Carbs
Allan, not sure there is a diference, I don't think merc makes a 'racing carb' as such. I have been reading a lot on this on S&F lately, trying to get rid of a bad acceleration bog on my engine.
If your trying new carbs on a 2.5 try a set of wh 15's or wh 20's these are old 2.4 big bore carbs and should work better on the 2.5 with it's more low end power.
Try searching on s&f carbs & T-Rex, he knows his stuff on carbs.
What kind of intake on your engine? Horizontal or vertical, how many reeds? Are you planning on any mods to it?