The other day I was out and noticed that in gear idling there was a slight miss when I pushed down on the hotfoot quickly. So yesterday I took out all the spark plugs and every one of them were fouled pretty bad. They were not wet. I also noticed with the plugs out that when I looked into the top port side cylinder, there seemed to be a layer of carbon buildup on the piston. I could see the middle of the piston where there was not any carbon which I assume is because that is the hottest and closest point to the spark plug. These plugs are brand new after the winter and ordered from one of the sponsors here sparkplugs.com (installed in March). I used the sparkplug finder on the website to make sure I was getting the correct plug. The gap was checked before install and all were gapped correctly. This winter a new oil filter was installed, fuel filters cleaned or replaced by mechanic, and a 10 micron water separator was installed. I always use ring free and marine stabil and yamalube. Mechanic checked out the O2 sensor and said it was good. The motor was on diagnostics and had 150 hours as of last August, and I would say now it probably has 160-165 hours. What could be causing this plug fouling problem?