The problem is likely that on a small engine the cylinder volume where pressure is built is relatively small (at least compared to a car engine). The compression tester hose increases the cylinder volume significantly, and as the piston only displaces a small volume the pressure that displacement can make is therefore lower. In automotive compression testers the one-way valve that allows the gauge to retain the highest reading until the pressure release valve is pushed is located near the pressure release valve.
Small engine compression testers have a one-way valve located at the end of each adapter hose near the cylinder. This hose still adds volume to the cylinder and the saw has to be pulled several times to build compression, but when cylinder pressure is released through the exhaust port the hose pressure is trapped in the hose by the one-way valve located at the end of the adapter hose near the cylinder, allowing for a true pressure reading. Multiple one-way valves require more seal maintenance (given there are more valves to maintain) and are more expensive due to greater manufacturing costs, which in an automotive application is pointless.
Depending on several factors and as a
general guideline, compression should be at least 125 PSI; on a new saw 135-145 PSI, higher on some models/brands, 155 PSI and above on ported saws.
Disclaimer: I don't build saws, just run them mainly, and not for a living.
The Mityvac MV5530 was suggested by other members, I bought one and it is a quality kit that works well.
$75 on Amazon.
View attachment 1168232