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Hi Experts,

The engine and plane were perfectly fine on flights this past Sunday and Monday, but then when going to start it on Tuesday it barely started and wouldn't get above about 780 RPM.

In trying again this morning I noticed (and you can see in the video) that the VPX (electronic circuit breakers) was indicating that all the current was coming from the battery, even after switching in the alternators (see the attached charging circuit diagram).

Here's a video of the start this morning: https://youtu.be/uB9-wM6izyY

I did try it again flipping the INT ALT switch which I think if it's down, cuts out the Internal Alternator from the circuit.  No difference.

Thoughts on how to debug what the issue is?

Thanks!

Craig

 

9353_1_FirewallCharging.PNG (You do not have access to download this file.)
  • Re: 914 Barely Starts and Won't Get Above 780RPM (Video)

    by » 2 weeks ago


    This does NOT look like an electrical issue.
    The alternators are not going to produce any meaningful output at 700 rpm.
    I see the right side volt meter rises up to 14 v but it never connects to the Battery (12.9) on the Left.
    At 12.9v, the battery IS getting some Generator assist, just not enough.

    Your MAP is stuck at 29.8 BEFORE and AFTER starting.  ???
    29.8 is Ambient Atmospheric Pressure.
    What engine is this?
    You may be running on 2 cylinders.
    I would look at the intake manifold and the fuel supply.


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


  • Re: 914 Barely Starts and Won't Get Above 780RPM (Video)

    by » 2 weeks ago


    This is a 914.  Yes, the MP is atmospheric, but would you expect much change if it's barely running?

    Also, I see all the EGTs are moving up, so it does look like all cylinders are firing.  Maybe the air intake is clogged?  Strange considering it worked perfectly just the day before.

     

    Fuel pressure seemed normal too.


  • Re: 914 Barely Starts and Won't Get Above 780RPM (Video)

    by » 2 weeks ago


    If the throttle was Closed, you would expect to see well less than 20"
    If the throttle was Open, you would not expect to see more than 29" until above 4000 rpm.
    If the Air intake was blocked, your MAP would be below 10"

    In any event, you would not expect the MAP to remain rock steady at 29.8 and not move an inch unless the engine was Stopped.

    Fuel pressure is only one part of the story.
    Is there Fuel in the Carb Bowls?
    Are the Carb Bowls pressure equalized?
    Are all 4 EGTs at close to the same Temp?

    None of this information is going to be found in the cockpit, you are going to need to get under the cowl.

    The fact that it worked yesterday has little bearing on why it does not work today.


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


  • Re: 914 Barely Starts and Won't Get Above 780RPM (Video)

    by » 2 weeks ago


    I don't know if the MAP I'm measuring is the wrong location or sensor, but I've never seen, ever, a MAP below ambient.  See the attached screenshot of an entire flight a few months ago.

    The EGTs from the start yesterday, as can be seen in the video, are between ~400 and ~600, so I think they're all firing.  They don't get higher than that because it's starting cold and the whole sequence doesn't last long.

    I don't know about the other things as I haven't gotten that far yet - and I'm hoping I don't have to go that far.

    Craig

     

    33339_2_Screenshot 2022-08-03 193756.jpg (You do not have access to download this file.)

  • Re: 914 Barely Starts and Won't Get Above 780RPM (Video)

    by » 2 weeks ago


    It would appear that you are NOT measuring MAP.
    MAP is the pressure in the manifold between the Carb and the heads, typically measured at the crossover tube.

    The Operators manual pages 5-3 and 5-4 show that the pressure should be

    39" @ 5800 rpm  (115%)
    35" @ 5500  (100%)
    31" @ 5000  (75%)
    28" @ 4300  (55%)
    24" @ 3500
    21" @ 3000
    17" @ 2500

    A 2000 rpm idle should show ~12-15"

    Everything below ~4500 rpm will be at or below Ambient.

    If you can not measure the MAP you can not operate that AirMaster prop you have installed.

    I would look at fuel starvation first.  Is the Fuel Shut-Off valve ON?
    Is there Fuel FLOW (Not Pressure) at the Carbs?


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


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