I'm having an issue with a Johnson 15hp motor that is driving me nuts right now. The motor ran fine a couple of years back when my dad used it on his 14ft aluminum V-bottom boat. This summer, we put the motor on a 16ft fiberglass flat bottom John boat (a "Big John" model) and the motor runs fine at very slow speeds BUT it slips and jerks really hard if you try to go any faster. It won't even go fast enough to make any wake without this problem happening. First thing I tried was changing the tilt of the motor, but no luck there. 2nd thing I did was removed and completely disassembled the lower unit. I found that the "Clutch Dog" was worn a bit on the foward side, but not on the reverse side, so I flipped it around (putting the worn side on reverse). I reassembled the lower unit and refilled it to the proper level with fresh gear oil. I was pretty confident that I had found my problem, but I hadn't. The motor still had the same issue on the next trip out. Then I moved on to the prop and tried drilling 2 small holes on each side and inserting pins that reach all the way to the shaft (just to see if the prop was slipping under load). NOPE!!! The prop is not slipping either. Now I have no clue what to do next. Any advice?? Thanks in advance.