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MG MGB Technical - Ignition problem

I've just replaced my carbs with a remanufactured pair and they seem fine, tick over 900, and plugs a nice brown colour. Driving steadily around town everything was ok. But when I put my foot down the engine would momentarily stall and the tacho would drop to zero.

I figured this was nothing to do with the carbs but an ignition problem. I've got an ancient Sparkrite semi electronic ignition ( it retains the points and condenser ) . So I remove this, connect the wire from the dizzy to the minus on the coil; and I've now got no ignition light and the car won't start. I tried putting the sparkrite back, but I've still got no ignition light.

Any suggestions please

Chris Cummins
c cummins

OK, I disturbed a fuse so the ignition light is back on, but I've still not got a spark.
c cummins

Year? A fuse shouldn't affect the ignition warning light, or cranking or starting on any year though.

Did you put the +ve on the coil +ve as well as the distributor wire on the -ve?

Could be the points, or the earth wire inside the distributor from the points plate to the body, that would affect both points and Sparkrite. However if the tach was dropping with the Sparkrite connected I'd suspect the 12v feed on the white to it more than anything. Check for 12v on the coil +ve, check it only drops to 10v or so when cranking (however these two voltage will be different on a rubber bumper car, which is why it is essential you give the year), and check the voltage on the coil -ve is varying when cranking.
PaulH Solihull

A few years ago an MGA here had remanufactured carbs fitted. The pistons had to be lapped in to the dashpot before they would work. As delivered they were an interference fit. Once that was done they were fine. I'm quite happy with my Boyer Bransden ingnition module, which is an amplifier relying on a trgger signal from points like yours. It does have the plus of not need a capacitor across them. (+ve earth one only)
Stan Best

Paul its a 76, It was an added inline fuse. I checked the +ve and there was more than 12 volts, my analogue meter only reads up to 12v and it went past that, it dropped on cranking, but I cannot get anything on it now. I must have disturbed a connection on the starter.

c cummins

Ok; I moved
the switch on the side of the tester hence the the no reading.
I decided to change the coil, I had a spare. When I took the old one off, I found it was the wrong way round! but it's worked like that for years. I tried the spare coil, ( the right way round ) still no spark, I put the original back on; no spark. I checked the points they look ok, but I don't get a spark when I flick them. The earth wire looks ok, but I will remove the dizzy to have a good look.
c cummins

Check the grounding strap in the distributer. When you stepped on the gas the vacuum pulled the advance plate and the connection to earth could be broken.
darnoc31

If there is no voltage on the coil +ve (should be two white/light green wires, and it could be 12v or 6v) with the ignition on then the problem lies back towards the ignition switch.

Late 76 was when the cars got an ignition relay - two by the fusebox and white/brown wires on the front of the 2nd fuse up. It *could* be fed from the relay, but there was a retrospective mod to put things like the coil and fuel pump back onto the ignition switch as they had problems with the relays. Before that power came directly from the ignition switch and there were white wires on the front of the second fuse up.

If the ignition light comes on with the ignition then there is 12v coming from the ignition switch, and if you have 12v on the 2nd fuse up at the front then it is reaching there. So it could be either an open-circuit ballast resistance (a length of resistance wire contained in the harness) or possibly an earth on the white/light-green coming from somewhere which is swamping it.
PaulH Solihull

I've changed the condenser and it starts now with out the sparkrite. I just assumed that as the points no longer needed the condenser to reduce burning, it would not breakdown.
c cummins

This thread was discussed between 21/05/2012 and 24/05/2012

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