On Jun 11, 10:12*am, wrote:
On Wed, 11 Jun 2008 06:57:34 -0700 (PDT), wrote:
On Jun 11, 5:45*am, "Jim" wrote:
"Robert M. Gary" wrote in ...

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

Set your timing and idle speed and mixture. Clean spark plugs. De-carbon
engine. Let idle a minute or two before shutting down.

There's been many, many tests on the snake oils they sell to decarbon
an engine. In summary, the only way to do it is tear it down.

The "snake oils" can be effective at preventing carbon buildup, but
you are correct that they don't do much to remove existing deposits.
The "steam cleaning" method of dripping water down the carb throat
while the engine is running will sometimes help. If it's really badly
carboned up, then yes, tear down is the only way to really fix it. I'd
try the steam first, as its very easy to try, and might solve the
problem. On a little 2-stroke air cooled engine, popping the head off
and removing deposits is quick and no big deal. On engines with
overhead valves, it becomes a bit more complicated.

My guess is that the motor in question may have been run for long
periods at very low speed. That's conducive to carbon buildup.- Hide quoted text -

- Show quoted text -

When my Jeep got close to 300k miles, it still had excellent
compression, but carbon buildup caused it to ping, too much
compression! I ended up pulling the head and cleaning the head and
piston tops, problem solved. While I was in there, I took the head to
the shop and had a valve job done and changed out the timing chain.