Plus 1 for the exup, mine stuck opens few years back and the bike was not exactly gutless but noticeably slower than before until over 5000rpm or so.
It's easy to check: take the cover off (carefully incase the bolts are rusted in.....) Turn ign on pulley should rotate to its initial position, then fire up bike, pulley should rotate as you rev bike. Normally fully open by 7k.
Loads of exup info on Pat's site (cartestsoftware)
Cheers
T