Or your primary throttle blade is to far open at idle lower your idle some, your cam and cylinder pressure it makes may not like the gas your using also.If it is actually "dieseling", there are several things you can do.
Dieseling or run on is caused by hot spots in the piston crown and or combustion chamber. Di3eseling is not caused by a continuing spark but the mixture being ignited by hot spots in the combustion chamber area.
If you are running a projected tip spark plug you can change to a shorter tip plug and maybe a heat range colder.
You have to be getting some fuel on shut off being drawed into to the cylinders check your carb it must be rich idle or main jet pull over on shut down.