Ink black exhaust water - soot?
I bought a 1979 pearson 32 about three months ago with a Yanmar 2gm20f. Until last week I ran the engine at around 2200 rpm cruising. Then I realized it wants to be run higher and recommended speed is 80 percent of max. So, the last time out I cranked her and all was well. Today went out and after five minutes noticed black stream in the exhaust water (not smoke). It may have been there from go but I didn't notice. When idled down no black. When run under high rpm with no load (neutral) no black. I took a sample of the black water and it is not oil, it's sooty. Engine doesn't seem unusually hot and gas who be fine. Don't know about filters, haven't checked.
Is it possible that the two hours max i ran engine at 2000 cruising over a couple of months built up so much soot in the elbow or muffler that i just need to blow it out by running at high (correct) speed? I didn't persist since black stuff coming out the back the boat freaks one out.
Bottom is clean and i don't think there's a prop issue (it's folding) and there is no rough running, trouble starting, speed fluctuations etc..