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MG MGB Technical - Starting problem for 76 MGB

Hello,

I have a 76 MGB that was working fine 4 days ago and won't start now. Battery is good, it turns over good. I checked the spark plug wire and I am not getting any spark. Also, checked the wire from coil to distributor and did not get a spark. I'm thinking it might be the coil. I used a bulb and the wire that is going to the ve+ lights, so with that connected, the other connector ve- has light as well. When I try and start it I am not getting any spark from the coil to the distributor. Could this be the problem to why it is not starting? Any help would be appreciated.
FJE Frank

Hello. If the bulb lights when connected to the wire to the positive terminal then the wiring is OK. When you test from the negative terminal and the bulb is lit the coil is OK, or at least it is not open circuit.

With the wire from the coil to the distributor connected, connect the bulb from the distributor terminal to earth, if it is still lit then that wire is OK. Does the light go out when the points are closed? If it stays lit then the points are not closing and there is your problem, if it goes out it could be the capacitor or something in the cap shorting the spark to earth. Something like a carbon track in the cap or the rotor button has failed.

Tony
Tony Oliver

Whoops. Just re-read your thread. IF you have no spark at the coil lead then it is not the cap or rotor. Most likely points or capacitor.
Tony Oliver

FJE, First things first, what type of ignition system do you have? LBCs under go many changes in 30 years.
RIC
RIC LLOYD

Ric, I have the original ignition system - 45d points type. I pulled the ignition wire from the coil to the distributor cap and put it about a 1/4 inch from engine block and got no spark when I tried starting the car.
FJE Frank

If you can see 12v i.e. the test-lamp lights on both sides of the coil then you have ignition voltage present and the coil is OK. As Tony says it will be the trigger i.e. the points.

If the points or condenser were short-circuit you would get a ground i.e. no voltage on the coil -ve, and a big fat spark when you tapped the white or white/light-green wire on and off the coil +ve.

If the voltage is present all the time when cranking, and not going on and off as it should, then the points are open-circuit. This could be the points themselves, but the points plate twists back and fore as you move the throttle (vaccum advance), which bends the points wire and the points plate ground wire back and fore, and these can fracture i.e. go open-circuit. If, when you have voltage on the coil -ve, but not on the points spring, then it is the wire between coil and points that is open circuit. But if, with the points closed, you have voltage on the points spring and the points plate, then it is the ground wire that is open-circuit.

Paul Hunt 2

If you can see 12v i.e. the test-lamp lights on both sides of the coil then you have ignition voltage present and the coil is OK. As Tony says it will be the trigger i.e. the points.

If the points or condenser were short-circuit you would get a ground i.e. no voltage on the coil -ve, and a big fat spark when you tapped the white or white/light-green wire on and off the coil +ve.

If the voltage is present all the time when cranking, and not going on and off as it should, then the points are open-circuit. This could be the points themselves, but the points plate twists back and fore as you move the throttle (vacuum advance), which bends the points wire and the points plate ground wire back and fore, and these can fracture i.e. go open-circuit. If, when you have voltage on the coil -ve, but not on the points spring, then it is the wire between coil and points that is open circuit. But if, with the points closed, you have voltage on the points spring and the points plate, then it is the ground wire that is open-circuit.

Paul Hunt 2

This thread was discussed on 15/10/2007

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