please let an old guy ramble. your motor runs on after shut off when its hot because there is enough hot carbon in the cylinders to fire any fuel it can suck in. if the throttle plate shuts tight enough and there are no vaccuum leaks, such as open vaccuum ports, missing or broken vaccuum lines loose carb mount, manifold leaks egr system, or any other way for air to get in the cylinder, it will not be able to run. thats why it wont do it when its cold, because the carbon is not hot enought to fire.
if you dont find any vaccuum leaks, make certain that nothing is keeping the throttle open, ie; choke linkage, fast idle cam, or as i once found, a dried up dead mouse.
try not to resort to running colder as you will only cause more carbon build-up and your gas mileage will be worse than it is now.
thank you for your time, and remember my signature.