78 MGB engine runs on

I read a post from two years ago about a person (Jimmy Wade) having a 78 MGB with run on problems after the ignition was turned off. He claimed to solve the problem by putting a diode between the alternator and the ignition light. Unfortunately he doesn't explain any more than this. Could anyone give me a more detailed explanation about this technique.

Lou

Lou; With the later MGB there can be two types of run-on. The first is common to many MGB's of all years, when the ignition is turned off the engine continues to try running. It will shudder and shake and may try running backwards. This problem is basically a fuel related issue, although it can be caused by ignition timing. From around 72 a anti-run-on vale was installed in the MGB to reduce or eliminate run-on. If you have this type run-on perhaps your car has been desmogged and the anti-run on valve removed. Or the carburetor may have been replaced with a Weber or SU's and the anti-run-on valve not hooked up. For more information on running on see:
http://www.mgb-stuff.org.uk/hammerframe.htm
Scroll down to contents, look for fuel, then running on.

The other type of run-on happens on later MGB, the ignition is turned off and the engine continues to run as it would with ignition on. This is an electrical issue. Information about adding the diode is here;
http://www.mgexperience.net/phorum/read.php?1,591425,597300
See Dean Raymond's last post.

The alternator light circuit may not be the cause of the problem. It can also be caused by some diodes that were added to the handbrake and maintenance reminder circuits. If you don't have a good color wiring diagram you can get one at:
http://www.advanceautowire.com/
On the left click on stock schematics, MGB and find the PDF diagrams, I think your car diagram is on page 33.

If you decide to add a diode to the alternator light circuit it may be easier to just remove the bulb. A diode or removing the bulb prevents the circuit from letting you know if your alternator isn't working. For more information go back to Paul Hunt's site;
http://www.mgb-stuff.org.uk/hammerframe.htm
In the index find, electrics, alternators and faults. I'm quoting his information on the diode in the faults section.

"For heavens sake don't do what someone said and fit a diode to 'correct' i.e. hide this problem. If you do you may well have stopped the warning light from glowing dimly at night but you have also stopped it telling you of complete charge failure. If you want to do that you might just as well unscrew and throw away the warning light bulb and save the hassle of fitting the diode!"

He is talking about a different problem but the suggestion to remove the bulb should do in your case.

Hope this information helps.

Clifton

Clifton, thanks for the reply. I recently changed the Zenith carb with a Weber 45 MCHH and the problem is slightly worse. I don't think that it's a carb problem because the engine doesn't shudder and shake when the key is turned off. The idle speed is 800 rpm and the mixture is good. The engine just keeps running for 5-10 seconds until it shuts off by itself or until I pop the clutch or turn on the headlights. I'll investigate more this weekend. Thanks again.
Lou

Lou...check your ignition switch also. Had the same problem with my 77B. Turned out to be the "slot" in the back of the ignition switch was a bit worn, and simply did not turn off far enough to kill the ignition. Just a hair breath away, but far enough to not make it. Had to remove the switch assembly from the the lockset, and make some "adjustments" to the switch position and alignment. You can test yours by pulling the switch and operating it by hand to see if it will kill the car each time you turn it off. hope this helps ...good luck