After motor sailing last weekend for about 4 hrs as I returned to the slip I started to lose power as I tried to increase speed. I backed off and the engine ran at the lower rpm fine, but as I tried to raise rpm it started smoking (white ) and rpms fell off. It stablized and after mooring I run it at high speed both fwd & rev no smoke no issues. Could it just been bad fuel or what?? I am at a loss, can't dup.