Try making a touch richer.

I recently had a similar problem with my truck when I kept trying to get it as lean as possible. After a while I ran into a dieseling problem. Made it a touch richer and it went away.

Your timing might also be a little advanced too.

Also look at your plugs really close. If they are running too hot they will cause dieseling. Older engines with alot of carbon build up will diesel too.

My wifes chevette did it really bad. We usually just floored it right after shutting it off. This would almost always stop the dieseling problem in its tracks.