CB 500 Four - engine problems

[K2-K6]

As already noted, 90 psi seems very low and likely to give problems starting.

That would definitely give questioning to the gauge, at least to check it's status. Easily done by trying on another engine that's running well to see if it looks low there too.

Ordinarily they are getting "softy spoken" in combustion terms as they get appreciably below about 130 psi, more noticeably going down from 120 psi, but below 100psi, then highly questionable in my experience.

As it runs, did it start easily?  That would suggest it's possibly higher than recorded.

The high revs just suggests that the carb slides are set too high from adjustment initially,, unless that's confirmed as different. It must get air from somewhere to maintain rom.

[Murf]

If your comprehension tester has a long hose between the connector at the spark plug hole and the gauge, the extra volume of that can reduce the compression ratio. On small capacity cylinders this can be enough to give a slightly low reading.  I noticed your readings were quite even but low on all cylinders.

Maybe a leak down test is the way to go now, this can tell you if the valves or rings are passing pressure and on which cylinder.  there is info of how to do this on the web it’s a bit more involved than a compression test.

[deltarider]

Personally I've never been able to conduct a succesful compression test, The last one was some 20 years ago and I had 5 kg. on all four. As long as my bike starts rightaway, performs as it should and recently shows a healthy 20-21cm Hg underpressure @ syncing, I see no reason to worry.