bad leak down numbers and oil usage

I want to add there has been no noticeable loss in performance, no strange noise or vibrations, nor has the economy changed with the oil consumption or the latest compression issue.

Definitely burning too much oil. Probably valve guides and rings. Low compression indicates it’s time for a valve job and ring set.

All....

The valve guides are not your issue, it might be a bad valve stem seal.  (in 30 years working on them it is extremely rare to fail a guide from wear) The only real way to know is to pull the piston and cylinder.  Follow the maintenance manual heavy if you do your own work.  Look for the ring end clearance, this tells you if it is ring wear.  Lapping the valves back to the same seats is very important.  

This amount of oil consumption on a Rotax 9 series engine is very high, I would not fly it until you resolve this.  In addition do an oil systems check, as shown in the installation manual, to verify that you have the correct crankcase pressure and suction.  Excessive crankcase pressure can cause the oil to force itself past the valve seals.  The excessive oil burn can carbon your valve seats and be the cause of your low cylinder pressure.  Oil burn also suggest your oil rings are perhaps contaminated and no longer sealing and scraping the oil as required. 

Just my 25 cent answer today.

Cheers

My first 912 UL had increasing oil burn that turned out to be a problem with the oil control rings. However, the leakdown measurements were all good.

Sadly, inspite of fitting new rings, the poblem got worse. I had the engine rebuilt twice without success and I finally called it a day and bought another engine. Had I known this was going to happen I would not have wasted my time and money on the first engine and put the money toward the replacement. The replacement engine has been fine and it has over 1000 hours on it.

With regard to the problem with the original rings, when they were removed you could see wear marks in the back of the ring where the spring rests. These were acting as a rachet and preventing the ring from expanding and sealing against the cylinder walls.

I suspect that all will become clear when you remove the cylinders.

This is the back side of the intake valve for cylinder #3, the others look similar. To me this indicates the valve seal is leaking. My plan is to lap all the valves without removing the heads (least invasive), and replace the valve seals.