Throwing out ideas…

Idle too high?
Belt too tight, not enough deflection?

Clutch spring and weights might be wrong, like too light of spring and/or too heavy of weights?

 

Fairly sure belt drive Kawasakis don't have a centrifugal clutch. The belt is supposed to go slack enough at idle for it to slip. Belts vary in length slightly though, so Kawasaki used shims between the pulleys to make the belt go slack when at idle. That is what several posts above are about. You need to shim between the pulleys to get the slack belt, fairly sure you shim the driven pulleys. Get the workshop manual which explains how to do it properly.

 

Can anyone explain how to shim secondary? I just installed a comet 103 and a gates belt and it is engaging as soon as I put it in gear.