If this is the only symptom, I'm telling you that it's the timing.
If the engine is louder than normal at idle it might be that the idle is too high.
Since I'm just a girl, I copied the following so you would pay attention.
Sarah <== Daddy really did teach me all about cars.
Symptom: Engine continues to run when you try to turn it off
Description of problem: The car seems to run fine except that it doesn't want to turn right off. After you have driven awhile, you turn the key to stop the engine and it continues to run for a few seconds. At first you notice that this happens once in awhile, and now it happens quite often.
Probable Causes:
1. One or more of your fuel injectors are leaking.
2. The engine's idle is set too high.
3. The ignition timing is set wrong.
TROUBLESHOOT YOUR VEHICLE
EDIT: Read Tom & Ray's article. Don't trust those guys. Pouring water into the carb is a VERY bad idea. I know I'm just a girl, so go ahead and try what everybody else says. When you fix the timing PM me, OK?