wrote in message
...
On Wed, 11 Jun 2008 07:30:43 -0400, "jamesgangnc"
wrote:

"Robert M. Gary" wrote in message
...
I have a Bayliner with a 4.9L 190 hp carb'd Merc engine. When I shut
down it diesels for awhile. The dealer says that the longer I let it
idle first the less it will do this but the issue seems to be getting
worse. The engine isn't running hot and the water impeller was
replaced about 8 months ago.
I've done a lot of car work but its been a long time since I've worked
on a carb'd & distributor car. I seem to remember that dieseling is
caused by high cylinder temps. I thought you could affect it with
timing but I might be thinking pinging. What should I be looking at
for my dieseling issue?

-Robert

I believe you posted about his problem before and I suggested you might
have
deeper problems.

Have you tried premium fuel? Check the timing. Is it raw water cooled or
a
closed system? Are you sure it is not overheating? A lean fuel condition
can also cause it.

Pinging can also be caused by high cylinder temps. Boats are a lot
noisier
than cars so you don't hear the pinging. Both pinging and dieseling are
indications that fuel is igniting without a spark. You need to sort this
one out because I've seen it cause melted pistons. One way to check that
is
to pull the plugs and look for aluminum deposits on the ceramic around the
tip.


Can also be caused by excessive carbon buildup, which would get worse
over time. Could also be the wrong spark plugs were installed.


Yes, carbon deposits can also cause it. One old trick to remove carbon is
to slowly pour water in the carb while the engine is running at a couple
thousand rpm. The water scrubs the top of the piston and combustion chamber
as it gets turned into steam. You have to do it slowly though. But I would
check the timing and try premium if he is using regular gas before anything
else.