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EGT3 peaks 1.5gph too early

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ve0kog

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I am trying to run my O-320 LOP at approx 65% power and the Cylinder #3 is causing a little bit of a trouble. It peaks much earlier than the other three cylinders and I have to stop leaning after about 1.5gph "gami spread" due to engine roughness. With that mixture EGT3 and EGT4 are running LOP but EGT1 and EGT2 are still at the peak. The #3 is running is very lean and its CHT is dropping below 300F range, while the other 3 CHTs are staying in the normal mid 300s.

Here is what the chart looks like

Do you think I may have an induction leak on #3? I have tried the "savvy aviation" induction leak test (also seen on the chart) but not sure if tells anything.


1740437260347.png
 
Well, it does look like #3 has an induction leak. Try this: level cruise flight, 9 gal/hr or so. Smoothly pull throttle to idle. Does #3 EGT go shooting up? If yes you have a leak.
 
Carb engines vs FI-----just might be the issue here.

What throttle position did you have for the testing?
 
Thank you for the ideas! The intake hoses are due for replacement and I did not know there was an O-ring flange option. Looks like a worthwhile upgrade.
 

Hi ve0kog, ​

My #3 is lean as well - I manage it using carb heat. The theory is the carb heat door causes turbulence that helps even out the mixture distribution. I have added some vortex generators on the carb heat door as well.

The carb main nozzle causes some effect, too. There are pepperbox nozzles that seem to aid in getting better distribution. I have a non-pepperbox on now that works pretty well, but we are making a pepperbox to try soon.

I run LOP almost all the time. Some of my info here. I looked long and hard for a leak on #3 and have not found it - but I still might.

Pete
 
ve0kog said:
I am trying to run my O-320 LOP at approx 65% power and the Cylinder #3 is causing a little bit of a trouble. It peaks much earlier than the other three cylinders and I have to stop leaning after about 1.5gph "gami spread" due to engine roughness. With that mixture EGT3 and EGT4 are running LOP but EGT1 and EGT2 are still at the peak. The #3 is running is very lean and its CHT is dropping below 300F range, while the other 3 CHTs are staying in the normal mid 300s.

Here is what the chart looks like

Do you think I may have an induction leak on #3? I have tried the "savvy aviation" induction leak test (also seen on the chart) but not sure if tells anything.


View attachment 81584
Click to expand...
No. As the rpm drops at the end down to idle, you would see the cyl with the leak have an egt higher than the other i.e. leaner.the #3 is a bit richer than the others at idle.
 
Mike S said:
Agree with the idea of intake leak, SDS has a kit that makes that a thing of the past. http://sdsefi.com/sdsaero.htm
Click to expand...

I can't seem to catch a raise of EGT when pulling throttle to idle.. but there is some minor blue and brown staining at the flange.. I haven't used mogas but the previous owner might have a while ago.

I've ordered the SDS kit. Will see if it makes a difference.

1741825889537.png
 
ve0kog said:
I can't seem to catch a raise of EGT when pulling throttle to idle.. but there is some minor blue and brown staining at the flange.. I haven't used mogas but the previous owner might have a while ago.

I've ordered the SDS kit. Will see if it makes a difference.

View attachment 82714
Click to expand...
You have the classic lyc lean at high power on number three. This is due to poor distribution in the plenum. Most with carbs have this on 3 and/or 4. Intake leaks create the most profound effect at idle/high vacuum, with almost no effect at wot. If you look at the right side of the chart, where you go to idle, #3 is right in line with all the others. If you had an intake leak, this is where you would see a major divergence.

If you really think you have an intake leak, it is very easy to confirm. While the engine is idling, spray carb cleaner in the areas you think are leaking. If the rpm rises, you have a leak where you are spraying. If not, no leak. A30 second test with reliable results. Auto mechanics having been doing this to find leaks foe many decades.
 
Last edited:
lr172 said:
You have the classic lyc lean at high power on number three. This is due to poor distribution in the plenum. Most with carbs have this on 3 and/or 4. Intake leaks create the most profound effect at idle/high vacuum, with almost no effect at wot. If you look at the right side of the chart, where you go to idle, #3 is right in line with all the others. If you had an intake leak, this is where you would see a major divergence.

If you really think you have an intake leak, it is very easy to confirm. While the engine is idling, spray carb cleaner in the areas you think are leaking. If the rpm rises, you have a leak where you are spraying. If not, no leak. A30 second test with reliable results. Auto mechanics having been doing this to find leaks foe many decades.
Click to expand...

Well, I wanted this to be an easy fix, but you are probably right, since both 3 and 4 are lean on the chart as you mention. These are 450 hrs intake hoses and gaskets, and oil drain tubes. Probably won't hurt to change them anyway before leaks develop, but won't hold breath for the mixture improvements..
 
I just wanted to follow up, that after replacing the intake flanges (SDS kit, thank you @Mike S) *and* a 500 hour service/re-timing of the slick magnetos, the problem with #3 peaking too early is cured. Now #3 peaks at the same time as #4, and has 0.5 gph GAMI spread with #1 and #2 (vs 1.5 gph before the maint). There was no backfiring during startup so far either. Progress made :D
 
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