912 Manual rotating propeller problem

I believe some of your lifters are bleeding down when the engine sits. I had a 914 that always did this and it ran fine and had good leak down tests.

Ed

王汉成 hello

When you rotate the propeller slowly the compressions should feel the same for each cylinder.  if you are saying one is lower pressure then it suggests some pressure leakage.  The only way to confirm this is with a cylinder leakdown test.  You can find this test in the maintenance manual line, MML, for your engine type.  Check www.flyrotax.com and search the technical documents for that manual.  It is free to download.  (checking of compression 12-00-00) 

Solutions might be as simple as run the engine for about 30 mins at high idle on the ground.  This will ensure that all the valves are seating correctly on the seats in the head.  This should also address any issue with rings having gums holding them from good contact to the cylinder walls.  In rare cases the 3 ring gaps might vibrate into line and give you a short term loss of compression.  The short run will allow them to move away from being all inline and it will correct itself if that is the case.  

I do not think it would be a lifter bleed-down issue as in that case the valve would tend to remain closed and not open as in the case of loss of compression in my opinion.

Cheers

Before you worry just check a few things. First each engine's behavior  can be slightly different just like individual people. For instance some engines gurgle in the oil tank with 3-4 prop revolutions and some take 20+. Some props rotate much easier than a prop on another engine. Some aircraft Mfg's may set things up a little different from each other.

With your issue I would warm the engine and do a compression test. If the results are okay then don't worry.

I suppose it’s possible the lifters that aren’t on the cam when the hot engine is shut down expand and hold the valve partially open causing compression loss.

More than once I did cold leak down tests and never found any leakage. I put over 1000 hrs on that engine before removing it for an unrelated issue.

Ed