2004 Merc TKS intermittently missing, running rich, running on.

[JordyBoy604]

New to me boat and it screams lack of maintenance. The 2004 mercruiser 4.3 TKS engine on cold startup requires some encouragement from the throttle and once running idles rough. The distinct "hiss" from the TKS carburetor is present and once warm stops. The problem I'm having is that when running it feels like I've dropped a cylinder intermittently. It's runs smooth and strong then feels like someone has pulled off a plug wire and I start to run with a dead hole. I can smell fuel and in this condition it will run on after shutoff however this will come and go as we cruise without a real obvious root cause. I've replaced the plugs, wires, rotor, cap, ignition coil, ignition sensor with no change in the symptom. I'm leaning towards now the TKS carburetor possibly being culprit but before I drop 1k on a new one I was hoping someone else may have experienced these symptoms that found a solution. Thank in advance

 

[alldodge]

With it hissing and then stopping this would saay the module is shutting off the extra fuel as the motor warms up, which is good.

What did the old plugs look like?
Do a compression test

 

[santino44]

Maybe just a dirty fuel filter?

 

[Rick Stephens]

New to you boat:
1) Compression check for a baseline
2) Pull the drive and do the annual maintenance
3) Replace the impeller, inspect bellows gimbal and u-joints

 

[dubs283]

before I drop 1k on a new one

Those carburetors can be rebuilt for far less than $1000

Good idea to check fuel pump pressure

 

[JordyBoy604]

Those carburetors can be rebuilt for far less than $1000

Good idea to check fuel pump pressure

I can get a reman for about 850 and I could rebuild myself for about 7 with parts from Merc. All of which I'm prepared to do ......if it's the problem

 

[Rick Stephens]

Rebuild it. Have patience. Be thorough. Very carefully set the float height. Run every passageway and you'll be golden.

 

[nola mike]

See if you actually are dropping a cylinder. Put a timing light on each plug wire when it's acting up, see if you're getting consistent spark. Don't think that would cause the dieseling though.

 

[Rick Stephens]

Dieseling is running rich and or timing off. Richness puts carbon deposits on the piston and head. Those absorb heat and stay red hot. Hot enough to ignite a compressed fuel air mixture. So just like a model airplane engine with a glow plug, your motor runs on after shutting off ignition. Scary part with dieseling is the motor can kick and run backwards for a bit. Only takes a tiny bit of that to suck up water out of the exhaust.

 

[Scott Danforth]

Dieseling is running rich and or timing off. Richness puts carbon deposits on the piston and head. Those absorb heat and stay red hot. Hot enough to ignite a compressed fuel air mixture. So just like a model airplane engine with a glow plug, your motor runs on after shutting off ignition. Scary part with dieseling is the motor can kick and run backwards for a bit. Only takes a tiny bit of that to suck up water out of the exhaust.

the last gasp of an engine that "diesels" or "runs-on" is the motor running backwards. this usually results in a hydro-lock situation

 

[MarcelSydney]

Dieseling is running rich and or timing off. Richness puts carbon deposits on the piston and head. Those absorb heat and stay red hot. Hot enough to ignite a compressed fuel air mixture.

I experience the same thing, usually after pulling someone around on skis and then stopping to pick them up. Having corrected the running rich, can the carbon deposits be cleaned off without pulling the engine apart. Do those engine carbon cleanering products that's you spray into the cylinders via the spark plug holes work?

 

[Scott Danforth]

I experience the same thing, usually after pulling someone around on skis and then stopping to pick them up. Having corrected the running rich, can the carbon deposits be cleaned off without pulling the engine apart. Do those engine carbon cleanering products that's you spray into the cylinders via the spark plug holes work?

to clean the engine of carbon deposits, spray about 2 ounces of water in the intake (engine running) and you steam clean the combustion chambers. GM top engine cleaner also works

in the mean time, if you need to key off quickly to pick up a skier. key off with the motor in gear. that will snuff the engine and prevent run-on

 

[Lou C]

The Mercruiser 4.3s did seem to have a run on (dieseling) issue because I recall seeing a TSB to correct or at least reduce this problem, I don't recall if it was specific to the 2bbl or 4bbl carb though. Interestingly I have never had this problem with my OMC 4.3 4bbl Quadrajet carb version. When warm it starts in a half a crank revolution and never has dieseled, ever. If you have a persistent carbon build up due to running too rich, I agree try to get rid of it, if you can't use premium gas it will be less prone to run-on.
I would pull the spark plugs and take a look at what you have.
If I recall correctly that Merc TKS carb is nothing more than a slightly modified Rochester 2bbl like we had on our '72 Chevy 350. With a cold start enrichment system so owners who never drove a carbed vehicle with an auto choke can start it. For shame, they made a simple design worse, and it would have been easier to just put out a video on you tube of how to start your carbed Mercruiser!
Those Rochester 2bbl carbs are very simple to rebuild, nothing like a Quadrajet. The very best rebuild kit is probably $25-$50 at most and a few hrs of your time. DO NOT spend a grand on a new one! You may have nothing more than a stuck float, bad needle valve, or gummed up air bleeds. Those are the usual causes for running rich.