Bench ran a YS .63 the other day (Break-in) and it ran great; Powermaster 20/20, 13x6 APC, "F" plug, about 10K running a bit rich. The following day the engine would turn over freely with zero compression- odd. So, after removing the valve cover I found the intake valve had less than zero clearance- the valve was actually held open causing the zero compression issue. Re-adjusted the valve and it now has compression. I've yet to run it again but I'm sure it will be fine.

Question: Why? My only guess would be a valve seat not fully seated (if it does indeed have valve seats) which, once seated from running the engine, caused the valve clearance change. Or, something holding the valve partially open when the valve were initially set at the factory. I suppose a valve stem could have stretched but that seems like a long shot.

Any ideas? There's probably something obvious I'm overlooking...

When the valve was clearance at the factory, the valve was obviously not down tight against the seat. One more reason to check the valve clearance after the first 1/2 hour of operation on any brand of four stroke engine.