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The KitFox Vixen has now found a warm home in my hanger and doesn't want to leave.  The aircraft with an early 912UL and 375hrs came to the shop with fuel flooding the float bowls of both carbs pouring out the vent tubes resulting in extreme rough idle/low rpm operation.  In the past year, she has had a five year rubber replacement, a re-engineered fuel system because she didn't have a return line.  Complete carb overhauls including all the floats, float needles, and associated hardware.  A fuel pressure gauge was added.  An engine monitoring system was added.  I've performed every fuel system test recommended by Rotax and known to man.  I am now looking elsewhere.  The differential compression check is excellent with a range of 84 - 86 at 87 psi.  Borescope inspection showed good crosshatch and good valves in each cylinder.  The new fuel pressure gauge reads a steady 4psi.  The new engine monitor does give a low EGT indication for #2 cylinder at idle.  However, when the throttle is advanced to higher rpm 3800 or better all the EGTs line up.  There is something going on in that #2 cylinder at low RPMs.  Like I said the compression is good 85 @ 87psi.  I have checked both plugs for gap and resistance, they are new by the way, both new spark plug connectors for resistance, and both cables from the coil.  Everything is within limits.  How do I go further and test the coils, DCDI boxes, and trigger coils/stators, etc.?   How about the valve train for the #2 cylinder?  Could there be a bent pushrod or weak or leaking lifter or weak valve springs?  Could sticking valves in that cylinder cause the rough idle and not be a problem at higher RPMs?  This engine doesn't fall in the serial number range of any of the recent service bulletins about these items.  Still doesn't mean there can't be a problem there.  What else could possibly be causing the low EGT, resulting in the extremely rough running at idle that smooths out at higher RPMs?  My frustration level is thru the hanger roof and I have to remind myself that nothing I did during the rubber replacement or other work caused the problem.  She came into the shop running like that.  HELP...!!!


Jeffrey Fritts, USAF (ret.)

www.flywwlsa.com

"In aircraft maintenance, good enough is not good enough."

  • Re: Low EGT at idle in one cylinder...

    by » 5 weeks ago


    Very interesting story.

    Would you like to tell the lower EGT value  2/4 side ?∆ 50°C above is acceptable at lower rpm.any how, Lower value of EGT indeicat  rich mixture. Or faulty slow running jet in carburetor.


    Thank you said by: Jeffrey Fritts

  • Re: Low EGT at idle in one cylinder...

    by » 5 weeks ago


    The low #2 cylinder EGT could also be due to a lean mixture or an ignition problem and that cylinder not firing.  At this time the idle is so rough, 2.0 IPS according to my prop balancing equipment, that the float needle on the right carb, cylinders 1/3, will not stay seated and the float bowl is flooding and pouring fuel from the vent tube.  The left carb, cylinders 2/4 is not.  Both carbs have been rebuilt to factory new standards.  If the mixture were rich on the left side wouldn't it show as a low EGT for both the cylinders on that side?  That is why I wonder if there might not be an intake manifold leak just on #2 cylinder or since the compression in that cylinder is good there might not be a valve train issue.  Remember this is only happening at idle and in the low RPM range.  Once the engine RPM is brought up it seems to run fine and all the EGTs line up straight across.  

    I just remembered an old mechanics trick for locating leaking intake manifolds.  With the engine running at idle you spray ether starting fluid around the suspected leaking area and the engine  RPM should pick up and run smoother at the point the vacuum of the intake manifold draws the ether in.  Something else, the carbs have been balanced using a dual pointer manifold pressure gauge, like from a twin-engine aircraft, and the needles move together throughout the entire RPM range.  That doesn't mean however that I didn't adjust the carbs to match each other including the possible manifold leak.  So they actually aren't matched at all but one side is working as it should and the other side is adjusted to compensate for the leak if there is one.  


    Jeffrey Fritts, USAF (ret.)

    www.flywwlsa.com

    "In aircraft maintenance, good enough is not good enough."


  • Re: Low EGT at idle in one cylinder...

    by » 5 weeks ago


    Are the probes new? If not swap the EGT probes and make sure it is not a probe that is starting to fail, they have a life span of about 200 to 500 hours


    Thank you said by: Jeffrey Fritts

  • Re: Low EGT at idle in one cylinder...

    by » 5 weeks ago


    Garrett,

    Yes, they are brand new and were installed with the AvMap EngiBox.  I had thought about switching the wiring at the EngiBox to see if the low EGT reading moved with the connections.  I just haven't gotten around to that test yet.  Thanks for bringing that up.  


    Jeffrey Fritts, USAF (ret.)

    www.flywwlsa.com

    "In aircraft maintenance, good enough is not good enough."


  • Re: Low EGT at idle in one cylinder...

    by » 4 weeks ago


    Hi Jeffery

    My suggestion would be to test the valve spring tension, particularly on cylinder #2.
    Recently I diagnosed a rough running early 912UL. Several of the valve spring pressures were about 50% below specifications.
    Replaced four dual valve springs and that engine runs nice and smooth. 

    Best regards 

    John Baker iRMT


    Thank you said by: Jeffrey Fritts, RotaxOwner Admin

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