• Re: Rough running/High EGT in Cylinder 2

    by » 3 months ago

    I stand corrected:  The Top and Bottom Plugs are paired.


    You are correct that there is one coil that fires two plugs simultaneously.

    When one spark is firing in the compression stroke of one cylinder the other plug is firing harmlessly in the exhaust stroke of the other cylinder.

    Nearly every modern engine uses this method commonly called a Wasted-Spark ignition for obvious reasons.

    It is simple, reliable, and inexpensive to implement.  No distributor needed!



    Both plugs Fire and Fail simultaneously.

    Unless ... UNLESS ...

    Unless the spark for the failing plug is being redirected to ground by a  path other than the spark plug.


    There are going to either be Zero sparks, or TWO sparks.

    The question being, WHERE is the spark occurring if it is Not at the Spark plug??? 

    Pull the 1B and 2B Plugs.

    If the 2B plug is cold and the 1B plug is sparking, the 2B spark is occurring somewhere that needs to be located.

    Is could be within the ignition coil itself and a simple coil swap could confirm that possibility.

    Or it could be jumping out of the plug wire to ground somewhere.


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated.

  • Re: Rough running/High EGT in Cylinder 2

    by » 3 months ago

    Thanks Bill.

    I will proceed along your suggested diagnostic path when next at the airport and revert.

    My understanding is normally both spark plugs in each cylinder fire simultaneously on every compression stroke of every cycle (to increase combustion efficiency etc).

    Is it correct that each Lane is exclusively responsible for firing only one of the two spark plugs in each cylinder (as a magneto in a conventional magneto ignition)? Or can each Lane  simultaneously take control and be responsible for firing both spark plugs in each cylinder simultaneously on each compression stroke if the other Lane fails completely or is turned off.   So, for example,  during 'run-ups' when one Lane is turned off,  does one spark plug in each cylinder stop firing on each compression stroke, or does the Lane remaining on take control and ensure both plugs continue to fire?

    My thinking is that if Lane A is exclusively responsible for firing 2B spark plug (which in my engine is not sparking when cranking motor over) and Lane B is exclusively responsible for firing 2T spark plug (which in my engine is sparking when cranking motor over), this may be consistent with the observations of engine operation detailed in A. in my opening post (re-iterated below and in more detail) .  

    A. with both Lanes turned on and engine started: engine runs a little rough with cylinder 2 EGT rising immediately after start, to reach approximately  170-200 F higher than the other three cylinders .  Neither Lane A or Lane B warning lights come on and there is no Dynon Skyview annunciation of engine alerts/faults.  If Lane A switch is turned off and Lane B left on,  the engine changes from running a little rough to perfectly normal/smooth operation, RPM increases by approximately 200 and cylinder number 2 EGT reduces to the same as the other three cylinders within 15-30 seconds.  If Lane B is turned back on again RPM reduces by approximately 200 rpm, EGT in # 2 cylinder rises again and levels off at approximately 200 F above other cylinders within 15-30 seconds and engine runs a 'little' rough.  If Lane B is then turned off and Lane A remains on,  the engine runs very roughly, RPM reduces by approximately 400 and cylinder# 2 EGT reduces quickly to zero. (ie no combustion in cyclinder # 2)





  • Re: Rough running/High EGT in Cylinder 2

    by » 3 months ago

     The MM and diagram seem to indicate as you say that only one plug fires when one of the lanes is turned off.  It looks like Lane A powers coils 1 and 3 and the four bottom spark plugs while Lane B powers coils 2 and 4 and the top spark plugs. However, there is probably more to the story that only electrical experts like Bill can figure out?

    "In normal
    Dual LANE Mode (Auto Mode), both spark plugs
    ignite each cylinder at the same time. In LANE A
    or LANE B Mode, only one spark plug ignites per


    25323_2_Ignition diagram.jpg (You do not have access to download this file.)

  • Re: Rough running/High EGT in Cylinder 2

    by » 3 months ago

    I might take a re-read or two, of this to get it straight...


    Lane A (Coils1+3) fires the Bottom plugs. Lane B (Coils 2+4) fires the Top Plugs.

    The coil are number based on the cylinder that they are mounted nearest.

    Lane A, Coil #1 fires 1B and 2B 

    Lane B, Coil #2 Fires 1T and 2T 

    Lane A, Coil #3 Fires 3B and 4B 

    Lane B, Coil #4 Fires 3T and 4T 


    The plugs in Cylinders 1 and 2 fire simultaneously (1B+2B or 1T+2T); one will fire the compression charge and one will be waisted firing harmlessly in the exhaust.

    One revolution later the compression and exhaust cylinder swap cycles.  Similar for Cyl 3+4.


    Now it gets even messier...

    The plugs on a single coil fire simultaneously but the plugs in a single cylinder Do Not.

    The A and B plugs (Top and Bottom) fire on alternate cycles.

    The firing order is  1-4-2-3-1-4-2-3-1-4-2-3-1-4-2-3 but the plugs fire A-A-A-A-B-B-B-B-A-A-A-A-B-B-B-B ...  Plugs (1B-4B-2B-3B-1T-4T-2T-3T-1B-4B-2B-3B-1T-4T-2T-3T)

    Keeping in mind that 1B and 2B originate in the same coil: The coils will be firing 1-3-1-3-2-4-2-4-1-3-1-3-2-4-2-4


    If lane A fails (Solid Lane Light), only the lane B plugs will fire. B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B, Coils 2-4-2-4-2-4-2-4-2-4-2-4-2-4-2-4

    - - -

    This is why the 912is Lane (Mag) check does not produce a noticeable rpm difference.

    The 912iS engines always fires just 1 plug; A, B or Both is still just 1 plug. Bottom, Top or Alternating.

    The 912ULS engines normally fires Both plugs so a Mag check firing only 1 plug produces a noticeable rpm difference.

    When the 912iS engine goes into single-lane mode it also slightly enrichens the mixture for extra power so that a Lane(Mag) check will often cause the RPM to Increase instead of drop.

    - - - It is as clear as Mud but it covers the ground!  undecided

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated.

    Thank you said by: Jon Tensfeldt

  • Re: Rough running/High EGT in Cylinder 2

    by » 3 months ago

    Thanks Bill. That makes sense and it correlates with George's experience. smile

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