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There are no such thing as sticky valves. If a valve is not returning to seated postion under spring load when hand cranking the engine then there is something wrong that needs seeing to BEFORE you attempt to start the engine. These engines have 3-3.3mm valve-to-piston clearance at TDC so if a valve is staying open enough to contact the piston and stop the engine from turning then its open a lot.

The most likely culprit as Tiny said is bent valve. If I were you, I would have a very good look for more evidence and prepare to pull the head and remove the affected valve.
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