Starter generator won't stop running
 

A guy I know asked me to help him get his 129 running and this is the first problem we came to. I had a new ignition switch, so I tried that with no luck. The s/g just keeps running after the switch has been turned. I have to disconnect the battery to get it to stop. Since I have been lucky enough to never have electrical trouble with any of the older units an I'm pretty slow in the electrical department, I really don't what the cause might be. Does it sound like the magnetic switch or the s/g is at fault? The only magnetic switch failures I have had are a failure to work at all.
Thanks

Start with checking the starter solenoid.

Would I just check to see if there is 12V present on the output side after I turn the switch off?

That's correct. However you should disconnect the cable going to the s/g at the post of the solenoid or at the s/g itself if you're suspecting shorted tips within the solenoid. With the key in the off or run position there should not be pos battery volts present on the output side of the solenoid.

the voltage regulator can cause this problem if the one set of points gets burned together

Agreed, if you remove the cable I mentioned and it still tries to start then the v reg is your issue with n/o contacts stuck closed

Ok, I went over tonight and we did the tests for the solenoid and regulator. Both checked out fine, so he suggested maybe it needed some resistance before it would stop. We snugged up the belt, it was just hanging loose on the pulleys, and it works just like it should now. Anyone else ever experience this? I have motored other generators, but not a starter/generator and they stop on their own when you take the current away.

Yeah, I've seen it happen on my tractors a few times. Could have been solenoid contacts or regulator contacts stuck. You may have jarred them loose, or taking the power away may have let them un-stick. It may happen again, it may not. My 125 did it to me once this summer, never did it again. I wouldn't worry about it unless it gets to be a frequent problem.