could valves be sticking

[Deadly300]

my valves havent been adjusted for at least a year and i was wandering if their sticking because it sputters and acts up for a good 15 min on start up . then its fine for the day.

[400exrider707]

How does a valve stick?:confused: They might be out of adjustment, but cant really see them sticking to anything.

[AtvMxRider]

The only way a valve will stick is if it is bent