Weak vs Good Compression

How are you doing the test?  I get more like 10 bar.  The maximum possible is higher than the compression ratio because of the temperature rise by rapid compression.  Full throttle, screw-in type gauge.  Preferably warm engine so piston rings are realistically oiled.

David is right 5.1 barG is about 80 psi, which is so low we doubt that it's the result of a proper compression test on a running engine.

That's sad!  The usual next step is a splash of oil down the plug hole to seal the rings and try again.

Hi Kit,
Your valves look somewhat pocketed. If a splash of oil in the cylinder doesn't improve compression,  you may need to get your hand in your own pocket again and get the head done. New valves, seats and guides aren't cheap unfortunately :-(
Regards, Al.

The seats are hardened and not likely to have suffered.  And guides affect oil consumption but not engine compression.
Are you certain the throttle slides are fully open? Both plugs out.  Gauge fitted in one hole.  Throttle wide open and kick several times until the reading is stable.  Repeat for the other side.  Oil in the bores must help, especially if the engine has not run.
And don't forget to check valve clearances.  A sticky valve will ruin the test.  Put some oil through the rocker caps onto valve stems and make sure they are working properly.

....I rebuilt a short while ago, in a standard compression test cold, showed 115 psi on both cylinders.
This was with only a few km on the engine.
Adding a little thin oil in the plug hole raised it to 145 psi.
It does have high compression pistons.
I'm at an altitude of 1000 m, so at sea level it will be higher.

.....that the pressure measured by a pressure gauge bears no relation to the stated compression ratio. The stated ratio is simply a theoretical number from the volumes of the chamber at bottom and top pf the piston.

The inlet valve isn't closed at BDC, it'll close at about 30d after. So it's still open until the piston is some way up the barrel.

Though the figure is low, if nothing untoward is found then it is more important that both sides measure close to each other. 

Hi Kit,

How are you checking your rings?
Normal process is with rings off the piston,  placed in the bore ('squared up' with the piston) and measuring the gap between the ends with a feeler gauge. Doing this at several heights of the bore will also give a clue to how worn the bore is. 

Regards, George 

You have the head off, its only a few nuts and the barrels will be off so you can measure/inspect  them properly, along with the cam and followers. If the engine has been run without air filters the bores/pistons/rings will most likely be worn. Get the bores measured properly with a bore comparitor, set with a micrometer. This will tell you how oval and tapered the bores are. Shoving the rings down the bore and measuring the gap gives you an idea of how worn the barrel is, just nowhere near accurate enough. Then there is the ring groove on the piston, how much vertical play is there between the ring and its groove, If you can move the ring up and down in its groove the piston is worn out (should be no more than a couple of thou between the ring and its groove sides) More than this will promote oil burning/lack of compression. New Hepolites have a ring groove clearance of less than 1 1/2 thou  Fit an air filter on rebuild otherwise all your hard work will be undone in a few thousand or less miles 
Be aware, just fitting a new set of rings will probably not totally cure the problem and not for long. How much of a step is there at the top of the bores? if you can feel a step (where the top ring is at the highest point) , you will need a stepped top ring set (sets of rings sold with a step in the top ring so the new ring doesn't hit the worn step, that's if they are available) otherwise if they aren't available you have to remove the step in the bore. If its like that it is easier to get the barrel bored and honed and new pistons 

Fit a decent air filter on rebuild. Those pistons have suffered from ingesting dust and grit.
I am amazed that people run their bikes without any air filters, but hey ho, they must like reboring their engines.