Have just balanced the Carbs on a 912 ULS installed in a BushCat. All balanced up nicely at 3500 and 1900 - needed some adjustment but now all good. I had the balance tube disconnected and long tubing into the cockpit and connected to the 2 Vac Gauges. At idle, if I pull the right manifold tube off the gauge and leave open to atmosphere, it causes the engine to almost die as you would expect and you have to quickly put your finger over the end of the tube to prevent it from dying but doing the same with the Left causes the engine to pick up significantly in RPM when left open!!

The only explanation I have for this is that the Left Carb must be running excessively rich and by opening the tube to atmosphere is sucking in more air allowing a better leaner mixture?  It doesn't blow any black fuel smoke from the exhaust and seems to run quite smoothly so this doesn't make much sense.

I was just going to check and adjust the mixtures but unfortunately to do this on the BushCat installation it is basically impossible due to a Stainless Steel Heat Shield attached under the Carbs. You would have to remove both Carbs to check/adjust it and I just can't believe it could be that far out - the engine and aircraft are only 2 years old and 55 hours TIS. Surely it was set properly at installation.  The 'only' way to be able to tune the carbs would be to drill holes in the heat shield plates under the mixture screws and even that would be very difficult to achieve in-situ.

The idle balance had to be adjusted only a tiny amount from the nominal settings and I would have thought that a mixture that rich on one side would have affected both the idle speed and balance significantly?

It runs fine and smooth but it really has me intrigued as to why the difference between sides??

Any ideas?


  • Re: Carb Balancing issue

    by » 4 days ago

    It’s been my experience that it’s better to leave the mixture screws at factory settings and adjust the cable tensions. Checking vacuum leaks at different locations won’t help with any balance issues.

  • Re: Carb Balancing issue

    by » 4 days ago

    I would leave the mixtures screws at factory settings. There is more to learn about what affects carbs and and why most people should leave factory settings alone in the carbs.If they are synced now then leave wellenough alone. Rotax has been doing this for decades andhas good reasons for doing things a certain way even when we don't see it. Millions of run hours is good solid data.

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

  • Re: Carb Balancing issue

    by » 4 days ago

    The only thing is, you cannot just assume that the "factory settings" on the idle mixture screws are actually where they are supposed to be.  One of mine was not in the specified position, and I have no explanation for how it got that way, having owned the engine since new.

    If you have never checked the idle mixture screw positions, then you have haven't done the sync properly!  And yes, I have to remove the carbs from their sockets to get at these screws on my installion too.

  • Re: Carb Balancing issue

    by » 4 days ago

    Hi Don,

    Not sure if you have enough clearance to get between the fuel drip tray and the carb body with one of the many tools like this one:


    I recently had reason to do some carb inspections on a brand new 912ULS (had only 13hrs TTIS) and found the idle mixture screws where not at the factory 1.5 Turns as specified. One carb was more like 1.75 turns and the other more like 2.25 Turns .

    Agree with Rogers comments, stick to the factory specs unless very good reason to deviate.

    As for one cylinder bank performing different to the other when opening/closing the line, did you observe much difference between the intake vacuum levels between the two?


  • Re: Carb Balancing issue

    by » 2 days ago

    Yes, thanks Roger - I was only intending to check that the mixture setting position was correct because of the strange difference between sides when I allowed the extra air to enter into the manifolds. With the carbs perfectly balanced as they are, I would have expected the engine to react almost identically to the extra air coming in through the left 'or' right tube when left open?

    I'm 'almost' sure I had no leaks on my tubes but I'm now wondering if one of my clamps was allowing a vacuum leak and even though my gauges were showing balanced, maybe they weren't??  On the weekend I will go and redo it and double check all the clamps for leaks.

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