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MG MGB Technical - Runs then dies! Pease help a novice

My 78 BGT has developed a fault which I cannot track down. I recently tried to fit a luminition lectronic ignition (magnetronic) kit and the specially designed Luinition coil. The car started ran for about 10 seconds then died and would not restart. I had not fitted the ballast resistor supplied as I understood the late B's to have resistance wiring in the loom.
fearing the ignition unit to be faulty I replaced the eletronic system with new points and condeser but still got the same problem. ie Starts, runs for a short while then dies. Interestingly the colder the enginer the longer it runs for before failing suggesting the problem is heat related? I have gone back to the original coil but the problem persists. The original coil is a 6V coil (original lucas) What tests can I try to isolate the problem?
Martin A Gunn

On my US 79 B, I would suspect the gas vapor recovery system if I had a problem like
this. A vacuum may be building up in the gas tank
or above the float bowls in the carbs (assuming
there's a connection to the SU carbs from the
cannister.) This could be due to a bad
anti run-on valve, clogged cannister, or clog in
the ELC line running back to the gas tank.

When the car dies, hold with an insulator the
spark plug wire near a ground, and see if
you're still getting a spark. I would doubt
the problem is ignition related since you replaced
most of the components.
Ronald

Martin
Reading though Ronald’s advise I will suspect that unless you have a re-imported car than what he is describing will not mean a lot. Thankfully, we never had to have emission controls on our UK cars.
But his answer did get me thinking as to what to look for. It would seem that what you have done so far is logical, however I would suggest that you check the high and low tension leads, which are attached to the coil. Both of these could be breaking down. Another area of the ignition I would suggest you check is the spark plugs. Although to be quite honest, I doubt if it is these that is a fault here.
Maybe it is the fuel pump. This is easy to check by disconnection one end of the fuel line and switching on the ignition. Ensuring the fuel goes into a container to stop spills. If there is fuel then try the carbs. Ensure the fuel in the float chambers is clean and that the fuel is getting to the air to mix. I would also advise that you check the amount of fuel being pumped to the carbs but I would not know how to do that. But someone here will know.
Have you changed the fuel cap recently? If this is not the right type then a vacuum will build up and no petrol will be pumped to the carbs. There is a vent line on later cars this may also be blocked. To check this out just take of the filler cap and try starting the car.
Good luck…I am sure that if any of these don’t help someone in the UK will point you in the right direction.
Ian Holliday

If the Lumenition coil came with a ballast then you should use that and not the original loom ballast, which means you will probably also have to run a 12v ignition supply to the ballast. This may seem pointless (!) but you can't guarantee that the new and old ballasts and coils are the same, and running one without the other would invalidate the guarantee, I suspect. If the instruvctions say nothing about use on ballasted and non-vballasted, 12v or 6v systems then contact the supplier. If they can't tell you send it back.

However if you have gone back to the original coil, connections, points and condenser then something else is wrong. If removing the filler cap doesn't stop it happening then you need to start from first principles and check the voltages at the coil +ve and -ve with points open and points closed. Do this when the engine will run and when it won't so you have a comparison. There is a step-by-step diagnosis on my web site (see either to the left or immediately above this post) - click on 'Spanners', 'Electrics' 'Won't Start' and 'It turns over but won't fire'.
Paul Hunt

When the engine dies, how does it do so? Usually if it stops dead it's electrical, if it splutters and then stops it's fuel.

Tim.
Tim Jenner

How does the tach act when the engine dies? If the tach suddenly drops to zero, it indicates a low tension ignition circuit problem. If it winds down with the engine, it is not a low tension problem but is either a high tension problem or a fuel problem. In the high tension system, only the kinglead/coil lead or rotor could cause such a problem. If it were a bad spark plug or single lead, the car would continue to run, but in a rough, out of balance manner. A partially clogged fuel filter will cause the symptoms you describe, so I would replace any that are in the system before doing any trouble shooting. Les
Les Bengtson

Les re your last comment the tach is behaiving oddly. It appears to stick at zero until I rev then is ok ( i always assumed this was simply a sticky needle) when the engine is running on idle. When the engine dies the tach seems to fall before the engine speed does. Any ideas what the porblem with the LT circuit might be?
Martin Gunn

Does your distributor have vacuum advance? With my
optical ignition (Crane, same idea as lumenition), I had a
problem with the engine dieing when vacuum was applied.
The breaker plate when moving flexed the wires to the
optical pickup causing a broken circuit. When this happened
the tach dropped suddenly to zero.
Ronald

Martin. For some reason, all of the RB cars I have seen lately (over the last 15 years) have a tach which must be tapped (sometimes rather hard) on start up. This was not true when they were new, nor is it common on the CB cars. Check out the article on ignition system troubleshooting off my website at http://www.custompistols.com and it should give you a starting point. If I had to make a guess, I would suspect the ground wire from the baseplate to the distributor housing or the wire going to the points, but only because they are moved when the car starts and the vacuum advance kicks in. Go through the system from point to point, using an accurate volt meter, and you should be able to eliminate any ignition troubles. Then, if the problem still exists, look to a fuel supply problem. Les
Les Bengtson

Does the ignition warning light also come on when the tach drops while the engine is still spinning? If so you are losing the 12v off the white from the ignition switch.

Does the Lumenition have its own pickup and two wires leading from it back to the electronics? If so then I doubt that one of the causes of this problem in a points system - internally fractured wires inside the distributor - is the cause in this case. If it only has one wire back to the electronics then it probably does use the ground wire that is inside the distributor. Apart from that it seems to me the only thing that is common to the Lumenition and points set-up is the loom ballast and 12v supply. As I said, do some voltage tests before and after the problem and tell us the results.
Paul Hunt

This thread was discussed between 17/02/2002 and 19/02/2002

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