What was the break in procedure? What premix are you running? Was the piston clearance machined to what was recommended? On the HS needles, I have carbed 787's skis and on both engines I don't run them shut. Not saying that's your cause, I don't know, you have a 951 carbed. I have both carbs set anywhere from 1/8 to 1/4 out, but never zero. It runs cooler when pegging WOT, which I don't like to like to do for long or very often. Actually each ski, I ran the HS needles at 1/2 out on both carbs for about the first 20 hrs of rebuilding, before tweaking them in some.
But I don't know about your carbs set up, doesn't seem it should be leaning out...