Valve Problem
I had a recent problem like that with a new Yamaha YEP201 3-valve horn. The Yamaha valve guides are held in place by the valve stem (it tightens a washer that holds the plastic/metal guide). If the stem has come loose at some time, the guide will shift deeper into the guide slot. This causes it to bottom out and the valve to stick about half way down. No amount of oil or cleaning will help. You need to loosen the stem, then remove the valve. You may need to unscrew the stem completely then reseat the guide in its proper place (there is a little plastic "nipple" that fits into a hole in the valve.) Be careful to seat the plastic guide properly, then push the it in so that the tab that fits into the slot is pushed in as far as possible. Tighten the stem. Now re-oil and reinsert the piston. This solved the problem on my student's YEP201. I believe the basic guide system on the YEP642S is similar and the symptoms you describe fit the problem I saw above.
Doug
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