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#2 cyl not working below 2300 RPM.

WingnutWick

Well Known Member
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

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9-C262-D52-78-A9-41-B8-AA01-6624-FBF7-B21-B.jpg
 
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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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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?
 
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
 
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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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
 
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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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.
 
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.

So I tried pulling the prop through as suggested above and I?ve got equal (and normal) compression on all cylinders. Looking at all the pics I snapped yesterday coming in with higher throttle settings engine runs smooth with #EGT higher than the rest by almost 200. As I pull the throttle (or mixture) it does ?fall off a cliff? like Dan mentioned. It does seem like an induction issue does it not?

Will pull the cowling off today and see what I can see around the intake. I?ll keep you all posted! Yesterday was a bad day for the RVs (my gf?s 6s EI started acting up which is why I switched to the 8 for the commute)

Thanks for the ideas folks!
 
So I tried pulling the prop through as suggested above and I?ve got equal (and normal) compression on all cylinders. Looking at all the pics I snapped yesterday coming in with higher throttle settings engine runs smooth with #EGT higher than the rest by almost 200. As I pull the throttle (or mixture) it does ?fall off a cliff? like Dan mentioned. It does seem like an induction issue does it not?

Will pull the cowling off today and see what I can see around the intake. I?ll keep you all posted! Yesterday was a bad day for the RVs (my gf?s 6s EI started acting up which is why I switched to the 8 for the commute)

Thanks for the ideas folks!

With the addition of these symptoms, I would put an induction leak at the top of my list of culprits.
 
If the prop pulls stiff on all 4 cylinders then the condition cleared itself or there is a significant induction leak. Boot, or more likely a dried out gasket. If it's the gasket then you might want to take a look at the SDS "no leak" induction kit and fix it once and for all.
 
+1 for SDS "No induction Leak" mod. My carbureted O-360 idles smooooothly at 450 RPM after installing the mod.

No affiliation, just a happy customer..... :)
 
Old trick

Here?s an old trick. With the engine un-cowled, and running at a setting where it?s rough, mist or spray alcohol at each induction tube joint, one at a time. If the engine smooths out, you?ve found a leaking joint.
*****Use all normal cautions working around a turning prop******
 
That?ll do it!

So took the cowling off and sure enough there was the intake flange sitting at the bottom of the manifold. Not sure how this happened but the two bolts managed to work their way out completely! Gasket nowhere to be found. New gasket and bolts and she?s back to running like her good ol? self! Very odd though, needless to say I checked the rest of the intake bolts.

Thanks for the help all!

Cheers
 
Great news and easy fix! Love this forum to read these situations to learn a little something every day and know where to start looking if/when something does need attention.
 
Great news and easy fix! Love this forum to read these situations to learn a little something every day and know where to start looking if/when something does need attention.

This site has truly been a blessing and fantastic learning tool for a first time RV owner as myself!
 
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