912 ULS Roughness

by David HEAL in General Tech Discussion 9 years ago

The 300-hour 912 ULS in my RV-12 has exhibited a bit of roughness in the 4,000 - 4,500 rpm range since new (all other rpms are smooth and the engine runs well). In an effort to smooth out this roughness I have:

- Balanced carbs and verified correct throttle movement throughout idle - WOT range. Carb vent tubes are open and properly positioned @ bale.
- Balanced prop (0.02 IPS @ WOT @ 5,000 rpm), tracked tips, and verified prop blade pitches equal with digital level.
- Installed new compliant carb floats.
- All compressions are good and engine starts and runs well.
- Gear box friction torque is within spec.
- The roughness does not appear to be airspeed or airframe related.
- FYI, I normally cruise @ 5,250 rpm (WOT @ 5,500' MSL @ 5,650 rpm).

Nothing seems to effect the roughness. Any other suggestions?

Thanks!

Re: 912 ULS Roughness

by Roger Lee » 9 years ago

Roger Lee

128 5.28k 4.98k

You are in the normal range that has a slight roughness from the engine design. It is also one of the leanest points in the throttle.
I doubt you have a real problem.

The 912 likes, needs and runs smoother at higher rpms. This is the way it was designed and is that way to get 100 hp and good torque in a smaller lighter weight package.
many types of engines have rpms that produce a harmonic vibration and many engine MFg's advise not to run in these areas for extended times. Short and or intermittent use is fine.

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

Thank you said by: David HEAL

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Re: 912 ULS Roughness

by David HEAL » 9 years ago

David HEAL

23 191 439

Thanks for the input. What would be the effect of relocating the individual carb vent lines on my RV-12 from their present "bale-mounted" location to being located within the respective K&N air filters? Might this somehow (i.e., lower relative air pressure within air filter?) improve the mid-range lean roughness by enrichening the mixture??

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Re: 912 ULS Roughness

by James Ott » 9 years ago

James Ott

22 334 392

I run my vent lines into the back of my K&N air filters. This is essentially the same as those folks that use the Rotax airbox. Been flying that way for almost 400 hours now with no issues.

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Re: 912 ULS Roughness

by Garrett Wysocki » 9 years ago

Garrett Wysocki

1 572 1.45k

I have switched over to the new SWIFT 100LL replacement fuel, it has eliminated a lot of the vibration and the engines just runs better, it made me realize I can stop thinking about that larger balance tube. Other Rotax 912 pilots are reporting the same, just talked with a friend today who talked with SWIFT at Sun and Fun and he said they are shipping the fuel in 55 gallon drums just pay the cost for the fuel.

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912 ULS Roughness

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