DEVELOPING MISFIRE

I own a 1978 MGB GT that I’ve owned for some years now but it has now developed an irritating problem. A Castrol additive is always added to the petrol to offset the use of unleaded fuel and this has worked well. But recently now and again the engine has begun to misfire, dropping down to three cylinders and then two. Occasionally when this develops the engine sometimes appears to be running on just one cylinder. At this stage I have to stop and let the engine cool down.

Once it’s cooled down I can then re-start the engine and continue on my journey. My first thought was that the coil was failing, but I have replaced this along with the points and condenser without success. I’ve also stripped and cleaned the carburettors without any positive results.

Do you think that the cylinder head is at fault and should I fit one suitable for the modern unleaded fuel? Allan Childs Having covered the possible failures in the ignition system I believe that the fault you are experiencing is with the fuel supply. I do not believe that the problem will require a cylinder head and the symptoms do not appear to be related to the valve seats, as would be the case with any unleaded adjustment related faults.

You have stripped and cleaned the carburettors and hopefully you have ensured that the float and needle valves are operating correctly. I would also have expected you to ensure that the needle valves are perfectly free and not sticking. SU carburettors are not normally prone to icing up, but in your case this is a distinct possibility.

I would ensure that the air intake is able to pick up from a warm area of the exhaust manifold. If this isn’t the case I would expect rectifying this should cure the problem. If pancake type filters have been fitted this may be the source of the problem.