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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.
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.
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.