I had forgotten to hold the throttle open, thanks!
Si now I have the throttle taped wide open, I am using two different compression testers to compare. I also have a nice MS460 saw I am comparing too. The two gauges only vary about 5psi, I am using a bolt and teflon tape on the threads to plug the compression release on both saws also. So apples to apples. The testers are also the 10mm thread in style with o-rings, not just a gage with a tapered rubber end on it. I can now pull the motor over fast until the gauge maxes out. The new one tank of fuel MS660 I get around 125psi max. with either gauge. The MS460 that runs like a top I barely can get over 100 psi?? The squish on my MS460 is like .045 and the MS660 I milled the cylinder base for a squish of .021. One of the compression tester is a Craftmans from the late 1970's and the latest compression tester I just bought on Ebay. Yeah, it was a low buck kit. Could both of these low reading be low quality compression testers??