1981 xs400SH reading 90 psi

[Brads400]

I recently bought this bike, and I was having trouble getting it to start with pods and stock carbs, but once I got the bike to start it would have no problems running or idling once warm, but while warming up I had seen blue smoke ONCE; leading me to do a compression test, and it read 90 psi on both cylinders exactly. Is that a problem? Aside from being hard to start, it runs fine, doesn't feel low on power or anything

 

[tstidham]

If it's been sitting a while, it could have some rust on the valves, valve seats or cylinders. Running it up some may improve things but if not I would do a few different compression checks. Stone cold, full warm and also a wet compression (spray a little oil in each cylinder). If the compression changes drastically between the dry and wet compression you likely have worn out rings. If there is no change and the reading is still low, it's likely time for a valve job. When I got my 78 running, it had 115 and 90. It ran but not well. I pulled the head, lapped the valves in, ball honed the cylinders and put some new rings on. Now she pumps 150 psi on both and runs awesome.