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How fast does internal engine condition vary with the operating regime?

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I have previously written that experiments suggest that spark plug condition depends most on the last few minutes of the engine’s running. So if e.g. you rev it for a bit before shutdown while leaning it aggressively, otherwise oily plugs are likely to be quite dry.

However I don’t think this applies to deposits in the combustion chamber generally. I had a sticky valve repaired recently and have been borescoping that cylinder regularly (after every couple of flights – it takes just a few minutes) in case the sticking was due to something upstream e.g. a lack of oil coming up the gallery (the #2 cylinder is the last one being fed off that LH gallery).

However the other day I borescoped the next one along (#4) and found that the exhaust valve looks just like it usually does, while #2 is still very clean:

Both have the same (great) compression, BTW.

I am still “running in” the newly repaired cylinder, so doing a mixture of ~85% and 65% power settings. The oil consumption is settling down.

Shoreham EGKA, United Kingdom
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