I have a Yanmar 3JH-2E with 374 Hours on the engine. The engine ran fine up until last weekend when I towed (under sail and power) a stranded fishing boat into the Marina. I fueled up my half empty tank (21 gals. added) from a fuel dock. I immediately saw carbon swirling in the prop wash. The engine would not rev up to 2400 rpm's with very insufficient power output. I supected that I had bad fuel but have not been able to confirm this since all of the filters were relatively clean and I changed all of them and have the same problem. I bleed the system of air, checked the prop for fouling and found nothing. I changed all the filters, checked the Air intake all ok. The engine starts well but will not rev up and has no power. The exhaust is full of carbon but not oily. I have not disassembled the exhaust elbow because my friends all say "its the fuel stupid." I removed the fuel intake from the tank and made sure it was free of obstructions alone with the tank vent. Any suggestions besides call a mechanic?