What you have going on sounds like what's called "engine run-on". What is happening is a couple of the cylinders are continuing to light after you've turned the key off. This can be caused by a number of things:
1) Crappy gas, or gas with too low of an octane rating for the amount of compression you have so it "diesels" after you've shut it off.
2) Carbon build-up in the combustion chambers and/or on the tops of the pistons. When these carbon deposits get hot, they glow cherry red and can continue to light a cylinder or two after the key is turned off.
3) A poorly running and/or adjusted engine in which the throttle plates on the carb are opened too far in an attempt to keep the engine running at an idle. When you turn the key off, the idle position is too far open so fuel continues to flow into the engine and hot spots in the cylinders will continue to light a few of them.
Before EFI really became popular, cars of the 80's and 90's had idle stop solenoids which completely closed the throttle plates to eliminate this same problem.
Timing has nothing to do with it because they key is off. No key = no spark. No spark = no ignition or timing.
Usually when an engine does this it will rev up at the very last minute (sometimes even backwards), which will push out a foul smelling cloud of unburned gas. When you go to fire it back up, it "can" be a little more difficult because of the reverse running of the engine.
My first guess would be to take a peek at the carb to make sure it is adjusted correctly. if it is, then try putting a little better fuel in it and try a pint of Berryman's B-12 Chemtool in the tank to help clean-out any carbon build up.
A quick way to remecy this if you can't find the problem is to simply shut the key off while it is still in gear. The load of the tranny will help stop the engine from free spinning as easily and it should shut right off.
