Yanmar 30GM30F - Black Smoke, Power Loss

[FlatHatter]

Awesome forum!

I'm the second owner of a 27HP Yanmar 30GM30F with about 670hrs. I was transiting the boat and the engine started losing power. The engine was cruising around 2800RPM and then wasn't able to go above 2000RPM without producing black smoke, plus the throttle was still at the previous 2800RPM setting. I scooped up some exhaust water and there was what looked like little bits of black carbon floating in it. Trying to get the engine to power up to 2800RPMs also left a black patch in the water, not oily though. Prior to the smoking, the engine ran for about 12 hours at 2800 RPM, then had it off for about 15 hours and then ran it again for 8 hours before the power loss was noticed.

From the books and forums and talking with the local Yanmar guy, he said the likely culprit is the mixing elbow. Does this sound about right? Anything else I might consider first?

Also, I am extremely new to diesel engine repair/maintenance but want to do everything myself. So the elbow in the photos below is what should be replaced correct?

thanks for the help!

[Microship]

One other thing to check, as I just went through this recently. Any high load on the engine can do this... from a line around the prop shaft to (in my case) bad fouling of bottom and prop. When the divers cleaned it, miraculously the black smoke over 1800 RPM stopped and I could wind it up to cruising speed as usual.

The other thing to check is any blockage of air intake.

Cheers,
Steve

[Chief Engineer]

Bottom First

[FlatHatter]

I just dove down and the prop and prop shaft look okay, and the bottom is still pretty clean of any growths. Also, pulled off the air intake and it was clean. Any other suggestions, things to consider?

Thank you!

[Chief Engineer]

Do you know what your theoretical hull speed is?

If you try to push you boat past that, you will get black smoke.