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Borescope Pics - yellow deposits

YellowJacket RV9

Well Known Member
Patron
Working on my condition inspection today, and borescoped all cylinders. All the exhaust valves look excellent (2 years, 200ish hours on the cylinders). Compressions 74,76,78,78

Only odd thing is the yellowish deposits around (not on) the valve face in in cylinder #4. Does not occur in any other cylinders. Any comments?

8iYecWVMnadH5Ebu9LjvuTWo3shV0brAuvmah_5LZ9wzUQXLcxlyQXgsNJ5oGsydqTLoo6hq_r1tLPKFmUbUWbHpTmeU9TSMekuKeRqpzezOXbsCh0Do58uI-hD19f256PQvl6-D6J_w8qHQOyswwtPmJvFB6OEO8OGpuqDM1FcLSX0LieArjS2g3wvYq6b_Jr46B1cJXysmSBKHLUsZ7bAJubZyypMINgYwoJfhNGY8mQ-RtnUEoA3C7VCF4U5rRmkslCZWuslvuuVnslNr_a7Rex5RJcXDPgsYTictZiSWXhVfI01zaXEoPaeMBW1nsRJmymv94nfcsK_urnsnKQd62X9q8A4v9TUlSLmLJt1pJ1Rhaq9r46uO2jc_tuWDN2NADN6x6MJuEA8B3eRpkX-qB0kgwydp8vNJC9N1ttEbDW28I8QWgBs2KbEjh1c_oGVVjQyCnWOMVLuuABtjk9ZfqCYWnMePZBaT2-53MQ1Q1azVevvL7Eqq7oMABFQeiaIzWA89B2KlgOJ1v1EzFvCx-Zc4zXzdnpK_iEkJy3VGYwHJUvB5TNnJ6dCl1oml3m0dh7NqMrAnJH3Rc3n8UKRCLyO4XecA7QQZ_odRThM5Gyf8qwIAtLZgShGciSpQ0jlItH6jio1pwgIF-gB2Fb_wlzFNHaWe=w640-h426-no

If the pic doesn't work: https://photos.app.goo.gl/KRN8ysdBQqb54amx7

Chris
 
I don't trust the color rendering from my Vividia-400 unit, don't know about yours.
 
What kind of EGT is that cylinder running compared to the others is it cooler? The deposits look similar to the stuff that accumulates in spark plugs. Any way that cylinder could be running richer? Is the engine injected?
 
I just reviewed the logs and #4 doesn't appear to be hotter or cooler than the others. It is carbureted, not injected, so mixture could certainly be different in that cylinder though. It did strike me as looking similar to the stuff I pulled out of the plugs.

Chris
 
Is that oil? Any chance you could touch it with your camera, remove the camera, and check for residue on the camera? That, or cobble something up to rub that area with a Q-tip?
 
Good call on the fuel. Alcohol in mogas can contribute to this. So can detonation. Look at spark plugs in that cylinder, especially too high heat range. Exhaust restriction in that cylinder, timing to far advanced.

We used to blame this on lead but I'm not convinced that's the issue. Maybe lead in conjunction with lean running for some reason.
 
What kind of fuel are you running?

Good call on the fuel. Alcohol in mogas can contribute to this. So can detonation. Look at spark plugs in that cylinder, especially too high heat range. Exhaust restriction in that cylinder, timing to far advanced.

We used to blame this on lead but I'm not convinced that's the issue. Maybe lead in conjunction with lean running for some reason.

I run 100LL exclusively, and for the last 10 hours or so have also started Decalin, although I have never had any lead fouling issues. I generally am pretty religious about leaning, both on the ground and in flight. I run REM40E plugs, and the #4 plugs looked similar to all the other cylinders when I removed and cleaned them. Should I consider plugs with a lower heat range?

I just checked the timing, and it is still dead on at 25*. How would I check for an exhaust restriction?

Given the poor mixture distribution I suppose it could be too lean, although the in-flight EGT's don't seem to suggest that. Engine runs like a top.

Thanks for the input

Chris
 
The exhaust valve presentation is solid. However, the deposits do not match the chart. Does the OP use mogas? Are these phosphor deposits from mogas? Small displacement Contis will sometimes get sticky valves from just Mogas (Jack Norris Luscombe publishings).

Is this the richest appearing cylinder from the plug deposits? I assume as it is #4 it is the hottest.
 
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I am pretty familiar with that info, however to me the valves themselves look great. I am more wondering about the areas around it. It very well could be nothing, or just lighting...

The exhaust valve presentation is solid. However, the deposits do not match the chart. Does the OP use mogas? Are these phosphor deposits from mogas? Small displacement Contis will sometimes get sticky valves from just Mogas (Jack Norris Luscombe publishings).

Is this the richest appearing cylinder from the plug deposits? I assume as it is #4 it is the hottest.

These cylinders (ECI nickel, post-SB) have never seen mogas. Due to the carb setup, 3/4 are richest at WOT, but leanest in cruise. Still all within normal limits, however. Never any signs of detonation. Spark plugs looked normal, with normal, small amounts of lead buildup in all of them. Color looked good with no oil fouling.

Chris
 
I missed 100LL only in post 9. Either the color rendered is off or it does seem noteworthy but in the context of a healthy appearing exhaust valve face.
 
Your engine is perfectly fine.

The yellow crud is probably Lead Oxybromide, made famous by the ATSB :rolleyes:

It probably means you had a pretty cool cylinder while taxing for a while. It burns off quite readily.

What you are looking for is any sign of eccentric scale burn. You have none.
 
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