my thinking was a used 14 might be more available and where he is more or less stock, the 15 may not run as well as the 14 as they seem a little more finicky to tune at least on a stocker. really no need to run a 15 on a stock bike, but good to always on the lookout to score a 15. maybe his pipe and airfilter are too restricted to handle the 78, but I can't see a 15 running a 60s jet unless its severely restricted on the filter, intake and exhaust he's using. those crank pipes are that good, huh? wow. is your head milled?