cylinder 3 much lower compression

[Dean]

why does the 3rd cylinder show low compression (70 psi) and the others are good (200-210 psi) on 2 different mk1 600 fzs engines all are cold readings as engines are not in bikes

[BBROWN1664]

put a bit of oil in the cylinder to see if its a sticky/knackered ring.
Could be dirt/crap stopping one of the valves sealing properly too.

[Gnasher]

It could also be you've got a valve/s not fully seating. 

[basher]

for me its cylinder 4 is really low, I suspect rings are worn but I wont bother replacing the pistons until winter