i had a 15 hp johnson that was doing the same thing. i cleaned the carb twice, same thing. took it to a local shop and they changed the jet in the carb. said it wasn't getting enough air at full throttle, which is where you run a 15 hp just about all the time. when i called him about it he asked me to pull the choke out when it started bogging down. if it started running better, it was starving for gas, if it didn't run better, it was starving for air. i dunno, but it ran great when i got it back. i sold it to the kids across the canal and it's still running great. good luck.
es le bon ton roulet