1GM-10 Black Sooty Exhaust

[Wunu]

We were beating into 17 knots of winds, waves and current yesterday. We were making 2.7 knots at about 3/4 throttle. We went up to full power to see what she would do (about 3.5 knots) and after about 30 minutes started to get a little smoke and black sooty exhaust water. We also lost some power without losing rpms. We trapped some of the exhaust in a bottle to confirm that it was not oily and once we throttled back down the exhaust coming out was clean. We did a few tests over the next hour or so and when we revved up to full we would get the black soot. Our power was all over the place as well after that regardless of how much throttle we were giving her. Sometimes 3 knots and sometimes falling to 1 knot or less without any change in wind or waves etc.
After returning to the slip (we motored about 8 or 9 hours total) we talked to a few folks and checked online. We think it is likely we had fouled our prop, which also has a small amount of growth on it. We had not experienced that before and did not know how it felt to happen. We reversed getting into the slip and may have released some seaweed. When I dove in with a mask there was a handful of weed loosely on the shaft at the prop. I also noticed a bigger ball of the same type of weed floating in our slip which may have come off when we reversed in.
So ultimately, we plan to change/clean the air intake, polish our fuel and proceed ahead.
Does anyone have experience with a problem like this? Do you think our engine should be able to pull us along against wind and waves at 3/4 throttle? (Until we started with diminished propulsion we were making 2.5 knots) Do you think fouled prop could be a reason for our engine to behave like this? Thanks in advance for your helpful advice.

[Astrid]

Could well be the case--rather like a heavily loaded tractor trailer rig having to pour on the fuel to go up a long incline. Now that you have unfouled your prop, take her out again and see if it is still doing it. If it is, your problem is somewhere else; if not, all well and fine.

[lorenzo b]

Start by checking you air intake filter and making sure the engine gets enough air. My gen set started belching black smoke and just removing the air filter cured it. If it's plugged it will be hard to pull it off if the engine is running due to the suction. You need a filter so get a new one.

[Wunu]

Thanks for the advice. It seems the engine has eaten the air filter!! Only the armature remained so we replaced it. We took her out Friday and she performed admirably, so we really think we had fouled the prop with weeds and she was spinning wildly with little resistance. We have learned about 500 things every time we have taken her out.

[ATC]

If you have an Yanmar, I would think the mixing elbow on the exhaust,
otherwise I would check for fuel contamination.