Yamaha outboard RPMs jumping

[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

[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.

[Blue_Runner]

I agree with Destroyer. Could it be that when you hit the sandbar it damaged the female connection where the splines lock in and it is slipping there? Not sure what it is called. Or your marks on the prop just coincidentally lined up? Keep us posted on what you find out.

[phatdaddy]

are you judging rpm increase by sound or actual tach readings

[Rusty5387]

Thanks everyone! I am judging it based on sound and on my tach. I will see if I can find a prop to test with before I go out and buy one. The other thing that was brought up to me was to make sure when we installed the new lower unit that it had a ventilation ring already installed or was I supposed to transfer that from my old lower when I took the prop off. A friend of mine had an issue with their ventilation ring and it caused the same issue on their outboard. Fingers crossed that's but I won't be able to check until sometime tomorrow.

[phatdaddy]

I was just asking because a wacky tach is also a symptom of electrical/charging issues.