rough running 912 ULS

by Walt Shipley in General Tech Discussion 9 years ago

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Re: rough running 912 ULS

by Walt Shipley » 9 years ago

Walt Shipley

3 5 0

I heard back from Lockwood on my ignition modules - both checked out okay. Joe at Lockwood
suggested I unscrew the idle jets from the carbs and clean them with carb cleaner &
compressed air. I did this, and after the modules came back, reinstalled them and started
the engine. Again I had rough running as I throttled up through 2800-3000 rpm. At 4000 rpm,
I got a drop of around 80 rpm on ignition B, and about 200 on A.

All in all, the engine seemed to run somewhat better than before. I also did a compression
test and all cylinders are normal.

Joe said he's seen this problem before, and that it usually "just goes away." Hopefully it
will do so in my case.

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Re: rough running 912 ULS

by Rob Seaton » 9 years ago

Rob Seaton

3 1.32k 240

You may have prop issues.
All 4 cylinder piston boxer engines have a rough spot around 3000-4000 rpm. This "rocking couple" is inherent and cannot be designed out.
If you have a 2 bladed Sensenich and the pitch has been set with the supplied stop pins then you may have a significant tracking and pitch problem.
The pins that come with the Sensenich are easy to use but it is also easy to get a slight variation between the two blades, depending on how hard you bump up against the pin. Due to the scimitar shape of the Sensenich blade a slight pitch difference will result in a significant out-of-track at the tips. This will cause rough running.
You need to check the tracking and set the pitch with a digital level, propeller protractor (not a junky plastic kind) or laser.
see this disscusion for more info: http://www.rotax-owner.com/en/rotax-forum/4-general-discussion/5313-vibration#10781

You also need to have the prop/spinner assembly dynamically balanced. Static balance is not enough.
Also, if money is no problem; 3 blade prop is inherently much smoother for the pilot.

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Re: rough running 912 ULS

by Roger Lee » 9 years ago

Roger Lee

127 5.2k 4.78k

I'm with Rob. Check the blades first and set them by hand and not the pins then get a dynamic prop balance.

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

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rough running 912 ULS

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