A buddy of mine just purchased a used boat and it runs well on muffs but when you put it in the water wont get over 4000 rpm,s and not doing over 15 mph. I have done a little research but not exactly sure where to start. Some say possible clogged exhaust, possible stopped up high speed jets, and I had one once that I believe the brain box was weak and caused this same problem. It's a 50 hp Mariner on a 16 foot Bass Tracker so it should be able to hit more than the numbers it's doing. Wont even get on plain. Any suggestions? Thanks