912ULS hydraulic lock

  You don't have to go that far... all the most current up-to-date manuals are right here on this site:

https://www.rotax-owner.com/en/support-topmenu/engine-manuals#912-series-engines

There is no need to surmise Air in the Oil within the Oil galleries producing a false "Burp".

One revolution of the engine moves about 3cc of Oil (or Air).

It would take about 10 engine revolutions( ~4 Prop Revs) to move 30 CC (1 oz.) of Oil/Air.

On the other hand, Each Cylinder displaces 400cc on every half rev of the engine.

If you turn it slow enough, the vast majority of the Air will leak past the piston rings into the crankcase and purge the Oil into the Tank.

3 or 4 turns of the prop will be enough to "Burp" the entire Oil volume of the engine if it had drained into the crankcase.

This is why you are instructed to hesitate at the top of each felt compression to allow it to bleed off.

A little bit of Oil in the cylinders makes a Huge difference in the static compression.

During a compression test, I squirted about a half seconds worth of WD-40 into each cylinder.

After the test, it was close to impossible to hand-crank the engine.

It actually felt like a hydraulic Lock except it was not a Solid Lock, it was springy and you could pull it through with effort.

When first trying to start the engine, The electric starter gave it half a turn and then came to a full stop.

Remember that Rotax has upgraded the starter at least once due to its marginal cranking ability.

After a few attempts, the managed to engine get the engine to get past that first cylinder and cranked until it started with a short cloud of WD Smoke.

Changing the Oil should not allow any Air into the hoses unless you purposely blew out the hoses.

The Oil Thermostat is OPEN when Cold.

The majority (But Not All) of the Oil bypasses the Oil Cooler.

By the time the Oil reaches operating temperature the Oil cooler has been slowly purged of Air.

At operating Temp, the Thermostat Closes and prevents Any Oil from Bypassing the Cooler.

The small amount of air in the system is harmless when you consider that the entire Oil volume goes around the circuit every 10-15 seconds.

Many many thanks for your inputs on this one guys. It has truly been tying my head in knots. 

Unfortunately I can't change the installation to what it should be as per the Rotax manual. What I can do however is try and help educate the owner (a student) about the ways in which his installation is different from the standard and requires a bit more care to be taken. 

The way in which you described the feel of pulling through the prop after squirting the WD40 into the combustion chambers and subsequently the inability of the starter motor to turn the engine over is exactly what this one felt like. I am now coming to understand that 'hydraulic lock' is when the engine goes solid. As you said this one felt hard to pull through but it never fully locked. Hopefully this also means that no mechanical damage was done by starting it up.

On this installation if you carry out an oil and filter change in the normal way the contents of the oil cooler and its hoses are fully drained also as the cooler is 'upside down' and allows oil to drain out of the sandwich plate when the filter is removed. Because the oil cooler is upside down it cannot be pre-filled with oil before re-fitting the oil filter but neither will it receive the full oil flow to purge the cooler of its air until the thermostat closes at 80 degrees C.

The reason for asking if it was possible to achieve a 'false burp' if air was trapped somewhere in the system was because the oil level after burping on this engine seemed to be weirdly 'unpredictable' and almost like some oil was 'hiding' somewhere! On the day that the engine was started up and oil flew out everywhere the student said that he had burped the tank. The oil level had been low at the bottom of the flat on the dipstick and indicated the same level after burping. This made sense to me as I assumed at least 300ml would have been taken up after its last oil change by re-filling the oil cooler, hoses and oil filter (on this installation all on the pressure side of the pump). I know that the difference between the bottom of the flat and the top is 500mls so I topped the tank up with 300mls which should have been well within limits. After checking the oil level again after running the engine however the oil level had shot up way over the top of the flat leading me to believe that somehow it had been overfilled. Since this happened I have been really wary of adding any more oil until I am 100% sure that the level on the dipstick after burping is correct. It doesn't seem to be consistent which I know probably just sounds plain daft!