As usual, it would be helpful if you provide what year Spitfire. As stated earlier the electrical systems in these cars are basic and straightforward. Is this car stock or has somebody fiddled with the ignition system, e.g. sports coil, modified distributor, etc.

In my experience most of the issues I encountered were distributor related, I still have the stock distributor with points, condensor, etc. on my TR6.

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'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor