outboard surges

Ok guys i have a question. I just bought a john boat for duck hunting. It has an older johnson outboard. I took it out for a fishing trip to test it and it ran great. Now i took it out bow hunting and when i give it more throttle it will "rev" or surge really high but not go any faster. It runs fine when going slow throttle? Any ideas?

Any update?

It sounds like the hub on your prop is spun, especially since the motor is running fine...

Your lower unit is a worm gear of sorts, it doesn’t slip, unless the lower unit is shot, and typically there is some noise associated with that...

The shaft coming out of your lower unit transfers power to your prop via the hub. Over time, the hub gets worn out, and the hub spins inside the propeller, and your RPMs go up, and you don’t go anywhere.

Motors that have a spun hub, operate normally under low speed / low torque conditions.

If I am a betting man, buy a new prop, and you’ll be good as new!