Re: 200 hp to 225 hp...
The 150 and 200 Mercs (if the engines we are discussing are classic series), they are both 153 cubic inch (2.5L) blocks. The only way to tell for sure what's changed is to compare the parts lists line for line for the two engines. If the bare block part number, pistons, heads, exhaust tuner, intake manifold, and reed block have the same part number but the carbs themselves are different then and only then can you say it's only the carbs that makes the difference. As for the displacement, even though two engines share the same block, porting is one of the ways power is achieved on a two stroke. Therefore porting may be different (and very likely is) so the blocks really are not the same. Changing the carbs is done to take advantage of the better breathing ability of the engine with more agressive porting. Since you put more air and fuel into the engine, you need to get rid of it as well so again, porting is an issue and that's where the exhaust tuner may or may not be involved as well. While carb change only may hold true on some smaller outboards, it is not usually the case on bigger engines. If it were, why would anyone buy a 200 when all they had to do is pop a set of carbs onto a 150. The engineers do some creative things to achieve power, but they aren't magicians. Your 200 and the 225 fall into the same category. The performance you will likely see in such a change is an increase in fuel consumption with an imperceptable increase in performance.
