HF Comp Tester Reads Low

I bought one of these for the adapter fitting, tried it and it read about 20PSI lower than my trusted gauge. Be careful If you buy one, then decide your Motor has Low Compression! get another know good gauge to verify the reading.
www.harborfreight.com/compression-test-kit-66216.html

nothing is made in USA anymore.

Do the brass spark-plug adapters each have a check valve in them? If not, there goes the compression.

I've seen other cheap ones without check valves. They always read very low. It's almost as if the person designing it had no idea what they were for or how they should work. :huh:

I guess I don't understand - what does the check valve do? Obviously, I don't design these :laugh:

DoubleDub wrote: I guess I don't understand - what does the check valve do? Obviously, I don't design these :laugh:

I would think it keeps the pressure from bleeding back into the cylinder when you stop cranking it over.

Without a check valve you're basically making the combustion chamber larger, thus lowering compression. With a check valve you're making the chamber larger until the pressures equalize at max psi, at which point you're not pushing any more air into the gauge.