Re: octane fuel
octane should not have any effect on exhaust, from 87 to 93. The detergents in the fuel and the quality of fuel determines that.<br /><br />With octane, ZmOz hit the nail on the head. with automotive motors having knock sensors, the computer will know the engine is knocking under throttle, as in the case with low octane fuel, and retard ignition timing to prevent the detonation. The effect is a loss of power. By running the level of octane needed, the engine won't knock and the computer won't retard ignition timing at all under throttle, and you get full power. That is the case with the Chevy LS1 motor in the F-body and Vette. They recommend 93 octane, but state 87 is acceptable.<br /><br />The statement about the computer not being able to adjust timing correctly does not apply to your situation- at idle or low throttle. The motor isn't going to knock at idle speed, so there isn't going to be any timing adjustment being done by the computer. That usually only happens when under load and your running 30+ degrees of advance. Also, I'm not aware of any marine engines having knock sensors, so I can't figure how the computer would ever know the motor is detonating. A knock sensor on a boat motor would give false readings whenever you started wave hopping.<br /><br />My guess is the motor is running pig rich and that's what's causing the black residue buildup at slow speeds. One solution might be to go one heat range hotter on the spark plugs, but whatch out you don't get knocking at high rpm's. Also check that you're running the correct spark plugs, and that none are fouled.
