General compression test question... applies to XJs too

Might be fun, but I'd need to buy a different compressor. Mine only does 110 PSI. The highest pressure on from HF that I found does 175... that still might not be enough to find the real pressure holding capability of the cylinders, but at least it would allow you to evaluate whether the seal is decent.

 

http://en.wikipedia.org/wiki/Leak-down_tester
lower pressure will work, it just takes a little more math
50 psi should work ok but would that be your psi's or a lowlander's

 

If you're up that high, maybe you should have a microlight instead of a motorbike. (joke, guy)