Possible causes for dieseling (run-on) are:
- carbon deposits in the cylinder
- low octane gasoline
- bad timing
- too rich of an idle mixture at the carb
- idle speed set too high
- high engine temperature or high temperature engine compartment air
- wrong heat range spark plugs or spark plugs with deposits on them
Carbed engines do not shut off the fuel supply when the key is turned off, because air is still flowing through the carb into the manifold. When air flow is present through a carb, naturally, fuel is drawn in with it. If there is enough heat (or a hot spot) in the combustion chamber, this fuel/air mixture will burn, which causes the engine to run-on even though the ignition system is not sparking.
Low Octane Gasoline[]
Try putting in an octane booster into your gasoline, or buying premium gasoline. This, however, just tends to mask the problem rather than really fix it; but, in the absence of anything else, this helps.
Bad Timing[]
Reset the timing by loosening the distributor bolt, using a timing gun on your harmonic balancer, and turn the distributor until the v-shaped notch on the harmonic balancer lines up with the notch on the flywheel, making sure to dial in the appropriate advance on your timing gun (the spec is in your engine manual, and is also usually written as the number of degrees before top dead center on your splash guard).
Idle Speed[]
Set the idle speed as low as possible to avoid run-on.
Idle Mixture[]
Make the idle mixture richer by turning idle mixture screw on carb out 1/8 turn.
Idling[]
Idle the engine for 1-2 minutes to let heat dissipate before shutting the engine down.
Throttle Adjustment[]
Adjust throttle cable barrels so hat the carb's throttle level returns against the idle speed stop screw every time the throttle is returned to idle. The throttle might be sticking open just a little.
Spark Plugs[]
Replace your spark plugs with a cooler version. Replace AC-MR43T with AC-MR41T (or Champion RV12YC or Champion RV91MC), or replace AC-MR43LTS with NGK-TR6 (or Champion RS9YC)
[this section is disputed...even though there is a Mercruiser Bulletin that states this as a potential solution, it should not be attempted until you are sure that the idle speed and timing is set correctly]
Fixing Carbon Deposits[]
Rumor has it that SeaFoam is the best thing to use when trying to clear carbon deposits, even though it does not succeed 100% of the time. Warm your engine, and pour small amounts of SeaFoam into the carb throat until your engine starts to stumble a bit, and then back off. This will generate lots of white smoke if it is successful in dissolving off the carbon deposits.
Hot Engine Compartment[]
Use blowers to clear heat out of the engine compartment prior to turning off the key.