my 1978 f-150, 400 engine diesels when i shut it off. my timing is not off. what is wrong

 

try running higher octane fuel

 

Your carb idle is set too high. You can find out about carb settings in the Fuel systems forum.

 

Hehe..Torque beat me to it. Idle is a bit too high, you will have to tweak it a bit to find the "sweet" spot, where it will idle fine but not diesel when shut down.

 

A good tune up will make it easier to find that sweet spot. An old engine that is down on compression and dying will be hard to get to run correctly.

 

I would say it might be a streched timing chain.

 

Carbon deposits on exhaust valve or combustion chamber will cause it when you combine too much idle air and no load on the engine ie. with an auto. Try shutting it off in gear or letting the clutch out in gear with the brakes applied. Sometimes works for auto's too.