Engine dieseling

[craig newton]

1996 Sterndrive 4.3lx 6cyl. 4 barrel carb with Thunderbolt ignition.
At idle, wot and everywhere in between it runs fine. The problem I am having with it only occurs when I turn it off. With the key in the off position, it keeps trying to run. Not in a smooth way, the engine kind of coughs, hiccups and jerks at times up to 15 seconds before it finally dies. Is that a gas or timing issue? What are some reasons for an engine to start dieseling. A couple times out I noticed a strong gasoline smell after filling the tank. I inspected the lines but couldn't find any leaks. I'd appreciate any advise I can get as to what I should be looking for. Never ran into this before with any motor.

 

[Earl Cordova]

What do the sparkplugs look like ?

 

[Lpgc]

Seems to me one or more of:
ignition isn't turning off properly
hot spots in cylinders due to carbon build up
plugs getting too hot (fouled / wrong heat range / over advanced timing)
low octane fuel

 

[Scott Danforth]

Things that cause run on

Rich fueling
Improper timing
Running hot

The big problem with run on isn't that it happens, it's that last gasp and death rattle when it finally dies. That is the motor running backwards and sucking in water

Check your plugs, if they are black, change them and perform a tune up

Check your timing, make sure it's to spec

Change the impeller

Shut off in gear if you need to use the boat prior to fixing the issue

Regarding the strong gasoline smell. How old is the boat. If the fuel lines are 20 years old or older, replace the rubber fill line, vent line, and the lines from the tank to the motor

 

[craig newton]

My thanks to all of you, I now have a direction. Gas lines, plugs, timing, better fuel.... I can handle that. hot spots in the cylinders, not sure what to do about that. make it last on the list and hope it's fixed before I get that far.