Merc 7.4's - Black "soot" discharge from exhaust when starting. Thoughts???

[Stee6043]

Greetings, CSR. I hope everyone had a great weekend on their boats. We had some stellar whether here in Michigan.

This past weekend (only my second on my new-to-me 340) I noticed a black discharge from both exhausts when starting my boat. It occurred on two or three occasions over the weekend. It’s definitely not oil. It’s a black soot almost, hard to describe. It only comes out upon startup, not during normal running. I tend to think it’s running rich and the black gunk is a fuel byproduct but my motors are MPI so why would they be running rich? And why both engines? Is this a sign of something going wrong? Or perhaps it’s something I can easily address?

She runs and starts fine but I don’t really love the small shot of black gunk when starting. I’m wondering if anyone here has an tips or advice for me. Thanks in advance for your time!

 

[Ten Years Gone]

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[MarkCrusei]

Both engines at the same time really makes it difficult but I would start with the fuel system.
Carburetor: Choke sticking or fuel mixture way off. Maybe a stuck or partially stuck float.
Low compression on 1 or more cylinders.

 

[sbw1]

Both engines at the same time really makes it difficult but I would start with the fuel system.
Carburetor: Choke sticking or fuel mixture way off. Maybe a stuck or partially stuck float.
Low compression on 1 or more cylinders.

Read the post......MPI engines.

 

[Stee6043]

Hmmm....perhaps I will talk to the marina service dept and see what they charge to scan the computers. The fact that both engines are doing the same thing is interesting.

 

[PlayDate]

Fuel injection has a short phase at startup called "enrichment" which in most cases increases fuel flow at the injectors to upwards of 50% of normal. As the engine warms up, the enrichment decays to normal. In your case since both engines are acting the same.....that would generally rule out a bad sensor and simply point to an overly rich startup fuel table in the ECMs. It could be a software update that wasn't applied or hopefully one that is available to cure it.

John