My 81 F100 has a 302 2bbl in, it runs good doesn't leak or anything, but for some reason it starting this after running when I shut it off. I've done a complete tune up on it and I'm not sure if it is a carb problem or a fuel pump problem. The fuel pump on this engine is on the left side of the engine.Thank you for any advise.

 

This will only occur on a warm motor that is idleing too fast when shut off. Try tapping the gas pedal to get the idle down before you turn the key off.
Spray some carb cleaner on the carb linkage and work it all back and forth until clean and operating freely.
Steve G.

 

If I understand you correctly your truck is dieseling*. My 81 F150 did the same thing and the fix for me was timing adjustment. It would diesel really bad if I turned the truck off with the A/C or heater running which as 8 Lug metioned would be due to high RPM.





*Dieseling or engine run-on is a condition which can occur in sparkplug internal combustion engines whereby the engine keeps running for a short period after being turned off, due to fuel igniting without spark. Dieseling is so-named because this is essentially how diesel engines operate (without spark).
An automobile engine that is dieseling will typically splutter then gradually stop rather than continue running as if the engine was not switched off at all

Potential causes
This condition can occur for a multitude of reasons:

• Built-up carbon in the ignition chamber can glow red after the engine is off, providing a mechanism for sparking unburnt fuel. Such a thing can happen when the engine runs very rich, depositing unspent fuel and particles on the pistons and valves. Similarly, non-smooth metal regions within the piston chamber can cause this same problem, for they can glow red. It has also been suggested that an improperly rated sparkplug can retain heat and cause the same problem.

• A carburetor that does not close entirely can contribute to running once the engine is off, for the extra fuel and oxygen mixture can combust easily in the warm piston chamber. Similarly, hot vaporized oil gases from the engine crankcase can provide ample fuel for dieseling.

• Incorrect timing.

• An engine that runs too hot or too lean may produce an environment conducive to allowing unspent fuel to combust.

• An idle speed that is too fast can leave the engine with too much angular momentum upon shutdown, raising the chances that the engine can turnover and combust more fuel and lock itself into a cycle of continuous running

Potential fixes

Items similar to carburetor cleaners and carbon cleaners have been suggested as partial remedies for attempting to clean the piston chambers and valves of engines that run too rich.

For those engines that have sharp metallic edges, it has been noted that poorly milled heads and blocks can contribute to this problem, so having the rough spots smoothed may help.

For those engines that run too hot or too lean, verify that all mechanisms in place to cool the engine properly function as they should. Replace the thermostat if necessary. Clean the radiator. Verify that all auxiliary fans engage at their proper temperatures, and ensure that the thermostatic sensors on belt driven fans engage as necessary.

In the case that there is too much angular momentum, lower the idle speed if possible.

Wiki

 

Do you have to turn an electric solenoid to adjust idle speed? If so, check to make sure the solenoid is functioning. The plunger should extend with the key on and drop with the key off. It's there to close the throttle plates when you shut off the engine. (Even a diesel won't run without air.)