Hi Paul

I wonder if you are getting plug fouling by carbon and oil ? This can be a bit of a vicious circle, as an initial misfire fouls up the plugs, which then makes the misfire worse. It's worse if the engine is run off load and never gets thoroughly hot. It's possible to have a faulty or lightly fouled plug which seems OK i.e. shows open circuit when tested at low voltage (with an Avometer or modern equivalent) but conducts at higher voltages. This can allow the ignition energy to leak away, and the voltage across the electrodes never quite rises high enough to make a spark. You could try swapping the plugs between the cylinders to see if the fault moves, in which case the finger of suspicion points to the plug rather than the magneto or the cylinder. If the fault stays with cylinder 2 then I would suspect a valve or compression problem.

Location: New Forest

Would a valve problem prevent the magneto from sparking on one lead?

Have you tried it with the plug out and earthed to the head?

Location: Devon

If you still have no spark on No2 swap the plugs around and see if the misfire moves with the plug.

Location: Devon

If having done this No2 is still not sparking the problem has to be with lead or Mag.

Location: Devon

Hello John,

I think you might be on to something here. At present I have the car on stands in my shed whilst I do some other work on it and I've been trying to fix this misfire problem at the same time. As a result it doesn't really get properly hot when I'm test running it. Also, and this may be the main point, I've had trouble with the SU's float chamber flooding and petrol running out of the jet. I've fixed the flooding problem but during that period the plugs were quickly fouled. I cleaned them and refitted them more than once but this repeated sooting may have weakened their resistance to earth. I take your point about the vicious circle. It's possible that the lead to No 2 had a bad connection in the distributor cap as well - also fixed when the new HT leads were fitted. This may have been more than one fault originally.

I'll start again with clean plugs and take it from there.

Thank you very much for your advice.

Regards
Paul.

Hi Stuart,

I've had the timing light showing a spark on No 2 when I tried a different plug so I believe it's either Ruairidh's theory of a hairline crack in the distributor cap causing intermittent earthing or John's suggestion of the viscious circle of plug fouling with a rich mixture in an engine that not's fully hot.

I can't spend any more time on it until the middle of next week now but I'll report back when I've had another go at it.

Thanks you very much for your help.

Regards
Paul.