I had looked into the 6 to 8 horsepower engines. The answer I got was
carb jetting. Otherwise exactly the same engine. I thought the 9.9 to
15 might be the same issue.

This was a couple of years ago, and probably is arguable, but this is
what my research showed me.

My next engine will be a Honda, a 15 (or so) will replace my Johnson 25.

Jim

IBNFSHN wrote:
There is a lot more to it than just carb jets. For instance a kit to upgrade
a 9.9 to a 15 runs you about $800.00. That is according to my local Johnson
dealer.