I have a different view on the oil check burping.
According to the Rotax Operators Manual,
1. Remove bayonet cap from the oil tank, turn the propeller
slowly by hand in direction of engine rotation several
times to pump residual oil from the engine into the oil
tank.
2 It is essential to build up compression in the combustion
chamber. Maintain the pressure for a few seconds to let
the gas flow via the piston rings into the crankcase. The
speed of rotation is not important but rather the continuous
pressure and the amount of gas which is transferred
into the crankcase.
3 This process is finished when air is returning back to the
oil tank and can be noticed by an audible gurgle from
the open oil tank.
4 Check oil level and add oil if necessary.
My understanding is that over time oil siphons from the tank back to the
crankcase
after the engine is shut down. "Burping" the engine uses compression to force
the oil in the crankcase back into the tank.
We operated our 914 for nearly 1300 hours until the compression in several
cylinders
got low enough that the engine no longer performed well. They were around
60/80 psi with a differential test and 85 psi with a conventional test.
For the last several years we were unable to get the engine to burp prior to
start,
even rotating it 50 or 60 revolutions! We quit trying and instead when shutting
down the engine, we checked the oil immediately since the tank was full
and the siphoning had not yet started.
Our new 914 burps after just a few turns and the compression is much higher,
it's
difficult to rotate.
Notice that Rotax says to rotate slowly and to maintain the pressure to let the
gas flow. That's exactly how we do it. You can feel the pressure drop and hear
the air flowing.
I'm surprised that burping after the last flight of the day and then checking
the
oil level the next day gives a satisfactory result. I'd think the oil would
siphon back to the sump during that time.
Just my thoughts and experience.
Jim Butcher
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