2008 4.3 ltr. TKS diesels when turning off

[Larry Baloun]

Engine runs smooth and performs well. Idle set at approximately 650 rpm. Serial # 1A312851.
Diesels badly nearly every shut down with regular fuel. Still diesels with premium fuel, but not as bad.
I noticed there is a vacuum hose fitting coming off the distributor ... but no hose attached. I assume this fitting is not there for decoration, could this be the problem?. If there should be a hose attached, where does it go to?
If this is not the problem, what else should I look for?

 

[Scott Danforth]

post a pic

there should not be any vacuum hose on the distributor, just some wires

dieseling is from improper timing, or running lean

is this a new to you boat?

 

[Rick Stephens]

The fitting is a vent with a 90 degree spark capture.

You are running rich. Carbon soot stuck to the piston and head, glowing red hot are what keeps the motor dieseling. Need to fix it. Dieseling can run backwards and end up sucking water into the motor.

 

[prc03sx195]

I have the same issue on my 2003 4.3L. just reset timing and still does it. Mine has a carburetor where yours might be fuel injection?

 

[Lou C]

I think Merc had a service bulletin on this problem, see if you can find it. I have the 4bbl Quadrajet on my 4.3 (pre-vortec) and never had dieseling even with the old cyl heads that I replaced 4 years ago. I wonder if gas is dribbling out of the main boosters or idle transfer slots as you shut it down, might want to take a look with the flame arrestor off when someone else turns it off for you. If that is the case you may need a carb rebuild and set up. Unlike an engine with fuel injection, when you cut the ignition on a carbed engine, the fuel system due to the float system in the carb can still deliver fuel at least momentarily. So you want to make sure that the needle valve is not leaking and if the carb uses idle transfer slots they are not over-exposed.
Is this a 2bbl or 4bbl?

 

[Larry Baloun]

post a pic

there should not be any vacuum hose on the distributor, just some wires

dieseling is from improper timing, or running lean

is this a new to you boat?

It is not new to me. It has always dieseled some but is getting worse

 

[Larry Baloun]

The fitting is a vent with a 90 degree spark capture.

You are running rich. Carbon soot stuck to the piston and head, glowing red hot are what keeps the motor dieseling. Need to fix it. Dieseling can run backwards and end up sucking water into the motor.

OK sounds pretty serious ...how should I fix.

 

[alldodge]

First check timing and make sure its at 10* BTDC in base timing mode

Being a TKS, there is extra hissing going on when cold. Does the sound stop within 10 minutes are so?

 

[Larry Baloun]

I think Merc had a service bulletin on this problem, see if you can find it. I have the 4bbl Quadrajet on my 4.3 (pre-vortec) and never had dieseling even with the old cyl heads that I replaced 4 years ago. I wonder if gas is dribbling out of the main boosters or idle transfer slots as you shut it down, might want to take a look with the flame arrestor off when someone else turns it off for you. If that is the case you may need a carb rebuild and set up. Unlike an engine with fuel injection, when you cut the ignition on a carbed engine, the fuel system due to the float system in the carb can still deliver fuel at least momentarily. So you want to make sure that the needle valve is not leaking and if the carb uses idle transfer slots they are not over-exposed.
Is this a 2bbl or 4bbl?

It is a 2 barrel. I have never searched for a service bulletin before where / how do you do that?

 

[achris]

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwivsNHy6bTyAhUX7XMBHTooBRIQFnoECAIQAQ&url=https%3A%2F%2Ff01.justanswer.com%2Foldnewenglandmarine%2F27437009-fea5-4619-b04c-92144e25bba4_Engine_Dieseling_or_Running_on.pdf&usg=AOvVaw3C3cHbO4nSoLFBmlKkLizI

 

[Bt Doctur]

10 oz of plain water poured slowly into the carb will vaporize carbon deposits in the engine. Other than that check the timing and drop the idle some more and enrichen the idle mix. May have to get the tool or remove the anti-tamper device