Flat Spot above 5k RPM in 5th

'71 cb450, put stock carbs on with one jet higher than stock on pilot and mains (38 to 40 and 50 to 55). Took her out for a ride and I can't get past 55mph, when I open the throttle further the engine stops pulling and the engine RPMs..flutter? The only way I can describe it is a modern day engine hitting the rev limiter. Pulled the plugs and they are on the rich side but not fouled or anything suspicious. Putting around town the bike sounds and operates great, clutch plates and springs are new so I doubt its slipping. Ideas?

It's too rich when slide lifts
Adjust float level at least 1mm higher.
450 'flat spot' is same as all the other CV carb twins of era. (350/360)