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Carbs will not synch

in 912 / 914 Technical Questions 6 years ago
Hello, I know there is a lot to read about this on here and I have read it all. Below is my issue.

Carbs will not sync pneumatically and my EGT right to left is 350f delta.

100 hours on 912ULS

1. I have gone through the carbs very thoroughly many times.

Weighed the floats all at 3.4 grams each (within spec)
Swapped choke between carbs (just a test)
I've swapped the slide/diaphragm assemblies to test (They are lined up)
I've Set float levels to 10.5mm ( I even swapped the float needle assembly between the carbs)
Verified all the stock jets and needle positions
Mechanical sync as per videos
Blown out every orifice with compressed air
Did compression check
Sprayed starting fluid at all interfaces while running to look for air leaks.



Engine runs pretty smooth, mag check seems normal. Power seems normal. Does not seem rough when throttling up or down through the mid range. At full RPM there is still 2 deviations on the vacuum gauges and adjusting the throttle position does not effect it much.

When I am trying to sync with the gauges, as I increase throttle, the left side rises and then works it’s way back down while the right side moves up pretty linearly as the throttle is advanced..

I have tried different needle positions between the two sides, I have messed with the air screws, I have swapped my test assembly to verify its not my tool.



Any info would be very appreciated.

  • Re: Carbs will not synch

    by » 6 years ago

    Hi Ken,
    Just for clarification is this a 912S (certified) or a 912ULS? What aircraft is it on? Did you do the required 25 hour warranty inspection? This sounds like a carb issue and from the post you may want to get some experienced help. Sounds like the carbs aren't properly setup and may have issues internally.
    A set of floats (2) should be less than 7 grams. Weigh each set separately. What is 34 grams? You shouldn't have swapped the chokes. Put them back and make sure the center punch mark on the end of the choke shaft is pointing towards the air filter side or towards what looks like a pipe coming out the side of the choke housing. Make sure the float armature is 10.5mm from the edge of the carb body to the top of the armature. You may have the diaphragms not properly lined up with the rubber tab in the carb rim indent. A mechanical sync only gets you in the ballpark. You then need to do a pneumatic sync.
    A call to a Service center or an experienced Rotax IRC would help you. This may be to in depth to do in writing. It may take a long time and generate a lot of questions from both parties.

    You're welcome to call me if you want.

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

      
    Report

  • Re: Carbs will not synch

    by » 6 years ago

    Typo, the floats are 3.4 grams each (within spec)

    The choke assemblies are back where they were, that was just a test swapping them


    The floats are absolutely at 10.5mm

    The dimple on the choke shafts are pointed towards the air filter.

    The slide diaphragms are lined up( I have swapped these as a test as well to see if issue followed the slide assembly)

    , I have read every post, and watched every video on this web site and have tried everything possible stated here before reaching out.

    The point of my post is that I am not successfully able to get a good pneumatuc sync so yes I am aware that they need to be pneumatically sync’d.

    It is not a certified engine and it’s on a Just Highlander.

    Thank you
      
    Report

  • Re: Carbs will not synch

    by » 6 years ago

    I just did an annual on a Just Aircraft Highlander and will do its rubber replacement here in a few weeks. I replaced the original vernier throttle cable setup. It was a PITA to try and sync and wouldn't stay where the pilot put it. Yours may need the same thing. We put in a McFarland throttle.The gauges should be helping diagnose the problem. Is it just one carb acting up? How do you have the gauges hooked up? Could there be a leak? If yes then it may need to be removed and inspected again. Something may have been missed.
    Were the carbs in sync when you did the 25 hr. warranty inspection? Did these just now get out of sync? Do you have experience doing the Rotax carb sync?
    I have lots of questions. Give me a call. Doing this one post at a time may take a long time.

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

      
    Report

  • Re: Carbs will not synch

    by » 6 years ago

    don't forget to check your throttle cables from each carb to the common point and ensure each one has exactly the same travel (stroke) and doesn't bind anywhere. The cable will pull the throttle closed, the spring pulls them open.
      
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  • Re: Carbs will not synch

    by » 6 years ago

    Sorry for the delay, I was at high sierra fly in camping.

    When I purchased the aircraft it had 50 hours on it, I doubt that a true 25 hour had been done to the engine.

    The first thing I did was put the sync tool on it because it was running rough. When I finally found and cleared a piece of debris under one of the float needles I was able to get a perfect sync and the engine has run great since.

    I just did a new panel last month adding EGT to both of the aft exhausts. In cruise flight I saw a 350 degree difference between the right and left (they are the exact same temp when cold before the engine is started). This then led me to putting the sync tool back on the engine. I am now not able to get it even close to syncing. The engine does run smoothly and everything is in the green on my engine monitor.

    I swapped the sync gauges and hose to see if the issue moved to the other side and it did not.

    The throttle is setup nice and tight as are the choke cables.

    At 5200 RPM in cruise, I am seeing 1340F on the left and 1000f on the right. All of the jets are stock and the carbs are setup exactly as the videos show. Mechanical sync is dead on. Nothing leaking out of the carb overflow in the hangar.

    Again, it seems to run pretty smooth and make normal power. The EGT's would indicated that the right carb is rich but I have gone through it many times with no change (even swapping the float needle assembly and floats between the carbs, no change).

    Any other ideas?
    Thank you again for the help.
      
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