Thread: Can you make a 1986 yamaha 2 stroke 9.9 into a 15hp

I moved this to the mechanics forum.

Most experts say .... no, or they say it's generally more complicated than a simple carb/reed swap. The exhaust system, cam shaft, and even the prop may have to also be changed. (https://www.smalloutboards.com/modifications.htm)
They also say you could be risking damage to the motor, and swapping motors may be more economical (unless you're a marine mechanic & can do the work, yourself).

Here's a discussion on a similar note from 2009: http://www.crappie.com/crappie/mecha...-15-hp-yamaha/ .... though the year of the motor was never divulged.

By the time you replace everything the cost may be more than the motor is worth. I did it to a 89 Johnson and didn't seem to make much difference.

I don't think the 15hp carb will be of much help on the bottom end but should give you some more rpm on the top end. A different prop may be a better answer for the getting the boat up on plane.

I had same problem .Added a hydrofoil and it easily planned off at 15MPH. I'll admit I was skeptical, but it made a huge difference.
Before you start swapping props you need to know what RPM your motor tops off at. If you don't have a tachometer, you can buy a "Tiny Tac" to check your RPMs. They cost around $10 and all you do is wrap the wire it comes with around a spark plug lead and position the tach where you can read it. It is small, about the size of a match box and has a digital display.
I put one on mine before I added a regular tach and it is dead on with the standard tach.