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This has completely baffled me, the original Starter burned out because of this problem so I've installed a brand new Powermaster 9502 and new Starter Solenoid.
Expert up the road tested the old Starter and said the Starter Solenoid on the Starter Motor was sticking.

When I wire up the new Starter motor beside the bike and earth it with a Jump Lead to the Bell Housing it all works fine.
But, when it's all fixed in place the Starter keeps spinning and is kept engaged on the Ring Gear.

The Bendix gear can't be sticking on the Ring Gear because when I switch off the Ignition - it disengages and stops spinning.

I've just installed a new FAST EFI but can't think that's linked to this problem...?

I’d guess the start switch is faulty. Pull the solenoid wire off the solenoid and make sure the power doesn’t continue when you release the starter.

It wasn't that - but you'd never guess the reason!
Problem's now fixed.