Thanks guys. The model number I took off my engine is "M18S"
I looked through the parts site and missed the fact about the carbs so kudos to aagitch, Joe and Matt.
My interest isn't to change it (unless the carb craps out and needs to be replaced

) it was to wonder if a engine designed for 18 hp will last longer as a 15 hp?
Thanks for the welcome Duke.
Nice site you guys have here.