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Following problem w/ rotax 912 UL.

 Background info:

 Removed engine/avionics from old plane I had for many years. Engine always ran great. 

Installed it in my new zenith build. First start in ran smooth for a while and then it started running rough, I noticed fuel coming out of overflow lines so I shut it down and cleaned the carbs out. Prop was installed for this run.

Then:

Went to start again. Lots of backfiring. Only running on CYLS 3 and 4. Pulled prop off.

Swapped pig tales on top module (in my case older engine four pin). Then it only ran on CYLS 1 and 2.

Swapped pig tails out on bottom module ran on two cylinders.

 Put it all back together ran on all 4 cylinders. At this time I didn’t have the prop on. let it idle while monitoring temps. Ran good. All cylinders go. Everything perfect.

 Put prop back on.

Tried to start it up. Only running on 3 and 4.

Swapped pig tails for top module running on 1 and 2. Change bottom module out with one that I know to work but will not ground (in other words wont shut off). No change

 I am stumped.

Please help.

 

  • Re: Only running on Cylinders 3 and 4 how is this even possible

    by » 4 days ago


    How do you know that it is only running on specific cylinders, EGT?

    The way that the ignitions are configured means that if one dies the other one should spark a plug in each cylinder. You could use some series spark testers to see if you actually have sparks occuring during the fault condition.

    It is more likely that there is a fuel related problem. I would inspect the carbs and look for debris in the float valve and jets.

    I agree that from what you have described it is somewhat perplexing.


  • Re: Only running on Cylinders 3 and 4 how is this even possible

    by » 3 days ago


    I checked the jets. They are clean and installed per page 73-00-00 page 25 in the manual. 

    As I research i suspect trigger coils could cause this. 

    I did have to pull them to get the ring mount on...

    However, I would think it would have run poorly from the beginning if that were.the case? 

    Because it ran fine then started to misbehave it makes me think I may have lost a ground somewhere, but I just don't understand where I should look. 

    Oh and yes, EGT and the old fashioned hand on headpipe test is how I determined which cyls were running a d which not. 


  • Re: Only running on Cylinders 3 and 4 how is this even possible

    by » 3 days ago


    Let's confirm we are looking at the same modules.

    You have two ignition modules.
    EACH module has a 4 pin connector, a Ground Wire and 6 Single-pin connectors.
    The 6 connectors at the module are White, Yellow, Orange, Brown and Two Blues.
    Plus the Black Ground Wire.
    Do we agree ???
    - - -

    The Black Ground wire should be Grounded.
    The Brown Wire should NOT be Grounded. (Mag Switch)
    - - -

    Either module is capable of igniting all 4 cylinders on its own
    You have two problems, Probably related.
    When you see the problem again, run a Mag check.
    I am guessing One Mag will have NO effect and the other will kill the engine.
    On second thought, Run a Mag check.

    Closely inspect your 4-Pin Connectors.


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


  • Re: Only running on Cylinders 3 and 4 how is this even possible

    by » 3 days ago


    Well yes and no. 

    I have the old style modules that look like this (attached). You are correct about the ignition switch, when this happened one switch had no effect. 

    Also by the way I have mine set up with two switches for each module to ground out the engine and a push button start, I had it this way in the old plane too and prefer it.  

     

     

    30653_2_Screenshot_20210914-214459_Brave.jpg (You do not have access to download this file.)

  • Re: Only running on Cylinders 3 and 4 how is this even possible

    by » 2 days ago


    The ignition module you picture is shown in the Heavy Maintenance Manual Sec. 74-00-00 Pg. 31, Fig. 74-21.

    If swapping the White and Yellow Single Pin. Connectors also swapped the working Cylinders from 3/4 to 1/2; This indicates that the module is Bad/Intermittent OR the 4 pin connector is losing the 1/2 trigger coil.
    Measure from pin 1 to 2 and from pin 3 to 4 on the 4 pin connector.  Both readings should be similar.

    The Mag switch test shows that the Lower Module is NOT Working at all. (BRN Ign. Wire Grounded ???)

    Swap the Modules completely. Upper to Lower.
    If the problem moves you have a bad module.
    If it stays the same, you have a wiring harness problem.


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


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