Yamaha outboard RPMs jumping

[Destroyer]

Originally Posted by Rusty5387

Hello everyone!! I recently put a new lower unit on my 1993 Yamaha 150. Unfortunately last year I hit a sandbar and damaged the old lower so I had to replace it. The new lower installed nicely and I changed all my filters and plugs on the motor for this year. Im currently in the middle of my break in on the new lower where I can exceed 3500 RPMs for the first 5 hours. So I was running arounf 2900-3000 so I was on plane and cruising along nicely when all of a sudden the motor RPMs jump up by around 400 but there is no increase in speed. The RPM jump only lasts for a few seconds and drops back down. I first thought since there is no speed increase that my prop was slipping. I could not see anything wrong with the prop so I marked the prop and the hub and took it out again. The motor did the same thing but after checking the marks they are still lined up so I do not believe my prop is slipping. My father inlaw thinks it could be just old gas that's been sitting in the tank sink July last year. Does anyone have any suggestions on what this could be? I do plan on putting a fuel treatment in the tank when I am able to get to the gas dock on Friday. Thanks for any help

I know that you said that the marks on the prop are in the same place but I would still put on a different prop to PROVE that it's not the prop slipping on the hub. I cannot think of another situation where the rpm's would increase without a corresponding increase in the speed of the boat. Everything else is connected mechanically to the head.