Dieseling

[Sorceress]

My new Bayliner 205 Capri, with the 4.3 V6 will not shut off. The dealer has been absolutely no help. They keep telling me they can't duplicate the problem. This is the carburated 190 HP. I understand MerCruiser also puts out 220 HP with MPI fuel injection on the same motor. The fuel injected motors will not run on due to a real fuel shutoff. Does anyone know what it would cost to convert over?

[blowngas]

most motors want to diesel when with ignition off is due to the carb butterflys open enough to provide air and fuel, and something else is hot enough to ignite the mixture----installing a cooler thermostat will help----you could mount a solelnoid from a carb that had one one it and wire it into your system so that when you turn on the switch, operate the throttle control, the solenoid extends to the idle position holding the butterflys open slightly to the idle speed and when the key is off, the solenoid arm retracts, closing the butterflys and stops any dieseling.-----you should be able to find the solenoid at any junk yard for a couple of bucks----a lot cheaper than trying to convert to efi

[SmokinLowriderSS]

I've seen a lot of threads of this very problem over the last few years. Sorry that doesn't help you but know you are not alone. In my opinion, many boat companies are asking too much out of that little V6, it gets hot running hard (and running pushing a boat is ALWAYS hard), then, if it is hot enough in the chambers, it will diesel since carbs provide fuel as long as the engine spins.
Have you tried a few minuites of idle after cruizing to someplace? That lets the engine cool down some. It's not "engine temp" so much as it is combustion chamber temperatures.
Can YOU manage to "cause" it with regularity? Perhaps a dealer's mech ought to go out for a test-run with you.
If you run a system with a thermostat, I would have the coolest one available installed (or do it myself) to attempt to lower the engine (and the cylinder head/combustion chamber) temps.
Perhaps, a slight richening of the idle mixtures could help, but too much could cause unstable idle problems and fouling of plugs as well.
I have no idea the cost of the conversion but I would suspect several thousand $$$ An ignition upgrade is likely to be part of it as well as all the magical computer-type parts and hi-pressure fuel pump to the injectors.

[Krumbsnatcher]

I have the same problem when running our 22 Tritoon WOT then turning the engine off. The only way to stop the dieseling is to idle around and let the cooler water bring the temp down so it does not pre ignite.
bad design by merc if you ask me.

[BUSBY]

Rio is right ... using a higher octane fuel will help. Now, I don't want to get into a big long debate with others here about octane and what it is or isn't ... but I do have a lot (and I do mean a lot) of experience with testing different octane levels in fuels and the result it has on engine characteristics.
A higher octane will have a higher resistance to pre-ignite. Cylinder temps, carbon deposits, sharp leading edges on vavle reliefs all can ignite lower octane fuels. If, as others have said here, you are having air/fuel pulling through the venturi's in your carb ... it will make it into your cylinders ... if your cylinders have an ignition source (hot spot) the fuel will ignite ...
Usually octane helps with preignition or "pinging" ... but will also help clean up any carbon build-up as well as reduce the diesel effect when shutting down.
Go down and get some Berryman's B12 Carb/Fuel Injection cleaner. They sell it at most parts stores in a liquid form ... run a can through with your full tank and see if it helps. We used to run a can through every 4th tank or so to make sure we didn't get any carbon build-up in the lake boat ... always ran like a champ.
Good luck!