The most common cause of sticking valves is varnish from stale gasoline. Gas sits in a tank for months (or years) and the lightweight components evaporate. The residue forms varnish, which gets dissolved when fresh gas is added to the tank. That stuff doesn't go away, however- - - -it ends up in the valve guides. After the first few startups of a "wakeup" engine and a cool-down, the next start sticks valves, bends pushrods, breaks rocker arms, etc. NEVER- - - -EVER wake up an engine from long term storage by adding fresh gas to the onboard tank- - - -use fresh gas from an auxiliary tank like an outboard motor tank, etc. Steam clean, or replace the vehicle tank before you use it to run the engine.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!