Internet forums have done more damage to the vintage Honda world than any other single thing. The following is an exerpt from a sohc4.net "expert's" declaration that it's okay for your SOHC Honda to have low cylinder compression because he contends, it's not really low; it's just that Honda's published numbers are not real-world, being derived using a tool no one has access to.

So you did a compression check and the numbers were lower than book value. Does it need an overhaul? First question you should ask, is “Does my test equipment lie to me?” Even if you assume the gauge is accurate (Hey, it’s brand new!), the means of connection can alter the readings.

Anyway, the volume of test apparatus adds to the chamber volume during compression, because its volume has to be compressed to deflect the gauge needle. So, instead of a 9.237cc combustion chamber, you’d have a 20.737cc total chamber with my compression tester, which effectively lowers the cylinder’s compression ratio from 9:1 to 4.1:1.

So, why are Honda published numbers so much higher? Looking in the Honda 500/550 shop manual, there is picture of someone doing a compression test. The apparatus has a very long, thin, rigid metal tube between the gauge head and the spark plug hole. Clearly this small tube apparatus was used so as NOT to add very much volume to the combustion chamber, which will result in a much closer representation of actual chamber pressures. I don’t have such a gauge. Therefore, I don’t know what the actual volume is added by the Honda test apparatus.

So, unless you have a test gauge like Honda or one that adds little volume to the chambers, your test result numbers will be lower than book values.

The numbers can still be meaningful even if they are lower. All cylinders should still be within 10% of each other.