The float bowl solenoid on the Intek OHV V-twins seems to be an anti-dieseling device, and from what I've read on the internet, it is not unusual for the solenoids to fail either partially or completely, causing non-starting in extreme cases, or fuel starvation under load in very mild cases. If your Vanguard engine has one of them, it does sound as if you will need to replace it - you cannot allow the engine to diesel when you switch it off, because it may cause damage.
