I've just spent a couple of months faffing around with exactly the same thing.
The first thing I checked was that the valve was rotating freely and gave it a good lube and it was better but not fixed. So, I took it apart again, and again and it finally clicked that the damn spindle had a slight lip and the valve was sticking in the bushing.
So, I took a file to it and filed it back to profile, confident I'd nailed it . . you can guess . . a bit better but still not fixed. The rev counter now only flicked around for the first few miles.
And then it stopped altogether one day and for a couple of weeks all was fine. And then I started to notice that it sometimes wouldn't pull properly after coasting down from motorway speeds and getting away from the lights at low revs, it sometimes bogged.
So, I whipped the cover off and guess what, the return EXUP cable had snapped. This meant that it was ok going up the rev range where the valve opens, but it would take a lot of vibration and gravity to get it to return to closed for low revs. I suspect a snapped pull cable would be more obvious because in that case it wouldn't rev out properly.
So actually, it wasn't the valve at all, it was the cable getting progressively more sticky that was causing the original problem. Mind, after 102,000 miles, I don't blame them for getting past their due date. I've had to do both throttle cables recently and the clutch went 2 years ago.
So, after nearly passing out at the £150 quid price of a new pair of cables, I bought WeMoto's 'Slinky Glide' pattern cables and fitted them a couple of weeks ago. Worked like a charm. Fazer back on stun (sorry).
Anyway, final point - check the cables are running freely. It might not be the valve at all...