[Richard]

If the voltage was too low to hold the fuel shutoff solenoid open the engine will do exactly as you describe. Before I replaced the coach batteries on mine, it would do the exact same thing when they were low. If I cranked the big engine, the batteries would merge, and the genny would stay running. If I slid the genny out and held the solenoid open by hand I could crank it like that too. Bottom line is I think your mechanic was pretty savvy.