Question1978 m g b roadster car runs fine for 30 mins then starts to miss and sometimes back fires if left to go cold starts fine then after 30 mins starts to miss and will not tick over have changed lots ie leads points condenser plugs coil at wits end can you help
AnswerHi Kevin. Not an easy one to trace.
I have a few suggestions:
1) Is the coil getting very hot? I notice you have replaced it, so I hope you have fited the right one. The 78 MGB should have a ballasted coil, so a standard 12 volt coil will have great difficulty in providing a good strong spark and could misfire as it gets hot.
2) Inside the distributor, attached to the screw holding the condenser to the baseplate, there should be a wire, and the other end of that wire should be a good earth. I have seen some where the wire has partially broken or is missing altogether. As the dizzy gets warm, the wire breaks down electrically and the car starts to misfire.
3) When the car starts to play up, does the rev counter do strange things? They are both supplied via a relay under the bonnet (hood) near the fuse box. A faulty ignition relay could cause the same symptoms. Look for a small can with 5 wires attached to it. Black, Brown, two Whites, and a Brown with a Yellow stripe. I would replace this relay and see if the problem goes away.
4) Have you checked the tappett settings? If a valve gap is too tight when cold, the engine will start and run normally. However, as the valve gets hot, it will expand. The valve then longer closes properly. A partially open valve would cause a backfire. Stop the engine, it cools down and the problem goes away.
There are other possibilities, but I have given you enough to start with. Feel free to come back for more. Any extra clues you can give me would be helpful.
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You have subsequently told me that the rev counter is not working at low revs. Maybe you also have a poor battery or an alternator on the way out. This might be a symptom of something else.
