I'm looking at a 15 hp 4 strk kicker for my 621. Question is this motor the same as 15 hp "bigfoot" . Is that motor the same as the 9.9 hp "bigfoot" except different carburation. How does the Pro Kicker differ or are they all the same?

I beleive the gearing is lower and the lower unit is beefer.

take a close look at the specs for the 15's, the standard 15 and the Pro-K 15 have the same cu. in., and lower unit ratio. The 15 Bigfoot has slightly less cu. in., but a lower gear ratio. I believe that the 15 Bigfoot is the older, "cast iron" 4-stroke, while the 15std and 15PK are a newer generation. In either case, they have much more displacement that the 9.9's

http://www.mercurymarine.com/engines/outboards/fourstrokes/enginesetfour_specs.php?ID=74

HRG