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Hey guys, started my engine today and it shot out a clump of black soot which floated on top of the water for a while. It seems like the exhaust water has a stream of this grit in it as well. It's relatively recent, doesn't seem to be running hot, etc -- what could be causing this?
It looks to me like it is unburned fuel mixed with some carbon deposits.
Does it do this only immediately after you start the engine or does it continue outputting this even after you are running it at speed? If the former, when your engine fires up does it get up to idling speed quickly or does it kind of chug for 3 or 4 seconds, slowly getting up to speed while it spits out this stuff?
I have a Yanmar 1GM, which I realize is a totally different engine. But if the governor works the same, this may be what is happening. When the engine is off the fuel rack is wide open, even with the throttle all the way back. While this may sound counter intuitive, this is because when the engine is off there is no opposing force from the governor spring, which relies on engine rotation to oppose the throttle spring in closing the rack. On my little 1GM, I find that if I don't give it enough throttle on start up, the fuel rack/injector pump will load up the cylinder with fuel in those first few seconds, which the engine cannot burn fast enough and simply spits out the exhaust. Then, once the engine gets up to speed (i.e., in maybe two or three seconds) these emissions stop. On the other hand, if I give it just a bit more throttle on start up so that it gets up to rotational speed more quickly, these emissions are greatly diminished or disappear entirely.
In any case, the only times I see this are: (1) When I start the engine without giving it quite enough throttle; and (2) When running the engine and I shove the throttle forward quickly from a low setting to a much higher setting. In both cases, the engine cannot burn the fuel as fast as the fuel rack/injection pump is sending it to the injector/cylinder, and it comes out as unburned fuel in the exhaust.