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RV12 Rotax 912ULS Engine Vibration

D

Dcheiman

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I have a 2017 model Experimental Amateur Built RV12 with the Rotax 912ULS engine. It currently has about 370 hrs on it.
It has the 2 blade composite Sensinich prop. I purchased this aircraft back in February 2024 and did a pre-buy and condition inspection. The logbook entry for the condition inspection says a carb balance, carb inspection and cleaning ,engine gearbox friction checked currently at 470 inch pounds.
When I am in cruise flight and enter the pattern to slow it down and pull the power back I get a noticeable vibration at around 4000 rpm and below.
Can anyone suggest where to start looking?

Thank you,
David Heiman
N463CS
 
I have the same issue. I own a Dynovibe so prop balance is free and I have checked that frequently. I have very precise equipment to balance carburetors so I know my vibration is not related to either of those. The carbs have been rebuilt, new motor mounts, pretty much everything anyone has suggested I have done. I am pretty sure mine is coming from the gearbox. I bought a new gearbox and will put it on next month. It will be an interesting test to see if the vibration changes.
 
This still feels like it could be a carb balance issue, as the mid RPMs are where an imbalance will be most notable. Gearbox friction check doesn't appear to be relevant, given that it sounds like one cylinder bank might be having issues. An ignition issue might be at play, but you'd notice it at all RPMs. I've done a run-up with a single spark plug that had been inadvertently left disconnected and it was extremely evident when performing the lane checks. I'd also recommend checking that the Vans-supplied carb return springs are installed properly (KAI Chapter 50).

If the pre-buy wasn't performed by an A&P with specific Rotax training, I'd recommend looking up the closest one on rotaxirmt.com and having them check things out - verifying the carb sync only takes a few minutes. Even though I had done my own balancing before, an experienced iRMT was able to get it nailed when I took mine to them.
 
Dcheiman said:
I have a 2017 model Experimental Amateur Built RV12 with the Rotax 912ULS engine. It currently has about 370 hrs on it.
It has the 2 blade composite Sensinich prop. I purchased this aircraft back in February 2024 and did a pre-buy and condition inspection. The logbook entry for the condition inspection says a carb balance, carb inspection and cleaning ,engine gearbox friction checked currently at 470 inch pounds.
When I am in cruise flight and enter the pattern to slow it down and pull the power back I get a noticeable vibration at around 4000 rpm and below.
Can anyone suggest where to start looking?

Thank you,
David Heiman
N463CS
Click to expand...
Classic symptom of gearbox needing attention...
 
Dcheiman said:
I have a 2017 model Experimental Amateur Built RV12 with the Rotax 912ULS engine. It currently has about 370 hrs on it.
It has the 2 blade composite Sensinich prop. I purchased this aircraft back in February 2024 and did a pre-buy and condition inspection. The logbook entry for the condition inspection says a carb balance, carb inspection and cleaning ,engine gearbox friction checked currently at 470 inch pounds.
When I am in cruise flight and enter the pattern to slow it down and pull the power back I get a noticeable vibration at around 4000 rpm and below.
Can anyone suggest where to start looking?

Thank you,
David Heiman
N463CS
Click to expand...
I have a 2011 RV12 912 ULS with about 370 hours as well. I also have a vibration range between 3200-3900 rpms that I've been chasing since I bought the plane. It had 107.0 hours when I bought it 3 years ago. I have balanced the carbs multiple times, checked carbs, plugs, ignition, prop pitch and many other things all to no avail. It runs smooth and perfect at any other rpm. I'm starting to think it just doesn't like this rpm range. If you do a web search, you will find some complaints about the 912 ULS running rough in this rpm range. I tried a lot of "try this" advice all with no help. So until I hear of something else to check, I will try to avoid that rpm range when possible. I have a couple of friends that also fly 912 ULS's and theirs also runs less smooth in that rpm range...
 
Post #3 above has it correct… The Bing 64 Carb has three air/fuel mixtures depending on throttle / manifold vacuum… idle, mid-range, and high-speed. Carbs get synchronized only at idle and slightly above idle speed. It's very important to have a smooth-running engine at this low speed as uneven power pulses are death for the gearbox. The gearbox can tolerate a fair amount of imbalanced power at higher speeds, but not at low speed. Mid-range is not adjustable... same for high speed. It’s possible the two carbs may not switch into mid-range jetting at the same engine speed... it’s a crap shoot. High speed is not a problem because the large volume air flow through the carb throat is equal on both carbs when throttle plate is open.
 
It's not unusual to have a rough engine between 3000 and 4000 rpm. One issue is that balancing the carbs is only done at idle; it should also be done at 2500-3000 rpm. I've also seen rough-running engines caused by dirty or old air filters or both carbs not aligned with the intake manifolds. Another issue after accumulating 300 hours the carbs have not been cleaned and new o-rings and gaskets installed. I recently worked on a carb with 325 hours which had a broken o-ring on the idle jet.
 
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