OK.............
I'm sitting here eating my lunch, and I re-read everything. AND.... I watched the vid. One thing to keep in mind is that these are 2-strokes, and it takes a little time (a second or so) to go from low RPM to high RPM, even without a load. In the vid... the person was quickly opening the throttle, and releasing it again in about a 1.5 second time frame. Is this what you are doing?? Are we chasing a 1 second delay from idle to full throttle? If that's the case, I don't think it's going to get any better. Now... if we are talking a real stumble... where you think it's going to die, but then caches itself.... then that is an issue. That's partly why I was asking if it's better when you roll onto the throttle. Basically... without an accel pump... you have to wait for the vacuum to catch up if you are instantly opening the throttle.
With that said... it can be massaged out. You can put in the next size bigger pilot jets, and then close the low screw to compensate.
Just an FYI... the 720 and smaller engines don't seem to have that delay because they don't have a counterbalance in them.... so, they have less mass to try to accelerate.
If I'm totally off base... just tell me... but I'm getting a feelling that we are chasing something that isn't a problem.
A great example is with a "Buell" motorcycle. (The XB series to be exact) Guys would come off of a jap sportbike... and be use to blipping the throttle at a stop light. (why... I don't know) But then they would get on a Buell, and do the same thing. Then... they would complain that they were fouling plugs all the time. Basically... the mass of the big V-twin could accelerate, and every time they would blip the throttle, without a load... they were sending extra fuel into the engine, and it wouldn't accelerate as fast as the jap engines would.
******EDIT*******
OK... we can only talk about so much here. Too much get's lost in translation, because a "STUMBLE" to one person... is a "FLAT SPOT" to someone else.
Go out , and take a vid (with good sound) of your ski, of exactly what it's doing. (on the water) Or bring it to me. (lol)