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11-16-2016, 03:07 PM
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Join Date: Feb 2011
Location: USA
Posts: 3,179
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Low CHY and low EGT on one cylinder of a carbureted O320?
I experienced a few seconds of rough engine operation at 2560 RPM while flying at 9500ft. OAT was 29F.
I then watched the #2 cylinder EGT drop from approx 1400F to 1190F. I also watched the #2 CHT drop from 280F to 200F.
It stayed like this for about 30 minutes while I circled the airport at altitude. Eventually the CHT very gradually rose to 230F and EGT rose to 1200F.
I did check carb-heat and had the usual a small drop in RPM with no rise of the course of one minute.
The CHT returned to normal as I descended.
Thoughts?
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11-16-2016, 03:32 PM
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Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,596
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Good puzzle to solve.
As you have a carburetor, the normal suspects associated with fuel injection are eliminated. Assuming you eliminated altitude impact on mags and sparkplugs, the symptoms you describe are consistent with a cylinder being way lean.
If you were anything other than WOT, you might have an air leak on that cylinder that is causing it to be lean. I'd first look at the intake pipes and gaskets. As the problem changed with time however, a simple leak might not jump out at you.
Carl
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11-16-2016, 03:38 PM
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Join Date: Oct 2008
Location: Utah
Posts: 8,151
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Devastator just tells you to fly more
An acting spark plug would be my guess.
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11-16-2016, 03:47 PM
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Join Date: Oct 2006
Location: Garden City, Tx
Posts: 5,145
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Spark plugs or sticking exhaust valve are the two most likely culprits.
__________________
Greg Niehues - SEL, IFR, Repairman Cert.
Garden City, TX VAF 2020 dues paid 
N16GN flying 700 hrs and counting; IO360, SDS, WWRV200, Dynon HDX, 430W
Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
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11-16-2016, 04:29 PM
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Join Date: Feb 2011
Location: USA
Posts: 3,179
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Wouldn't a bad spark plug result in an incomplete burn and thus an elevated EGT - similar to running rich.? Am I mixing up the effects of combustion?
The idea the cylinder went excessively lean would explain both drops. Since this is a carburetor, is the likely culprit the intake pipe? Is there an easy test for an intake pipe leak?
Less than 10 hours ago we did a borescope and the engine is "exceedingly clean" because I burn so little 100LL - probably not more than 20 gallons in the past 80 hours. This would not support a stuck valve (but would also not exclude it 100%).
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11-16-2016, 05:00 PM
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Join Date: Dec 2009
Location: Boulder, CO
Posts: 4,435
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I'm not an engine guy but I'd think that the plug or a sticking valve could explain it. They could clear themselves before landing, except perhaps the wire.
In both cases, they'd show up as low EGT and low CHT.
My experience with bad plugs is that they are very likely to lower the EGT for a short while. Sticking valves, in my experience, seem to show up on run-up, clearing up as the engine warms up.
The plug gives varying (that is, non-constant) EGT temps. The valve shuts down the entire cylinder until it starts working again. It's either on or off.
Now of course other indications are possible for these defects and perhaps other causes for your engine. This is merely my own experience.
Dave
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11-16-2016, 05:54 PM
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Join Date: May 2014
Location: Fort Worth, Texas
Posts: 436
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Sticking Exhaust Valve Thread...
You can search the forums, there are several great threads out there about this issue.
Here's the latest, a thread I started last month and just wrapped up yesterday:
http://www.vansairforce.com/communit...d.php?t=143125
Take a look, it could be the problem.
__________________
Rob Schroer
RV-7/N75WV
YIO-360-M1B
New Braunfels, Texas (KBAZ)
VAF Monthly Donor
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11-17-2016, 04:38 AM
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Join Date: Feb 2011
Location: USA
Posts: 3,179
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Thanks Rob.
I searched the forums and read the link you provided.
I will investigate the possibility of a sticky valve but suspect - given other information - the intake is a more likely cause.
I am looking for suggestions on how to test for a leak on an intake tube.
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11-17-2016, 07:22 AM
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Join Date: Aug 2016
Location: Asheville, NC
Posts: 2,104
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Use a compression tester
Quote:
Originally Posted by humptybump
I am looking for suggestions on how to test for a leak on an intake tube.
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You can tape over the air intake and use a compression test adapter to pressurize the cylinder on the intake stroke with shop air. The intake valve should be open with the piston at the bottom of its travel. Then use soapy water solution to inspect the entire induction system for leaks.
__________________
(2020 dues paid)
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11-17-2016, 07:55 AM
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Join Date: Feb 2011
Location: USA
Posts: 3,179
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Quote:
Originally Posted by snopercod
You can tape over the air intake and use a compression test adapter to pressurize the cylinder on the intake stroke with shop air. The intake valve should be open with the piston at the bottom of its travel. Then use soapy water solution to inspect the entire induction system for leaks.
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Thanks for the test procedure. I'll have to take a closer look at where I will have the necessary access of the intake tube between the carburetor and the cylinder.
Update: there is a very useful video for testing the induction tube: https://www.youtube.com/watch?v=CfG4QGv-r64
Last edited by humptybump : 11-17-2016 at 08:03 AM.
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