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  #1  
Old 12-31-2018, 07:17 PM
romaja romaja is offline
 
Join Date: Jan 2016
Location: Vancouver, Washington
Posts: 30
Default Rotax 912 ULS Engine Vibration Solved

I built a Rans S-20 Raven in 2016 and installed a new 912 ULS engine. 302 hours on the Hobbs. Early this year I started experiencing an engine vibration at cruise power settings. Very Frustrating. Troubleshooting went as follows:

1. Tested with new carbs
2. Tested with new ignition boxes
3. New Prop spinner
4. Dynamically balanced prop.
5..New spark plugs, several times
6. Balanced carbs, several times
7. Repitched prop before dynamic balance
8. Compression check, compression leakdown check
9. New floats

Apparently there have been problems with some gearboxes. I read a post on the yahoo S7 forum that sounded hauntingly familiar. This person even replaced pistons and cylinders troubleshooting this vibration at a considerable expense. His problem went away after he replaced his gearbox. I pulled my gearbox and sent it away to have new gears pressed in. The vibration is now gone! After considerable expense and a great deal of trouble shooting, the problem has been solved. I wanted to share my story in hopes that it might help someone else with similar symptoms.

Best,
Jim
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  #2  
Old 01-01-2019, 12:02 AM
NinerBikes NinerBikes is offline
 
Join Date: Dec 2018
Location: Granada Hills
Posts: 133
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I thought this was on gearboxes on the 912 iS models, circa 2014 or 2015 builds? I've seen mentions made of keeping the engine speed up to 2500 rpm to keep wear to a minimum on the dogs inside that gear box. What rpm's do you normally run and warm up at?

Last edited by NinerBikes : 01-01-2019 at 09:38 AM.
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  #3  
Old 01-01-2019, 05:53 AM
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snopercod snopercod is offline
 
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Location: Asheville, NC
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You may have dodged a bullet there, Jim.
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  #4  
Old 01-01-2019, 08:07 PM
romaja romaja is offline
 
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Location: Vancouver, Washington
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Quote:
Originally Posted by NinerBikes View Post
I thought this was on gearboxes on the 912 iS models, circa 2014 or 2015 builds? I've seen mentions made of keeping the engine speed up to 2500 rpm to keep wear to a minimum on the dogs inside that gear box. What rpm's do you normally run and warm up at?

When starting cold, I idle at 2000 rpm for 2 minutes, then to 2500 until warm. Yes, they had problems with the 912is but no mention of the 912ULS. No unusual wear on the dogs upon inspection.
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Last edited by romaja : 01-01-2019 at 08:19 PM.
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  #5  
Old 03-01-2019, 01:24 PM
NinerBikes NinerBikes is offline
 
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Location: Granada Hills
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Quote:
Originally Posted by romaja View Post
When starting cold, I idle at 2000 rpm for 2 minutes, then to 2500 until warm. Yes, they had problems with the 912is but no mention of the 912ULS. No unusual wear on the dogs upon inspection.
Fuel you run, and brand of motor oil that you run? Thinking out loud, if the motor oil also needs some gearbox lubricant supplement, much like what Mobil 1 does for motorcycle gearboxes, or with their racing motor oils. See Rotax list of approved motor oils, with certain fuels.

Last edited by NinerBikes : 03-01-2019 at 01:36 PM.
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  #6  
Old 03-01-2019, 04:37 PM
romaja romaja is offline
 
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Quote:
Originally Posted by NinerBikes View Post
Fuel you run, and brand of motor oil that you run? Thinking out loud, if the motor oil also needs some gearbox lubricant supplement, much like what Mobil 1 does for motorcycle gearboxes, or with their racing motor oils. See Rotax list of approved motor oils, with certain fuels.
I use Aeroshell Oil Sport Plus 4. Also, Rotax oil filters.
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  #7  
Old 03-01-2019, 10:08 PM
NinerBikes NinerBikes is offline
 
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Quote:
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I use Aeroshell Oil Sport Plus 4. Also, Rotax oil filters.
You run the Shell because you burn 100LL or you burn Mogas?
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  #8  
Old 03-02-2019, 08:29 AM
Driftdown Driftdown is offline
 
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Location: Clearwater, Florida
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I run Aeroshell Oil Sport Plus 4 and use Rotax oil filters because . . .
that’s what Rotax recommends, regardless of the fuel used.
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  #9  
Old 03-02-2019, 09:26 AM
waterboy2110 waterboy2110 is offline
 
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My second engine had significant vibration issues especially compared to the first one. I chased down all the stuff mentioned in the post but also would add:

Loosen the exhaust and re-center it. Make sure it's not breaking the springs.
Go through the engine mount bolts (all of them, not just at the engine).
Make sure the bowman cables have nice clean runs especially at the carb.
Try Vans new carb springs.

Honestly, I think the new carb springs did the trick. It's a much more conventional design that doesn't rely on hooks. My exhaust has run through quite a few springs but none since repositioning it. It's a known problem.

For me, it was easy to use my ADAHARS to troubleshoot the problem. At 5200 RPM the engine would set up a resonance that would send the ADHARS offline. When the autopilot was engaged the plane would bank and dive.

Rotax SL912-010 speaks to the known vibration issues on the 912.

You should also be measuring blade track and testing the friction torque for the gearbox. Don't let the gearbox run at minimum torque.
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  #10  
Old 03-02-2019, 06:43 PM
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Piper J3 Piper J3 is offline
 
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Quote:
Originally Posted by waterboy2110 View Post
Honestly, I think the new carb springs did the trick. It's a much more conventional design that doesn't rely on hooks.
This sounds like a more plausible explanation for a mid-range vibration issue. The 912 is really two separate engine powerplants that share a common crankshaft and prop reduction gearbox. Each half of the engine is independent of the other and controlled separately by its own carburetor. Carbs can be synchronized easily at idle especially with the crossover vacuum tube. Likewise, full throttle is well synched with each throttle plate nearly wide open and main fuel jet likewise nearly wide open.

Mid-range is where asynchronization is most likely to occur. Synchronization in mid-range is largely dependent on how well the throttle plate follows the displacement of the throttle cable via the return spring. Cable stiffness, bend radius, weak return springs, missing return spring – all have effect on mid-range smoothness, or lack thereof…
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