RV12 Rotax 912ULS Engine Vibration

[Dcheiman]

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

 

[seagull]

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.

 

[NORDO Industries LLC]

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.

 

[Special Delivery]

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

Classic symptom of gearbox needing attention...

 

[Jcurry]

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 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...

 

[Piper J3]

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.

 

[SPRV12]

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.

 

[Jcurry]

or both carbs not aligned with the intake manifolds.

Do you mean not sitting perfectly vertical?
Thanks

 

[mbell]

If one of your propeller blades has the pitch set incorrectly that can cause a vibration. Easy to check.

 

[JP4]

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'm going to bet it's carb balance. I had the the same symptoms on my 912uls on my Rans S-7S. It used to have a horrible vibration in the mid range. I bought a Carbmate and spent half a day tweaking it until it's mostly smooth throughout the whole range. It's very smooth at idle and above 4k now and has just a little vibration 2,500-3,500. I think I can still get it balanced a little closer, but that's not an rpm I'm spending any time in anyway.

 

[Duncan65]

I'm going to bet it's carb balance. I had the the same symptoms on my 912uls on my Rans S-7S. It used to have a horrible vibration in the mid range. I bought a Carbmate and spent half a day tweaking it until it's mostly smooth throughout the whole range. It's very smooth at idle and above 4k now and has just a little vibration 2,500-3,500. I think I can still get it balanced a little closer, but that's not an rpm I'm spending any time in anyway.

Probably not.
This is a classic 912 feature (in the situation described) as the gearbox transitions from driving to driven. Some engines do it, some don't. I experienced it less than a dozen times during 700 hours and gave up worrying about it (just move the throttle a bit to kill it).

D.

 

[jhunter206l3]

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.

Hi Seagull. I see you changed your motor mounts recently. Are the RV12 motor mounts the Lord J-3608-1 rubber mounts and do I need 8 of them for my RV12?
Thanks ,
Joe

 

[seagull]

Hi Seagull. I see you changed your motor mounts recently. Are the RV12 motor mounts the Lord J-3608-1 rubber mounts and do I need 8 of them for my RV12?
Thanks ,
Joe

I got them from Vans, part number EA 22002-15. They are $10, you get both rubber pieces for one mount. You need to order (4) of EA 22002-15 to replace all the motor mounts.
I Googled the number you posted "Lord J-3608-1", the mounts for the -12 don't look anything like those. It looks like this, two pieces that fit together.

 

[jhunter206l3]

I got them from Vans, part number EA 22002-15. They are $10, you get both rubber pieces for one mount. You need to order (4) of EA 22002-15 to replace all the motor mounts.
I Googled the number you posted "Lord J-3608-1", the mounts for the -12 don't look anything like those. It looks like this, two pieces that fit together.
View attachment 66738

Thank you Seagull your help is greatly appreciated I'll order them from Vans.

Joe

 

[cactusman]

There is also 4 aluminum engine mount bushings that you probably already have....if you are gonna do the work i always say might as well order all the hardware nice and fresh...YMMV....there was a S/B for soft isolators a long time ago... but as others have mentioned - make sure your blades are properly angled...carb balance....dynamic prop balance...gear box - maybe gear box....you can rent a puller and send that off for rebuild....