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  #1  
Old 11-16-2016, 03:07 PM
humptybump humptybump is offline
 
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Default 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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  #2  
Old 11-16-2016, 03:32 PM
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Carl Froehlich Carl Froehlich is offline
 
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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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Old 11-16-2016, 03:38 PM
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Devastator just tells you to fly more

An acting spark plug would be my guess.
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  #4  
Old 11-16-2016, 03:47 PM
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airguy airguy is offline
 
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Spark plugs or sticking exhaust valve are the two most likely culprits.
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  #5  
Old 11-16-2016, 04:29 PM
humptybump humptybump is offline
 
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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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  #6  
Old 11-16-2016, 05:00 PM
David Paule David Paule is offline
 
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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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  #7  
Old 11-16-2016, 05:54 PM
Aggie78 Aggie78 is offline
 
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Default 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.
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  #8  
Old 11-17-2016, 04:38 AM
humptybump humptybump is offline
 
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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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  #9  
Old 11-17-2016, 07:22 AM
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snopercod snopercod is offline
 
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Default Use a compression tester

Quote:
Originally Posted by humptybump View Post
I am looking for suggestions on how to test for a leak on an intake tube.
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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  #10  
Old 11-17-2016, 07:55 AM
humptybump humptybump is offline
 
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Quote:
Originally Posted by snopercod View Post
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.
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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