Black stuff on startup '94 Merc 7.4L Bravo Three, 400 hours

[yimstiz]

When I first start it, black stuff shoots out. Mechanic says it's carbon buildup. It's not burning oil (no smoke, never need to add oil). He asked if I need to pump the throttle a lot to start when cold and the answer is yes. It cranks for a bit, then starts and runs like a champ. No sputtering of any kind. But the black stuff looks like an oil slick that even a passing Coast Guard noticed and asked about at the dock. This is my 3rd season with this boat. Use premium gas only and use Sta-Bil fuel stabilizer as I get gas from the auto gas station. See video link to driveway startup yesterday (first start up after heated winter storage).

Thanks for any help diagnosing this issue.

 

[GA_Boater]

Do you have to pump the throttle for warm starts?

The motor is overly rich on start up. Assuming you have a carb, check the choke and make sure it works. Advancing the throttle one time should set the choke closed.

Pumping the throttle squirts extra gas from the accelerator pump and makes a rich mixture for starting. A choke makes a rich starting mixture by blocking air so you don't get the carbon blow out.

If you have fuel injection, never mind.

 

[yimstiz]

I don't pump on warm starts, only cold. It's a carburetor. The black stuff in the picture has dried up and it feels like ash. So it must be overly rich, as you say, right? If so, that's a huge relief. I'll try advancing the throttle one time as you suggest to see what that does on a cold startup. I'll post the results...

 

[Bondo]

So it must be overly rich, as you say, right?

Ayuh,..... Welcome Aboard,...... Yes, that's fuel carbon, you have a carb problem,.....

 

[QBhoy]

I’d be tempted to run a strong concentration of seafoam through her. About 3-5l of fuel in an outboard tank and a full bottle of seafoam. Can’t tell you how impressed with this stuff I am. It’s the real deal. Go on YouTube and look at guys hot soaking methods ekth this.

 

[porscheguy]

That’s carbon/soot from running way rich. Carb rebuild time.

 

[Lou C]

In the vid was the engine just started, or was it running long enough to warm up? If just started, I'd guess the choke vaccum break is not pulling the choke open enough (usually a fraction of an inch) after a cold start. If it was warmed up the choke may not be opening up enough, after the temp gauge hits 160 it should be nearly all the way open. If this is a Merc/Weber carb with electric choke make sure its getting a full 12V.

 

[yimstiz]

In the vid was the engine just started, or was it running long enough to warm up? If just started, I'd guess the choke vaccum break is not pulling the choke open enough (usually a fraction of an inch) after a cold start. If it was warmed up the choke may not be opening up enough, after the temp gauge hits 160 it should be nearly all the way open. If this is a Merc/Weber carb with electric choke make sure its getting a full 12V.

It was about 1-2 minutes after initial start so I guess you could say it was still cold (temp outside was about 65F-70F I'd guess). I don't know what kind of carburetor it is. I haven't tried starting it since I first posted this question so that's why I haven't updated this post yet.

 

[Lou C]

See if you can take a vid of it when it starts...the choke unloader or vaccum break as its also called, should pull the choke open a bit right after it starts. Then the electric heater element should have it all the way open in about 5 min run time.