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  1. #1
    Senior Member Paul's Avatar
    Join Date
    Dec 2011
    Gilbert, Arizona

    Default Compression Drop

    Hey guys, I'd appreciate any feedback or suggestions on a recent drop in compression on two (even) cylinders.
    Some background:
    I have 280 hours on my CCEX. (flying since January 2015)
    Compressions at time of last annual, this past spring, at 215 hours were great 78/79/78/78
    I always lean out, while taxiing and flying.
    Temps have always been super stable....Oil around 155 to 160, CHT's around 360 to 380, EGT's around 1000 to 1200
    Oil changes have varied between 25 hours and 35 hours.

    So here's the current situation...
    Changed the oil this week after 35 hours and it came out super black.
    Checked compression and I have two cylinders in around 67.
    Other two cylinders are 77/78
    Escaping air is coming from exhaust.
    Pulled valve covers and A&P working with me observed what he thought is some carbon buildup on the exhaust valves.
    cylinder walls look great
    no metal in the oil or oil filter

    So my questions for the forum...
    1) Does this all add up to the same conclusion for you....that the drop in compression is due to blowby on the exhaust valves?
    2) Is it possible the problem is the push rod clearance that needs adjusting (as suggested in the 340 Engine Problem thread) ?
    3) If there is carbon deposits on the exhaust valves why would they appear so suddenly? (my oil was not this black on the last oil change)
    4) Why carbon deposits only on two cylinders?
    5) What do you suggest I could do to get rid of the carbon deposits if that's indeed what the problem is?

    Thank you,
    Last edited by Paul; 11-18-2016 at 09:09 AM. Reason: Just adding some detail

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