Righty - my GK71B will zip up to an indicated 100mph in 4th gear, pulling around 11000rpm, with no problem. But in 5th, it is reluctant to rev out and 6th is just overdrive. I have changed the gearing (dropped from a 46t to 41t rear sprocket) but no different, which leads me to believe it's some form of electrical restriction (if indeed it is restricted). The neutral switch housing has 7 pins on the exterior, but only 1 wire (from the switch to the CDI) and only 1 internal contact.
Bike is running a speedo-healer so it's not restricted there - any ideas why it won't rev out?

Dropping the teeth on the rear will make it not help at all, you need to go the other way and fit a larger rear sprocket.
Its like on a push bike , the larger the rear or smaller the front makes it easyier to peddle.