Re: convert johnson 25hp to 30hp
As someone said, you can compare parts numbers over at the BRP parts site. But there's another question you might want to ask before investing the time it will take to find out:
How much is 5 hp worth to you? Let's say the difference is just the carburetor, and nothing else. You're in the $300 territory for the carburetor assembly. Add a few other parts that are likely to be necessary, and you'll be quickly pushing $500.
But, that's not all you need. The 25 and 30 hp engines have different part numbers for the crankcase and cylinder assembly, so you're out of luck anyhow.
But, even if the cost was just $500, would that be worth it on a 10 year old outboard? I'm guessing that the 25 and 30 hp ones have about the same value on the used outboard market, so the easiest way to switch from a 25 to a 30 is probably to sell yours and buy someone's 30 hp.
Modern outboards are pretty much running at peak performance. It's not easy to hop them up, and doing so is rarely worth the time, even if it's cheap enough. You'd gain maybe a few MPH, but darned few.
