1974 RV12, 320 hours.  SW Florida hot season. This plane always ran smooth with EGT's very close (10-30F diff.)

Several weeks after first experiencing rough running, problem persists.  Runs rough IN FLIGHT ONLY, and RIGHT EGT ALWAYS goes very low.  Much worse at high power.  Carbs have been synched twice.  Right carb removed and reinstalled 3 times finding no defects.  All plugs replaced, right side plug wire ends remade, no water or debris found in carbs or in gascolator or on gascolator screen.  Probes, R & L are correctly wired, and R just replaced and still shows identical behavior.  Test flight today:

Conditions 85F, high humidity. Field elevation 30 ft.1. 4000 rpm runup, ignition check drops: 140 L/130R.2. W.O.T. on ground: 5170 rpm, drops within 30 rpm.

Climb out on takeoff immediately after above numbers: 5230 rpm, 1370 L/1240 R.  Pattern level flight at 4700 rpm, 1400L/1315 R.  Climb starts rough running: at 1800 ft, 5100 rpm climbing, EGT's 1420L/1211 R.  Level flight 2000 ft, 5200 rpm 1420L/1211 R Rough running.  Full throttle immediately runs even rougher.  However, reduced throttle, level flight 1900 ft, 4800 rpm, egt's level out some: 1390/1320. Not smooth, though.

After this flight, on ground 4000 rpm ign. check: egt 1220/1257, mag drops 130/90. WOT 5130 rpm, 1200/1220!

The right carburetor is most likely, but what could cause this?  Obstructed main jet?  Leaking diaphagm?

Thanks for any help.

  • Re: 912 ULS Rough Running Low EGT - Solved!

    by » 3 months ago

    I suggest to you that.

    Weight the floats in dry condition.it should be less than 7gram

    Main jet on Right side is surely obstructed.

    EGT on higher Rpm is 40 to 50 F difference

  • Re: 912 ULS Rough Running Low EGT - Solved!

    by » 3 months ago

    Thank you xing.

    We also have weighed the floats twice. Under 6 g.  I thought of the main jet, too.  Will try that.  But then why would it not drop on a full throttle ground run?

    To clarify, the symptom has always been the Right EGT dropping immediately, with the Left staying where it usually is.  

  • Re: 912 ULS Rough Running Low EGT - Solved!

    by » 3 months ago

    1974? The RV-12 had its first flight on November 9, 2006.

    Need a little more info and things to consider and rule in or out.

    When was the last carb overhaul?  Not just remove and inspect. If it's been several years it's time especially since this cropped up.

    Check fuel filters, fuel lines and banjo bolt debris downstream of the fuel pump that goes to that carb.


    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-574-1080 Home (TRY HOME FIRST)
    520-349-7056 Cell

  • Re: 912 ULS Rough Running Low EGT - Solved!

    by » 3 months ago

    for EGT drop on Right side.

    i suggest check the RUBBER FLANGE ASSEMBLY may be crack that catch air. 

    all  other connection for carburetor Right side i.e O-ring under Intake manifold etc.

  • Re: 912 ULS Rough Running Low EGT - Solved!

    by » 3 months ago


    Thanks for the reply.  Plane is 2014.  Plane and carbs have 320 hours as per first line of post. Carbs both overhauled about 30 hours ago.  Fuel line to this carb flushed into bottle to check.  No debris.

    Strangest symptom:  happens in the air only, not at WOT on ground.

    Xing Wang:  will check rubber connector today.

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