Remove the valve adjustment covers to see if any of the 8 valves have too much clearance;that's the stuck valve.
I would say you'll need to remove the head if it has a stuck valve;remove the springs and valves(if that's the problem),then 'work' the fit on all the valves(check each one carefully for a bent valve)until they all have a nice,smooth movement in the guides(and some nice synthetic oil on the valve stems),w/o binding.
That engine has relatively low miles,after sitting so long.. those parts may be binding(over time all the oil drips down away from the top-end:no lubrication..
)and can seize later if you don't get them all smooth(possibly work a soft,used fine brass .22cal bore cleaning brush by pushing it up and down by hand once or twice)Nice the way they need.
Got a pic of the bike ? is that the one with the white w/ orange stripes and the factory Yamaha 1/4 fairing ?
