It could be like Flanny suggested - the main jet. And it could be something else. Maybe it is...
I have also found that very subtle obstructions in any of the atmospheric vents - float bowl, slide diaphragm, or fuel tank can affect high speed running. Make sure all vents flow air well with little restriction. These vents must be able to equalize pressure adequately. The diaphragm on top of the slide must be installed correctly also. Check for cracks in slide diaphragm cover which I have seen happen once before on a CV carb (KLR) from over tightening.
One time on my Ducati 900 SS I was having a similar problem (at 115 mph + speeds) , and found that it was being caused by high wind speed pressurizing the float bowl vent tubes on the FCR carbs. I had removed the small plastic triangular plenum chambers that the vent tubes went into. I did this while stripping all "unecessary" items off the bike. However I did not realize that this small useless looking item was very necessary to keep wind pressure from acting on the floatbowls at high speeds.
BTW, my 640 LC4e will pull 110 mph indicated with the 17:42 gearing
(it has: FCR 41 with a 138 main in Denver, internally altered SXC pipe)
1)Diaphragm vent/seal problem and the slide will not go all the way up
2)fuel tank vent problem and the fuel won't come out of the tank
3)floatbowl vent problem and the fuel will not meter properly.
So check the vents, that might be it.