Carb balancing on a ROTAX 912

H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

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FastEddieB
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Carb balancing on a ROTAX 912

Postby FastEddieB » Thu Jul 06, 2017 3:38 pm

As background, I had jumped into a thread on the topic on the VANS forum and gave bad advice concerning looking for slack in the cables at idle. I was corrected and blamed my past experience balance BING carbs on airhead BMW motorcycles. A good example of both the Law of Primacy and Negative Transfer.

I just did a carb balance on my Sky Arrow as part of this year's annual condition inspection, and posted my learning experience back to the VANS forum. I thought it might be of interest here as well...


As a follow up, I just did my carb balance as part of my Sky Arrow's annual condition inspection. It's not on my checklist, and I did not get around to it last year and figured it was about time, though my engine did feel about as smooth as always.

Everything is a little tougher with a high-mounted engine and a pusher prop. This time, I hooked up long extension hoses using some oxygen hose I had and ran it up to the cockpit:

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It appears I was overdue:

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Anyway, the Law Of Primacy is still strong with this one - in spite of the previous discussion in this thread, needing more throttle on the left side led me to adjust the barrel adjuster to take slack out, which, of course, is backwards - you need to screw the adjuster in to allow the springs to pull the throttle lever towards a more open position. All opposite to my BING experiences on BMW's.

I was wondering why my adjustments - in both directions - seemed to be having little or no effect. I'd loosen the lock nut, screw the adjuster in or out and tighten the locknut again. I was missing the fact that the adjuster on these carbs are not threaded into the support arm - I thought they were. After that "Aha!" moment, it only took two tries, adjusting the nut on the barrel side, to arrive at this:

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It appears about 1/2" off in the photo, but the two vacuums were tracking pretty perfectly throughout the rpm range. I think there may be a touch of parallax error in the photo as well.

Thanks for the input and corrections - I think it finally got through to me!
Fast Eddie B.
Sky Arrow 600 E-LSA • N467SA
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3Dreaming
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Re: Carb balancing on a ROTAX 912

Postby 3Dreaming » Thu Jul 06, 2017 5:45 pm

Your checklist may be different, but most of the Rotax powered airplanes I work on look to Rotax documentation for the engine. Checking the carb balance is on the Rotax 100hr/annual checklist.

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FastEddieB
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Re: Carb balancing on a ROTAX 912

Postby FastEddieB » Fri Jul 07, 2017 7:49 am

Thanks for the input.

Here's the relevant part of the Sky Arrow checklist:

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In a year where I only fly around 40 hours, I don't think skipping a carb balance is the end of the world. On bikes it's generally done "on condition" - you know when things are out of balance.

That said, doing it every year regardless has the added advantage of keeping the procedure fresher in my mind, so I plan on doing that from now on.
Fast Eddie B.

Sky Arrow 600 E-LSA • N467SA

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TimTaylor
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Re: Carb balancing on a ROTAX 912

Postby TimTaylor » Fri Jul 07, 2017 12:07 pm

FastEddie, the only problem I see is you're not flying enough.
Commercial Pilot Airplane Single & Multiengine Land; Instrument Airplane;
Sport Endorsement Airplane Single Engine Sea;
Flight Instructor Airplane Single And Multiengine;
Ground Instructor Advanced Instrument

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FastEddieB
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Re: Carb balancing on a ROTAX 912

Postby FastEddieB » Fri Jul 07, 2017 2:40 pm

TimTaylor wrote:FastEddie, the only problem I see is you're not flying enough.


No argument from me. Just flew .5 on a return-to-service post-annual test flight. May head over to Gainesville for a fly-in tomorrow. And Karen wants to return to Kitty Hawk - she was under the weather when we were just there in our travel trailer.

Image

"We really need to fly more often" is a common refrain around here!
Fast Eddie B.

Sky Arrow 600 E-LSA • N467SA

FastEddieB@mac.com

TimTaylor
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Re: Carb balancing on a ROTAX 912

Postby TimTaylor » Fri Jul 07, 2017 2:48 pm

I'm flying up to KGMU for $240 burger Monday, weather permitting. Should arrive around 12:30pm.
Commercial Pilot Airplane Single & Multiengine Land; Instrument Airplane;
Sport Endorsement Airplane Single Engine Sea;
Flight Instructor Airplane Single And Multiengine;
Ground Instructor Advanced Instrument

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drseti
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Re: Carb balancing on a ROTAX 912

Postby drseti » Fri Jul 07, 2017 4:53 pm

I like your signature line, Tim. But, dont you think incorporating ALL of 14 CFR in every post is a bit of overkill?
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof. H. Paul Shuch, Ph.D., CFII, LSRM-A/GL/WS/PPC, iRMT
AvSport of Lock Haven
fly@AvSport.org
http://AvSport.org
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Re: Carb balancing on a ROTAX 912

Postby TimTaylor » Fri Jul 07, 2017 5:59 pm

No, it saves me a lot of grief.
Commercial Pilot Airplane Single & Multiengine Land; Instrument Airplane;
Sport Endorsement Airplane Single Engine Sea;
Flight Instructor Airplane Single And Multiengine;
Ground Instructor Advanced Instrument


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