I took my boat out on Saturday for the first time in about 3 months. It's a 2007 Yamaha 90 2 stroke engine with very very very low hours. I have good rpm and everything was running the way that it should but i was only able to go about 10 mph while in full throttle. the lower unit is turning because the water make very good waves but the boat is not moving fast at all. what could this be? Please help.