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Occasionally my 4.3 L will "diesel" when I shut it off.... i.e. run-on. Sometimes it will shake pretty severely before shutting down. Sometimes it takes me 2-3 times off turning the key back on, then off to get it to shut down smoothly. It generally happens when the engine is hot. Anyone else experienced this? Is there a solution? Thanks!
Up the octane go to premium fuel
is it carb or injected if its carb does it have an electric throtle kicker if so then it may be stuck or try resetting the idle fuel mixture a little leaner if its rich at idle it will tend to load up as you turn off the igniton so adjust the carb very carfully and be sure to ballance both sides same number of turns on the mixture screws from all the way in and set it on the lean side at idle
Thanks Fixit. It is the carbureted version of the 4.3. I will try those adjustments when I get the boat back in the water. Hopefully that will help. It's been doing it for a couple of years now, and I've not been able to fix it.
The most common reasons for run on, are,
Idle too high
Timing advanced too much.
How does timing affect run-on? If I kill the ignition, doesn't that take timing out of the equation?
Its an overload of fuel in the cylinders. When the engine is hot, the temperature in the cylinders is still hot enough to ignite the fuel when you shut it down. Mine will do the same thing if I shut it off right after a hard run. Let it cool down at idle for a couple of minutes at idle before shutting it down. It cools the cylinder temp and burn off that excess fuel.
timing affects runon buy cyld temp advanced timing = high temp witch will cause carbon in the cylnder to glow red and preignition will cause the run on you can also try a chemical called X66 it is a delco product that will safely dissollve carbon when injested into intake and allowed to sit for a few hours warning be carfull will smoke heavy and if put in to fast can hydrolock the engine
Virtually all of the production marine carb motors do this. Ditto what others have said. Let the motor idle momentarily before shutting it down. Higher octane helps as well.
My 4.3 carb'd does the same thing, even with high octane. Let us know what works for you...be our guinea pig!