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Mag check: Drops OK, but left mag rougher?

in 912 / 914 Technical Questions 13 years ago
About 500 hours on 912UL with all recommended service bulletins met FAIK.

Have owned it for last approx 80 hours. Runs great, sounds good, passed annual including compression check by A&P in April. Compression WELL within specs. Performance as expected.

When running mag check at 3800 RPM drop is about 100 when turning off left mag, _about_ 125 or 150 when turning off right mag. And when running on just left mag(right mag off) the sound is different, with a bit of vibration that's not there running on just right mag.

So SOMEthing is different on the left side ignition system.

CFI who is also an A&P suggested something about maybe a plug that's part of the left-side ignition system is not up to spec.

Visual exam of all plugs about 15 hours ago showed them all looking very clean and nice.

First:
Is this anything to be concerned about or just within the normal amount of variation one might find?
Second, if it's not quite normal, any ideas of how serious this is, what the cause might be, and what the cure might be?

All thoughts appreciated.

Al

  • Re: Mag check: Drops OK, but left mag rougher?

    by » 13 years ago

    Hi Al,

    A 100 on one side and 125 on the other is fine. It is well withing limits and the higher the rpm you do a drop at the wider the gap might be. I wouldn't worry about a 25 rpm difference.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

    Thank you said by: Al C
      
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  • Re: Mag check: Drops OK, but left mag rougher?

    by » 13 years ago

    Hi Roger. Much thanks for replying.

    A minor comment is that it's more like 100 down on one side and 150 down on the other (rather than 125 down.)

    It's not really the slight difference in drop itself that I have some concern about and was asking about.

    It's that with one mag off the engine is definitally having a bit of a vibration different and stronger than when running on just the other "better sounding" side. If I HAD to speculate I might wonder if it was running slightly out of balance because one cyclinder was not generating quite the same power stroke as its opposite(?).

    It just does not run as smooth and seem to be so-to-speak purring quite as nicely with one side of the mags than with the other.

    I'm not overly concerned -- It's run fine this way for over 100 hours (and as far as I know for the 400 hours on it before I owned this 912UL).

    I guess, short of specific suggestions about determining what the cause of the difference is and remedies if called for -- It would be comforting to hear from other owners that they have noticed similar differences in the quality/sound/vibration of the left vs right single mag run up.

    Al
      
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  • Re: Mag check: Drops OK, but left mag rougher?

    by » 13 years ago

    Al,

    You do not mention how many hours are on the plugs. If you are running 100LL I replace the plugs at 100 hours, with auto fuel every 200 hours.

    You can swap the plugs from one ignition to the other and see if the problem follows or install a new set gapped per the manual.

    Next is a good carb balance.

    If that does not take care of your rough mag drop, check the resistance reading on your plug caps. They should test 4.5-5.5 ohms. I have experianced several times this year excessive or rough mag drops atributed to high resistance as the plug caps age. Most of the bad plug caps show on engines with 1000 hours or more.

    If all is right on your engine the mag drop should be smooth and consistant from side to side.

    Jim
    Thank you said by: Al C
      
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  • Re: Mag check: Drops OK, but left mag rougher?

    by » 13 years ago

    Just a followup FYI for anyone interested.

    The slight vibration/roughness I reported earlier (in my first two posts in this topic) when running on one mag during mag check runup is now gone.

    Only explaination I can give is I put an 8 ounce bottle of STP carb/injector cleaner into the tank with the 15 gallons of fuel on the previous autofuel (only kind we use) fillup. Maybe that did it.

    Can't say for sure if that's what did the apparent fix, but glad that symptom, however mild, is gone. I speculate there was a little gum/varnish or something in one carb that got cleaned out.

    ----
    That brings up a related question: In general there seems to be a pretty reasonable rule around here of "don't use any additives (oil, fuel, whatever) that aren't officially tested and approved."

    I'll post that related-question in a separate topic in case anyone cares to read or respond to THAT.




    Al
      
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