"The most important factor is
"The most important factor is the difference between the readings from cylinders. Closer to equal, the better and 10% or more indicates trouble.
Best done with the engine warm, throttle wide open, all spark plugs removed and ignition disabled (pull the safety laynard and may need in forward gear to achieve full throttle).
Some Yamaha's give different pressure target values between upper and lower cylinders, but this is unusual and Jon would have mentioned it, if it were in his Honda manual.
If you see a 'low' reading cylinder, squirt some oil in and do it again, if improved you know it's a ring issue and if not, a valve issue.
A better test is a pressure test where a special calibrated fitting applies 80 psi test pressure and a gauge measures that +/- 70 psi or so is maintained. This is done with the piston locked at top dead center on the power stroke. With this method you can hear the air escaping through valve or crankcase."
