86 f150, 300, brand new 1 bbl carb. Just got my emissions stuff put back on. Problem: truck diesels when I turn it off 70 % of the time. I figure I need to adjust the fuel mixture, but am not sure if I need to go leaner or richer, and how much of either one I should do. Please help if you can. Thanks

 

Have you verified the timing is correct?

 

I thought that too, but my mechanic told me it didn't have anything to do with the timing. So, I am under the assumption that the timing is ok.

 

Dieseling is usually caused by idling too fast. Is the idle speed correct? Is it coming down off the fast idle cam after it's warmed up? Does it have a anti-diesel solenoid to close the throttle fully when ignition is shut off? Is it functional?
Idle mixture should not make a whole lot of difference in run-on, start with it out about 1 1/4 turns and then adjust for smoothest/fastest idle when engine is thoroughly warmed up, then set idle speed to spec.

 

Dieseling has a lot to do with engine temp because of the way the computer adjusts the engine speed for temperature. If your thermostat is stuck open the engine does not get hot enough and the computer will keep the RPM's high. When you shut it off it diesels. There is a quick check you can do. There are two temperature sensors one for the gage in the cab and one for the computer. the one for the computer is at the outlet of the water pump. Start the truck and warm it up. Turn it off. Disconnect the computer sensor plug. Start it back up the engine RPM should be about 1600 RPM. while it is running replug in the sensor the computer should bring the engine back down to normal RPM.

84 F150 TFI