When I first opened up the cylinder head cover of a GPz900R many years ago I was surprised to see something I first saw in the late 70’s on a Honda CB750… “Rocker Arms” sitting over the camshafts, but with two valve adjustments on them due the the 16 valve head. Having screw adjuster rather than shims makes valve clearance adjustment so easy.
However, over time the valve clearances on most GPz900R’s become tighter and tighter till there is no clearance left. This is due to the wearing down of the valve itself and not the valve seat as many believe.
Refer to the picture below, it should be obvious which is the worn valve!
Looking at it side on you can see how the lip of the valve is gone.
Perhaps one reason the valves wear first is the toughened valve seats used to meet strict Emission Laws in the US of A and low quality fuel.
Regardless of the reason, replacement of any valve requires the removal of the cylinder head and a lapping in of a new valve. You can buy valves from anywhere. I bought a set from wemoto for one of my GPz900R’s and I believe the GPzZone has them as well.