Lycoming O-320E2D dark Spark Plugs

LongEZ320

LongEZ320

Filing Flight Plan
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Nov 1, 2025
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Franco
In the last 10 hours, I’ve had the exhaust pipe for cylinders 1 and 3 black.

The spark plugs for these two cylinders are black, and the spark plugs for 2 and 4 are very clean.

We checked the timing and everything concerning the E-MAG installation, but all is perfect. We changed the spark plugs and separated the cable from the magneto to the spark plug, but it’s still the same. The compression two months ago was good.

Has anyone had a similar problem?

Thank you very much, and regards
Franco Laganà

Long EZ

OK-SIM

Lycoming O-320E2D
 

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That engine likely has a Marvel Schebler carb, or Precision Airmotive or some other permutation of the MS. There are a bunch of ADs on that carb, among them one against the two-piece venturi. That secondary venturi comes loose and can do weird things. It's a long shot, but if that carb hasn't had the AD work done (it's on an E-AB, right?) the venturi could be cocked and sending fuel up the left side of the carb bore, overfuelling #'s 1 and 3 cylinders.

The accelerator pump nozzle is on the other side, or I'd suspect that. The pump check ball springs can break and let the engine suck extra fuel out of that nozzle.

Which cylinders have primer nozzles? Lycs typically had three of the four cylinders primed, but if only the #1 and #3 nozzles weren't plugged, and the primer wasn't locking properly or there was debris trapped under the needle in the primer, it could be sucking extra fuel out of those nozzles.
 
Dan Thomas said:
That engine likely has a Marvel Schebler carb, or Precision Airmotive or some other permutation of the MS. There are a bunch of ADs on that carb, among them one against the two-piece venturi. That secondary venturi comes loose and can do weird things. It's a long shot, but if that carb hasn't had the AD work done (it's on an E-AB, right?) the venturi could be cocked and sending fuel up the left side of the carb bore, overfuelling #'s 1 and 3 cylinders.

The accelerator pump nozzle is on the other side, or I'd suspect that. The pump check ball springs can break and let the engine suck extra fuel out of that nozzle.

Which cylinders have primer nozzles? Lycs typically had three of the four cylinders primed, but if only the #1 and #3 nozzles weren't plugged, and the primer wasn't locking properly or there was debris trapped under the needle in the primer, it could be sucking extra fuel out of those nozzles.
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Thank you for your answer; it's a very interesting explanation. I've installed Marvel–Schebler MA-4SPA™. I must find the AD. regards Franco
 
followed the suggestion and changed the Marvel Schebler carb, but nothing happened. The problem with the exhaust pipes for cylinders 1 and 3 is still the same, and they are still dark. I really don't know what to do now.
I hope someone can give me another suggestion.
Thank you very much,
Franco
 
Magman said:
Consider an intake leak on 1-3 side.
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I would bet more on a leaking primer sending extra fuel to those two cylinders. A lot of O-320s had three cylinders primed, and maybe the OP has #1, #2, and #3 primed, with the #2 nozzle clogged. Not unusual to have clogged nozzles.
 
I agree the primer would be checked first.

However;if that doesn’t fix it my next step would be to press check

intake system.

Might be other suspects too.
 
I don't have primer, and the compression for all 4 cilinders are OK.
 
LongEZ320 said:
We checked the timing and everything concerning the E-MAG installation, but all is perfect. We changed the spark plugs and separated the cable from the magneto to the spark plug, but it’s still the same.
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So to clarify, the plugs you show in Post 1 are the top and bottom plugs from cylinders #1 and #3?
And you have two E-Mags installed, each firing one plug in each cylinder?
 
Are those spark plugs all the same part number? Or are the ones from the #2 and #4 a colder plug while #1 and #3 have hotter plugs that burn the carbon off?
 
Bell206 said:
So to clarify, the plugs you show in Post 1 are the top and bottom plugs from cylinders #1 and #3?
And you have two E-Mags installed, each firing one plug in each cylinder?
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Yes, the NGK spark plugs are both black for cylinders 1 and 3. One E-mag firing: 1 and 3 up, 2 and 4 down.
 
LongEZ320 said:
Yes, the NGK spark plugs are both black for cylinders 1 and 3. One E-mag firing: 1 and 3 up, 2 and 4 down.
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It almost appears as though there is a "blockage" in the sump intake spider altering the fuel mixture flow to the cylinders, especially since you have no primer and you swapped the carb.
Have you had any of the intake tubes off as part of this troubleshooting? If not, pull one or all and look inside the sump spider.
Was there any engine work performed prior to the previous 10 hours when you noticed this problem?
 
Bell206 said:
It almost appears as though there is a "blockage" in the sump intake spider altering the fuel mixture flow to the cylinders, especially since you have no primer and you swapped the carb.
Have you had any of the intake tubes off as part of this troubleshooting? If not, pull one or all and look inside the sump spider.
Was there any engine work performed prior to the previous 10 hours when you noticed this problem?
Click to expand...
Yes, I'll do it tomorrow, and I'll change the intake air gasket; maybe are damaged.
 
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