"Robert M. Gary" wrote in news:cd4b44fc-45a5-4bda-
:

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


A carb pours fuel air mixture into the beast no matter what every time it
sucks on the intake. So, inside the hot cylinders you just ran like hell
for 12 miles to get back quickly, there are glowing coals of hot carbon
on the walls, on the head, especially on the spark plug which is the
hottest object in there, the center post....all above the flashpoint of a
gas/air carb's mixture output.

So, when you shut it down, these glowing coals of coke ignite the mixture
the carb is still pouring down its gullet, causing dieseling and run-on
after shutdown. The dealer is right. If you idle it, the coals cool off
basking in the intake's fuel-refridgerated air gulp down its gullet. No
coals, no unwanted ignitions....no dieseling....

A simpler way is to simply cut its fuel-air intake off with a blocking
plate over the intake. The fuel air mix can't get to the coals.
Sometimes you can get the choke to do this for you if it closes far
enough. Try choking it while it's dieseling. But, don't be in such a
hurry. The dealer's way is correct.....let it idle a few minutes, then
shut her down....or slow the hell down to no-wake speed before you are
now and RELAX....ENJOY THE VIEW!

Diesels are better, anyways....No fumes to go BOOM! in the bilge!