Spark plug

·         Doron Mishor

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My engine is Jabiru 3300, SN 33A831 (solid lifter), total 250 hours, Sonex AeroCarb.
Since 150 hours (or so) there is a new phenomenon happening in my engine: At warm up, the engine become rough, #3 EGT is very low, #3 CHT is lower than others.
Mechanical checkup shows:
1, Spark plugs 3L and 3R are covered with heavy soot deposits.
2, Compression leak down check #3 (cold) 80/72.
Replacing 3L and 3R spark plugs solved the problem and the engine run smoothly.
This phenomenon is repeated itself every 5 to 10 hours.
To eliminate electrical problem, 2x Distributor Caps and 12x High-Tension leads replaced with no help

I do not know what to do, please devised

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·         Doug Smith

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Hi Doron,
If it's happening to both spark plugs on that cylinder then it is unlikely that the problem is electrical as the two ignition systems are totally independent... but just in case, can you confirm that the plug gaps are right (Note that even new plugs usually need to be adjusted before fitting)? The only other electrical item that might be worth checking is the gap between the coils and the magnets on the flywheels but I think it's unlikely that is causing your problem.

What is the EGT on that cylinder when it has new plugs fitted (cruise and max power)? If that cylinder is running very rich it could simply be spark plug fouling. What fuel are you using?

Regards,
Doug.

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·         Doron Mishor

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Hi Doug,
Yes I do set the plug gaps by the book (0.55 - 0.6mm / 0.022" - 0.024"), I use auto fuel 95 octane no ethanol.
In this link you can see engine performance with bad 3L and 3R spark plugs; EGT #3 is higher than others for the first two minutes (warm-up). EGT #3 is un normal from minute 09:00 until end.
www.savvyanalysis.com/flight/1568058/6ff...fe-9f5f-817fb3f8036f 

In this link you can see engine performance with NEW 3L and 3R spark plugs
www.savvyanalysis.com/flight/1568061/d71...3d-b1dd-3125f8cb0046 

If you need, I can send you many more similar links
Doron

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·         Doug Smith

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Thanks Doran,

Interesting plots. Is each one a flight? The EGTs vary quite a bit in the flight... sometimes a bit high but often pretty low (about 622C). Have you looked at any of the other spark plugs? Are they black too?

I'm still thinking that cylinder #3 is running a bit rich - rich enough to foul the plug after a few hours operation. If that's the case then the other plugs - particularly for #4 - should also look pretty black.

I'm not very familiar with the AeroCarb: do you have in-flight mixture control on it?

Regards,
Doug.

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·         Doron Mishor

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Hi Doug,
These are two separate flights I chose to show the difference between engine performances before replacing the spark plugs and after with new plugs. All other spark plugs are OK no soot deposits light color.
The AeroCarb is very simple slide door carbi with in-flight mixture control manufactured by Sonex. www.aeroconversions.com/products/aerocarb/# 
Is it possible that too rich mixture will cause a spark plug stop working?
I think so too, #3 is very rich, I wonder what could cause one of the six cylinders to be so rich 🙁

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·         Doug Smith

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A mixture that's too rich can wreck spark plugs for sure. Most of the time your EGTs are quite even between the cylinders so it might just be that you're setting it a little too rich in the cruise or at low power?

It's normal for the EGT (and mixture) to be different between cylinders. It's not usually a problem, except when the differences get too large. There are several tricks with the Bing Carburettor that can affect EGT between cylinders - I'm not sure if any of them will work on the AeroCarb. With the Bing we try and make sure the air coming into the carb is as smooth and straight as we can - minimal turbulence, swirl etc. It might be worth asking Sonex if there are any tricks to help you?

Regards,
Doug.

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·         Doron Mishor

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Hi Doug,
My EGT probes position is 100mm down from the exhaust manifold mounting plate, the JEM3302-5 Engine Installation Manual call to Change EGT probe position from 100 to 120mm (which I didn't) maybe my EGT readings are not correct?
Also, What is a normal EGT changes when you do mag check?

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·         Doug Smith

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100mm should be ok, I would not expect that to change your readings significantly.
Well, doing a mag check on the ground you should be a bit richer than cruise I would think - around 650 - 700C perhaps? It would vary a bit depending on propeller, temperature etc.
Regards,
Doug.

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·         Doron Mishor

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Hi Doug,
Is it possible that this issue caused by an oil entering the combustion chamber through a inlet valve?
If so, how can we check it out?

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·         Doug Smith

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I would say that it is possible but it should be pretty obvious... if there's too much oil in the chamber and it's fouling the plugs you can normally see it as a shiny, oily, carbon, nasty contamination of the plug tip. If there's enough oil to do that then there is a good chance you'll see blue smoke when you start the engine or after it's idled for a while.

You said earlier that the plugs had "sooting" when they failed - can you share an image?

Regards,
Doug.

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·         Doron Mishor

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Hi Doug,
Here is a link to my plugs images:

goo.gl/photos/7S3XATgxJFtkZDGg8

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·         Doron Mishor

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Hi Doug,
I took high quality photos:
Spark plug 3L failure - is only 5 Hours,
Spark plug 3R failure - is only 5 Hours,
Spark plug 1R Good - this is 85 Hours plug for your reference as good plug.

www.dropbox.com/sh/b615sp1zpseub0g/AACZ1...3PrK0KhTHjwMOea?dl=0 

Doron

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·         Doug Smith

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Hi Doron,
The failed plugs do look a bit oily to me.
Does the engine use much oil or blow blue smoke (particularly on start or after it's been idling on the ground for a while)?
The next step might be to have a look at the back of the intake valve to see if it has burnt oil deposits, but I will see what the gurus think first 🙂
Regards,
Doug.