RE: Welcome to Club SAITO !
G'day I have twice had brand new Saitos suddenly have no compression at all despite where the throttle was set. On both occasions, the engine in question was fresh out of the box. On both occasions I had put the engine in a model and taken it to the field to give it some running in before flying. Both times, the engine had fired and been run briefly and then just decided to have no compression. Both times I turned the engine over by hand and could not feel any problems so I simply applied an electric starter. With one, the compression came back immediately. The other took a couple of applications of the starter but both returned to normal compression.
Why?
Well, both were new and so possibly were dry but I had primed both and had turned them over by hand with the glow connected until I felt a "bump" so there was some fuel in there. Both engines never suffered from this problem again and I started them and gave them some running and then flew them. I wondered if a valve had stuck as a valve stem was too dry or whether a small piece of something was propping a valve open. And of course, new rings never give the compression when new that they will once bedded in.
The first to do this was a 72 and the second was a 100. Both have turned out to be fine engines.