Dieseling can be fuel, ignition, or carbon buildup. Start with ignition system. If you don't know how old the components are, replace them. Pull and inspect spark plugs. If they are carboned up, clean or replace them. Inspect the wires for damage or cracks, if they are damaged, replace them. Remove distributor cap and rotor. Look for carbon arc trails and burned contacts. Clean or replace if necessary. Check and reset ignition timing.
Now to the fuel system. Make sure fuel lines are in good shape. Replace fuel filter if not done recently. Remove fuel filter and dump contents onto paper towel in a bowl. Look for debris or water. Carburetor is likely somewhat dirty. Use a fuel system treatment (I like seafoam). Put cleaner in gas tank and run it. If it doesn't help, pull carburetor and clean/rebuild as necessary. If you use seafoam, it will help clean carbon off the intake valves which can help too
