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Disconnected soft start input on 912ULS

in 912 / 914 Technical Questions 11 years ago
Hi:
I have a 912ULS with the soft start CDIs. I had them connected, but I didn't like how the engine started on them so I disconnected one.
My question is:
If I disconnect the soft start signal input (disconnected at the terminal block on the firewall) should I ground this input to the CDI to prevent false triggering?

I recently had a situation while doing the preflight mag checks that made me think the disconnected input was triggering the soft start function. The input was just open circuit disconnected. I've grounded it now, but I was wondering if that will damage the CDI.

  • Re: Disconnected soft start input on 912ULS

    by » 11 years ago

    Hi Scott,

    You should really leave them connected. You negate the advantage of the soft sart system. Now you have one ignition firing at 4 degrees before TDC and one firing at 3 degrees ATDC. One running normal from the first start nd the soft start side on a delay of 5-7 second. The whole idea is to start with less stress, wear nd tear on the engine.

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

      
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  • Re: Disconnected soft start input on 912ULS

    by » 11 years ago

    Roger: Thanks for the quick reply. Actually, I have disconnected both the soft starts. But my question really was whether grounding these inputs to stop false triggering would hurt the CDIs.
      
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  • Re: Disconnected soft start input on 912ULS

    by » 11 years ago

    Hi Scott

    All you need to do is remove the 12volt input to the CDI unit and this will prevent the soft start sequence. I'm not sure if grounding them would damage the modules, I doubt it very much.

    Hi Roger

    Just one point about leaving one soft start connected. Both ignitions would fire at either 4 degrees before or 3 degrees after top dead centre as it is the flywheel hub that controls this timing. The only thing that would happen is 1 module would go straight to advanced timing and one would be held on retarded ignition for a few seconds which would be a complete waste of time.
    I agree that they both should be left connected.

    Mark
      
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  • Re: Disconnected soft start input on 912ULS

    by » 11 years ago

    Hi Mak,

    Good point. (timing). I let that one slide right past me.

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

      
    Report

  • Re: Disconnected soft start input on 912ULS

    by » 11 years ago

    Hi all and thanks for the advice.
    However, I disconnected these inputs because the engine was hard to start with them connected. When the engine was cold, I had to pull the choke all the way on to get it to run and when the soft start cut out, the engine would jump as bad as it does without the the soft start.
    When the engine was warm, I would have to grind away on the starter for 10-15 seconds to get the engine to catch and then it wold start like the old radial engines, seemingly on one cylinder at a time and slowly catching more until WHAM the SS would cutout and the plane would nearly take off from the increase in RPM. Nope, I don't like the soft start much at all. Apart from the above, my engine runs fine.
    So reconnecting the soft start is Not an option for my airplane.

    In any case, I think I worked out the answer to my question with regard to grounding the inputs.
      
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