I concur with the high idle being the culpret. And as HotRod has stated you can play with your idle settings if you prefer a higher idle. My old 79 Monte had an A/C idle up solenoid that mounted to the carb and "bumped" the idle when the air was on. You could adapt one of these to set your base idle and when the key goes off it would lower the idle too low to diesel on. You probably need a 12 volt system though,
Scott
