Hello,
I know that this is an old thread but as you have shown empirically:
1. Adapter/hose volume matters! Anything in the adapter in front of the Schrader valve that either increases (usually) or decreases the effective size of the compression chamber will affect the compression reading. Obviously the smaller the displacement of the cylinder the more pronounced the effect will be. This is why most of these gauges work alright for large displacement car engines but not so much on smaller saws and brushcutters.
2. The physics behind different Schrader valves is as follows: To get cylinder pressure into the gauge you must provide enough pressure to overcome the spring in the Schrader valve PLUS the pressure already in the gauge on the other side of the Schrader valve. Therefore if it takes 25 pounds to open the Schrader the gauge will read 25 pounds low! You can demonstrate this with an air compressor and suitable regulator just don't connect the gauge. Increase the regulator pressure until the valve opens and you'll have your answer.
Class Assignment -
If you have a suitable air compressor and regulator take a moment to go perform what we'll now call the Schrader valve pop-off test and report the results back here. Inquiring minds want to know which Schraders are the best for this application!
Thanks,
Steve
