The thing I see so often now days is problems related to the alcohol gas we now use, wreaks havoc on carburetors. If you allow alcohol gas to sit in a carb for extended periods, it doesn't make a varnish like old gas did, rather a white residue, possibly the sugar in the alcohol. Anyway, this makes the plunger in the anti diesel solenoid stick when it's pulled in (run position). When you remove power from solenoid, whether by turn off ignition or by removing wire to test, plunger should release and block fuel flow, engine should run only for a few moments, but so ofter the plunger is stuck from gas residue.