Why does my truck deisel?

[dominator1]

I have a 1986 C10 CD with a 305 that keeps deiseling every time I try to shut it off, old owner has bypassed some of the vacuum when they changed the carb to an Eddie 1406, and timing is supposedly in order according to a mechanic I brought it to (I don't have a timing light or anything like that). I'm not particularly familiar with older trucks and my dad is more familiar with the even older ones than mine. Mine has HEI. Just replaced plugs and wires and tightened down the intake manifold bolts that I found were halfway out on the passenger side while doing the wires. Was hoping that would fix it but still deiseling. Please let me know if you have any idea as to what might be causing it.

[VileZambonie]

Idle too high for starters. As far as not owning a timing light and relying on a mechanic for these trucks, you better find a good one.

[dominator1]

Idle too high for starters. As far as not owning a timing light and relying on a mechanic for these trucks, you better find a good one.

ill try turning down the idle, it does seem to idle a little fast when warm but i didn't want to turn it down at first cause it seemed like it wanted to choke out when cold

[Shifty]

If it's not the idle, by guess would be timing too far advanced.

[MIKE S]

Timing would have no effect since the ignition is off when it diesels. To high of an idle or unmetered air as in vacuum leak. If you have heavy deposits in the combustion chamber that can also be a factor as they will glow and cause ignition. Using very low octane fuel can be a problem. 

[Geezer77GMC]

Once upon a time our mowing tractor ran out of gasoline, the 5 gallon can was empty. Clever wife grabbed the chain saw fuel can and added enough to get the mower back from the field. It ran just fine, but wouldn't shut down for maybe 2-5 minutes. Obtained straight gasoline, filled the 4 gallon tractor tank, and still ran fine, just dieseled quite a while after the key was turned off.
This repeated for several weeks until the fuel tank was drained & flushed several times using straight gasoline. Only a small amount of oil, kerosene or diesel fuel in the intake mix can cause problems, check for oil getting into the fuel/intake system. Has the fuel system been flushed/rinsed out with anything other than gasoline?
Is there excessive oil consumption? Valve stem seals or poor piston ring sealing could be an issue.
Many beautiful after market valve covers don't have oil separators where the pcv valve installs like the factory engineers designed into their engines. GM didn't spend the money to put separators there for no reason.

[JohnnyPopper]

Couple of things:

As mentioned, too high idle. If it chokes out when cold, adjust the choke so it works properly. Then turn down idle when at operating temperature -OT.

If you have carbon build up, when at OT, you can slooowly introduce water into the carb, keeping it running by hand, it will blow out the carbon. (not for the faint of heart- if you haven't seen it done, DON'T DO IT)

Finally, while you haven't said what type of trans you have and assuming it's an automatic, as a last ditch, turn off the engine while in gear.

[dominator1]

Once upon a time our mowing tractor ran out of gasoline, the 5 gallon can was empty. Clever wife grabbed the chain saw fuel can and added enough to get the mower back from the field. It ran just fine, but wouldn't shut down for maybe 2-5 minutes. Obtained straight gasoline, filled the 4 gallon tractor tank, and still ran fine, just dieseled quite a while after the key was turned off.
This repeated for several weeks until the fuel tank was drained & flushed several times using straight gasoline. Only a small amount of oil, kerosene or diesel fuel in the intake mix can cause problems, check for oil getting into the fuel/intake system. Has the fuel system been flushed/rinsed out with anything other than gasoline?
Is there excessive oil consumption? Valve stem seals or poor piston ring sealing could be an issue.
Many beautiful after market valve covers don't have oil separators where the pcv valve installs like the factory engineers designed into their engines. GM didn't spend the money to put separators there for no reason.

fuel tank and filter were just replaced and only had gas in it, doesnt drop any oil level

[dominator1]

Couple of things:

As mentioned, too high idle. If it chokes out when cold, adjust the choke so it works properly. Then turn down idle when at operating temperature -OT.

If you have carbon build up, when at OT, you can slooowly introduce water into the carb, keeping it running by hand, it will blow out the carbon. (not for the faint of heart- if you haven't seen it done, DON'T DO IT)

Finally, while you haven't said what type of trans you have and assuming it's an automatic, as a last ditch, turn off the engine while in gear.

it is auto, and i have been having to shut it down in gear when at OT and it will still deisel in gear, turned down the idle earlier tonight so we will see how it goes