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  #11  
Old 08-06-2017, 03:43 PM
sf3543 sf3543 is offline
 
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You could also verify that the plug for the primer inlet is still there. I have seen that missing before.
Also, check for cracks between the spark plug holes and verify the injector is tight.
You didn't say what engine, but some Titan cylinders require regular inspection to verify that the head isn't loosening. Just sayin'
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  #12  
Old 08-06-2017, 03:53 PM
flyinga flyinga is offline
 
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It sounds to me like you have a leak in either an exhaust valve or intake valve on #1 and the #3 cylinder exhaust or intake valve is open so you are hearing the air leaking out of the #1 valve, through the intake or exhaust manifold and out the open valve in the #3 cylinder.
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  #13  
Old 08-06-2017, 09:13 PM
VA Maule VA Maule is offline
 
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In the first post you said you can hear air @ the spark plug, remove that plug feel the seat with your finger dose it feel good and smooth or snaggey/ruff .A louse pug can erode the seat also if plug seized to the helicore insert it could have backed out a bit. Temporarily for compression check only replace copper washer with an O-ring with a good slavering of fuel lube for a good seal and do the differential pessure test again.
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  #14  
Old 08-07-2017, 12:17 AM
tim2542 tim2542 is offline
 
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You might want to bleed the hydraulic assemblies and check the valve lash on those cylinders. It's possible they were not set correctly and with normal wear the lash is gone and the valves are not seating fully.
If I'm not mistaken, this is suppose to be done after the first 200-300 hrs anyway.
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  #15  
Old 08-07-2017, 06:34 AM
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Carl Froehlich Carl Froehlich is offline
 
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I appreciate the thoughtful responses.

I too assumed the intake valve on #1 was the culprit (not fully seating). Reading reports of this happening on Lycoming engines however all had the same pre-conditions - the engine ran very rough. This engine runs smooth as glass and considering the EGTs and CHTs are tracking with the other cylinders, we conclude #1 is making power.

I do know that Continental did a test on what happens when piston rings are filed short such that the compression check is 0/80. The engine still produced full power.

The airplane owner (a Lancair 360) has sent all the data to the engine shop that built this engine for their recommendation. I suspect we'll be pulling that jug just to make sure.

Carl
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  #16  
Old 08-07-2017, 07:05 AM
VA Maule VA Maule is offline
 
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I know of 2 lanceairs from Dogwood. One's a world record setter, the other is an AME for NASA is it one of these gentlemen?
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  #17  
Old 08-07-2017, 07:16 AM
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Carl Froehlich Carl Froehlich is offline
 
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Quote:
Originally Posted by VA Maule View Post
I know of 2 lanceairs from Dogwood. One's a world record setter, the other is an AME for NASA is it one of these gentlemen?
Yep - the Lancair 360 guy.

Carl
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  #18  
Old 08-07-2017, 08:30 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Carl Froehlich View Post
I appreciate the thoughtful responses.

I too assumed the intake valve on #1 was the culprit (not fully seating). Reading reports of this happening on Lycoming engines however all had the same pre-conditions - the engine ran very rough. This engine runs smooth as glass and considering the EGTs and CHTs are tracking with the other cylinders, we conclude #1 is making power.

I do know that Continental did a test on what happens when piston rings are filed short such that the compression check is 0/80. The engine still produced full power.

The airplane owner (a Lancair 360) has sent all the data to the engine shop that built this engine for their recommendation. I suspect we'll be pulling that jug just to make sure.

Carl
You are correct. Leak down testing is not a good test for gauging performance issues. In the automotive world the go to test is a dynamic compression test, measuring the actual compression, in PSI, produced by the cylinder while spinning. Leak down tests are a secondary test to help pinpoint specific problems and provide a general guide to cyl wall / ring interface condition. As you have seen, a cylinder with poor leak down results can still produce solid dynamic compression when the source of the leak is ring/wall. This is because of the static environment. Once the piston is moving, the leakage is reduced and also less relevant.

The fact that you were making good power on the cylinder was the reason I suspected stuck rings. They typically still produce enough power to be unnoticeable.

Larry
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Last edited by lr172 : 08-07-2017 at 08:38 AM.
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  #19  
Old 08-07-2017, 09:50 AM
BillL BillL is offline
 
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Quote:
Originally Posted by lr172 View Post
You are correct. Leak down testing is not a good test for gauging performance issues. In the automotive world the go to test is a dynamic compression test, measuring the actual compression, in PSI, produced by the cylinder while spinning. Leak down tests are a secondary test to help pinpoint specific problems and provide a general guide to cyl wall / ring interface condition. As you have seen, a cylinder with poor leak down results can still produce solid dynamic compression when the source of the leak is ring/wall. This is because of the static environment. Once the piston is moving, the leakage is reduced and also less relevant.

The fact that you were making good power on the cylinder was the reason I suspected stuck rings. They typically still produce enough power to be unnoticeable.

Larry
The compression test (rotating) is a less repeatable and less sensitive test than a leak down test. I always counted the compression strokes to compare cylinders.

If it was a stuck ring (certainly one of a general list of possibilities), the "leak" would not blow air out another open spark plug.

Let's be careful about supporting idea that a bad leak down test is not relevant even if a compression test is good. While one might fly home, a small leak certainly may not affect idle or power - yet, but - it should be heeded as something to investigate. "Investigate" means it can produce false results, but that needs to be verified. In this case, it could be a piece of carbon under the intake valve or a tight valve at the limit of the lash adjuster. A list of things could cause a tight valve.

Good thing to get the engine builder involved at relatively low hours.
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  #20  
Old 08-08-2017, 02:22 PM
bjohnson1234 bjohnson1234 is offline
 
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Quote:
Originally Posted by dtw_rv6 View Post
Mike Busch claims that compression readings have no correlation at all to an engines ability to make full rates power. It only tells you if the engine will consume oil.

Continental says to borescope the cylinders to determine their condition. If that is okay then fly for (??) hours and then redo the compression test.

Don
See https://www.aopa.org/news-and-media/...ance-borescope
A compression test measures static compression. You slowly bring up the pressure and hold it, but that's not what happens when your engine is running. During the compression stroke the piston is moving very rapidly and while I'm sure the underlying physics are more complicated, there essentially isn't time for enough air to leak. 5/80 is pretty low and if you can't bump it higher by pushing on the prop then you might have a more serious leak (like the loose plug that someone mentioned), but if the borescope looks good and the engine runs fine then you are probably just going to go through a lot of oil. There might another problem that someone more knowledgeable can point to, but a compression test is not a reliable or repeatable test and shouldn't be used as the basis for airworthiness.
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