converting 20hp to 25hp

[stevo]

just got a cracking late 80s early 90s 20hp yamaha 2 stroke this is the same motor as the 25 what is needed to convert it to a 25hp thanks.

[Poly]

You need to go through the parts lists for both and spot the differences. On the early 2000's model you need to replace the carbs, the exhaust manifolds, and adjust the timing.

[Northern Pike]

I did that with an Evinrude, same vintage. A check of parts lists, and inspection of a 25 proved the only different parts were the intake manifold and main jet. The difference in manifolds was only the inner diameter.
I removed the restriction in the manifold inlet by grinding it larger with a Dremel (insert a greasy rag to catch the filings), then installed the jet specified for a 25.
Power increase was noticeable and it ran well for many years after.

[Northern Pike]

When researching this, I also spoke to an outboard motor mechanic, who was a hot-rodder and a racer. He explained, its simply a matter of balance; getting more fuel in, in the right proportion. That's why a bigger jet (more fuel) is combined with a bigger inlet (more air).
A further increase to 30 would have been possible, but necessitated a bigger exhaust outlet which would have involved removing the power head, so I didn't bother. Again, a matter of balance: more fuel/air in, needs a bigger hole to get the exhaust out.