Bad Earth??

Bad positive connection. Have you checked the starter cable connections including the solenoid. Is the solenoid bracket adequately earthed to the chassis or engine (don't know where it is on a Spit)? Could be a duff solenoid. Is the starter motor fully tightened up including the long bolts through the middle (don't laugh I've had this happen)? Are the starter motor brushes OK?

Have you had a good look at the starter motor. I had a problem a bit like yours on a P4. I finally found it was oil on the commutator collecting carbon particles from the brushes and shorting across segments. (I had fixed a leak some time before, but hadn't noticed where the oil had gone).

You can usually eliminate a bad earth by checking whether the choke and throttle cables get hot when you operate the starter. (Check with a wax candle, *not* with your fingers!)

Jim

or if for some reason you don't have a candle in your toolbox use a voltmeter or low voltage bulb connected between engine and chassis

Matt B

Thanks all. It was the starter motor brushes.