's it going again. (He can test this method before the car fails to be sure he is familiar.)

If that works you have isolated the problem. The standard supply to the coil follows a fairly tortuous path around the tacho to the ign switch via several connectors which would be worth investigating. Or it may be the ign switch contacts failing. A high resistance in any of these could rise with time/heat and lower voltage/go open.
Rich

Roger-
Are we correct in assuming that "won't start" means that it cranks but doesn't fire, or does it not crank when warm? Two different fish-kettles!
I have serious doubt about the "pro" working on this, since as Allen has pointed out, and I have implied, the power to the coil is quite simple. Allen's suggestion of a jumper direct from any brown to the coil W or + terminal will prove it, by bypassing all of the following: Brown goes direct from solenoid to Ign switch, with a bullet connector there. Through switch and out as White, (double) bullet there - frequently bad. To (double) bullet - also frequently bad - at rear harness breakout, to coil +. These bullet connectors, if bad, do generate a lot of heat - I burned my hand on the ones at the switch last year. There is nothing but the rubber insulating sleeve holding the bits in contact when this happens. Perhaps when the rubber gets hot it loses contact
Well cared for has little to do with it - these connectors die from age related stress cracking, as well as corrosion.
While I am not very familar with Lumenition since they are not too common here, I just had a 77 ballasted system car here with Lumenition. It still had the ballast and 6V coil, and has had Lumenition for at least the last 15 years. Pretty sure it works with either 6 or 12V.
Despite all this talk of spark, this does sound a lot like a fuel feed problem - either pump or blocked vent causing a vacuum in tank.
FRM
FR Millmore

Many thanks folks- I think we all are pretty well of one mind. I shall await "the call" armed with your advice. Will come back when there is a result - might not be this weekend 'though - the owner is a City man too and may have other things on his mind.

Roger - Central London
RMW

72 originally had a coil with rivetted spades. These can work loose and one of the symptoms can be hot running problems. Later coils had threaded studs and nuts for the spades. If there is 12v on the coil +ve with the ignition on and the points open, but no 12v on the coil -ve, then the coil or the connections to it are indeed the problem. If the coil +ve shows 12v with the points open but zero or very low volts when the points are closed then there is a bad connection back through the ignition switch. 12v on the coil -ve irrespective of whether the points are open or closed indicates an open-circuit through the points, distributor ground wire, or connection to the distributor. If the LT conditions are all correct it could be a faulty HT winding in the coil, distributor cap or rotor. If a timing light (the clip-on inductive pick-up type with separate 12v and ground connections) shows flashing during cranking on the coil lead but not on any of the plug leads it is the cap or rotor breaking down, change both. No flashing on any HT lead is probably a bad coil.
Paul Hunt

This thread was discussed between 28/06/2005 and 08/07/2005

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