Generator starts motors when engine stops? Help?

The Lucas generator in my '49 Morris LC3 starts motoring when the engine stalls or I turn the key off. I have to press the starter button to get it to stop motoring.
It was positive ground, which I've since swapped over to negative. It had the same problem in both configurations.
The generator does generate when the truck is running.

Is it the generators fault? Regulator? Or something else entirely?
Any suggestion where to start troubleshooting would be greatly appreciated.

 

far as I know if its motoring its getting power to the Gen, look at the regulator cut out maybe

 

The cutout is sticking for some reason, maybe the contacts are partly welded together or it may just need readjustment.
Take the car to an Auto Electrician.

 

That makes sense. Thank you both.

 

fixed?

 

I think so.

I pulled the cover off the regulator. There was no sign of any arcing or welding on the contacts.
I took the top contact off and bent the piece of metal that breaks the contact up a little bit and reassembled.
Only started and killed her a few times but it seems ok.