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  #1  
Old 06-27-2019, 03:22 PM
WingnutWick's Avatar
WingnutWick WingnutWick is offline
 
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Location: Fresno, CA (KFCH)
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Default #2 cyl not working below 2300 RPM.

Flying on my normal commute today in my carb’d O-360 RV-8 with 2xPmags, everything running smooth per normal until I began a descent and pulled power. I first noticed a little vibration and then saw that the #2 cylinder was very cool, EGT and CHT falling off. Playing around with it a little on descent right around 2,300 RPM would be the cutoff RPM: above that it would come back on and engine would run smooth. Below that it would fall off again and the slight vibration would come back. Pulling the mixture back (still in the normal running regime) would induce a similar effect. Cycling the the ignitions didn’t have any effect other than the normal drop in RPM while on one ignition only. On deck at idle, running a little rough and #2 definitely not firing. Of note, not sure if it’s related, when I went to shut off the motor on deck it took a little longer to come to a stop than it normally does (as if I was shutting it down via ignition vice the key).

Any ideas??

Thanks all!

Wingnut

https://drive.google.com/file/d/1uJ0...w?usp=drivesdk

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F-18E/F, Mk.58, CF-5
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Last edited by WingnutWick : 06-27-2019 at 04:12 PM.
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  #2  
Old 06-27-2019, 04:27 PM
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Toobuilder Toobuilder is offline
 
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Location: Mojave
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Default

Stuck valve or broken ring lands. Pull the prop through by hand and I'll bet one cylinder is soft.

Do you have the jumpers in the Pmags?
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

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RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C

Last edited by Toobuilder : 06-27-2019 at 04:29 PM.
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  #3  
Old 06-27-2019, 04:29 PM
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WingnutWick WingnutWick is offline
 
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Default

Quote:
Originally Posted by Toobuilder View Post
Stuck valve or broken ring lands. Pull the prop through by hand and I'll bet one cylinder is soft.
Would this be RPM dependent like that?
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Last edited by WingnutWick : 06-27-2019 at 04:34 PM.
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  #4  
Old 06-27-2019, 04:34 PM
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Toobuilder Toobuilder is offline
 
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With enough RPM even a weak cylinder will generate enough compression to fire. Could be as simple as a chunk of carbon holding the valve off the seat. A compression test will pinpoint the source of the leak. Maybe just have to stake the valve. Done a wobble test lately? How many hours on the engine?
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
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RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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  #5  
Old 06-27-2019, 04:49 PM
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Carl Froehlich Carl Froehlich is online now
 
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Default Been there, done that

Had the exact same thing happen to me on a rental plane back in the day. The cause was a lost bolt in that cylinder?s intake tube where it connects to the cylinder. At high RPM there was enough suction so the tube sealed on the cylinder. As soon as power pulled back the cylinder was lost and the engine bucked.

Carl
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  #6  
Old 06-27-2019, 06:20 PM
sblack sblack is offline
 
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You know it's number 2. No number of opinions here will substitute for pulling off the cowls and investigating. exterior inspection for leaks or cracks, compression test, borescope, pull the jug etc etc. The usial drill.
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Last edited by sblack : 06-27-2019 at 06:22 PM.
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  #7  
Old 06-27-2019, 06:53 PM
rv7charlie rv7charlie is offline
 
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Quote:
Originally Posted by Carl Froehlich View Post
Had the exact same thing happen to me on a rental plane back in the day. The cause was a lost bolt in that cylinder?s intake tube where it connects to the cylinder. At high RPM there was enough suction so the tube sealed on the cylinder. As soon as power pulled back the cylinder was lost and the engine bucked.

Carl
Had a more extreme version of that symptom with an O-360 many years ago. During cruise flight, pulled power back to start a decent, and engine went full rough. Pushed the throttle back in; smoothed out. Obviously, kept power up until in the pattern at next fuel stop. Ran very rough on the ground. Found that one cylinder's 1/8" NPT pipe plug (to fill the injector port) had decided to depart the engine.

In my case, near-full throttle kept the mixture rich enough in that cylinder to run OK. As soon as MAP went down, that cyl went very lean.

Charlie
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  #8  
Old 06-27-2019, 11:37 PM
lr172 lr172 is offline
 
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I would also look for an induction leak. On a carb'ed engine, they tend to exhibit little to no symptoms at WOT and get worse as the throttle is closed and vacuum increases. The leak would need to be significant to cause your symptoms, however,

Larry
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Last edited by lr172 : 06-27-2019 at 11:40 PM.
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  #9  
Old 06-28-2019, 04:46 AM
Southern Pete Southern Pete is offline
 
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HT lead on the way out?
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  #10  
Old 06-28-2019, 06:08 AM
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DanH DanH is offline
 
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Look at the EFIS photo above. As you further close the throttle plate, does the EGT reduce in step, then fall off a cliff? If so, it's an induction leak...#2 is way lean of peak and eventually goes too lean to fire.
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