Well, I have a YS50st that has about 3/4 gallon through it. I am still a bit rich for break in, but I noticed today that I can now take the starting wand and just turn it over. I pulled it out of my T-Rex 600 and I can turn it over from the crank with the plug in, I mean nearly no compression. I am going to send it in tomorrow, but I have been flying airplanes for 20+ years and have never seen an engine loose compression like this if it had not been in a wreck (dirt ingestion) or had been run lean. After each flight I did the backplate touch test and I could hold my finger to the backplate as long as I liked, it was warm but not hot, I think that I was about 1 5/8 on the main running CY 30%. It had very good compression all yesterday when I flew it and the first flight today, but the last flight I noticed that the starter just flung it over and then turned it over by hand with the wand, it would NOT have done this yesterday or on the first flight. Any idea's? I looked at the piston/cylinder and there is no scoring. I am going to send it in tomorrow anyway, I understand that a ringed engine may not have a lot of compression until broken in, but I see no reason to suddenly loose compression during break in when run rich and not leaned out.