Atomic 4 Slow forward
Agree with the fouled prop.
However, if this doesnt clear the problem..... Check the following:
1. dirt in the emulsion tube in the carburator.
2. The very beginnings of a blown head gasket. If you have closed loop freshwater system you can get some specialized dye from an automotive speed shop to check for excess CO/CO2 in the cooling water. Obviously, if your raw water cooled this wont work.