You're not the first to try that upgrade, 15hp carbs are scarce at best. But, depending on the year of your 9.9 you need more than the carb to do the conversion.
yeah i know, the real kicker is that the company i work for threw out a 15hp evinrude engine (late 60's early 70's i think) about a week before I found out that you could do the conversion. I've been trying to rid myself of my pack-rat ways so i didn't jump on it.
Someone else mentioned here (and I believe it) that the mid-70s era 15s were more like 12s. On those, the only difference was the carbs. Later models incorporated a different exhaust tube, even later models introduced some spacers at the intake.
The 15 is essentially a 9.9 enabled to turn more RPMs at WOT.
Honestly, I don't think you'll really notice much difference moving up to the bigger carb.
