I have a 1980, 70 HP Mercury Outboard that used to run fine. I take good care of it, I change the oil in the lower gears every year and I changed the spark plugs this spring. It ran great for most of this year until today when it seemed like it didn't have the get up and go it used to have. I use it alot. About 15 hours a week, usually sking. I also pull two skiers most of the time. Do you think that's too much for the engine? I don't have a speedometer, but I can tell the boats not running as fast as it used to. Anybody have any thoughts? 