912UL Issues

[68niou1]

I just ordered those today myself. I have been trying to get rid of the vibration/rough running and this is next on my list. Just to recap here is what I have done-

Rebuilt carbs, new needles with Viton tip, new floats, etc.
Rebuilt gearbox.
Synced many, many, many, many times. Bought the throttle adjuster from Highwing LLC that has been helpful.
Changed plugs

Anything I have forgotten? Thing runs glass smooth from 2500 up- pulls 5200 on climbout, just rough running at 2500 down. Very annoying on base to final

[Av8r3400]

Okay. I'll continue my saga here instead of spreading to the other "prop balance" thread.

I worked on my plane through the week to this morning. I removed the gearbox again and re-shimmed it per the instructions in the "heavy maintenance" manual I downloaded from Rotax. Per this procedure it took an additional .2mm (.008") shim to properly align the parts. During the work I took some additional photos of the "spauling" I noticed on the dog hub and the mating portion of the driven gear. (Photos below)

This week I also modified my engine mounts per Kitfox service bulletin 54A. By shortening the sleeves inside the rubber isolators (which were in very good condition). My sleeves were 1.25 (1-1/4)" long. I shortened them to the recommended .930" long. This tightened the motor mount considerably.

I thought that for sure the motor mount modifications would have solved my problem. It didn't.

I test ran the engine and noticed a significant improvement. So much so that I decided a test flight was in order. After reassembling the cowl on the plane I started it and taxied out. During the taxi I noticed the motor was still somewhat rough in the 2200-2800 rpm band, not as much as previously, but still not "fixed". Higher and lower throttle settings were smooth as can be.

Take off and climb were just as good as ever with a 1200 fpm climb and good power. Turning down wind I throttled back to 4000 rpm with no ill effects. Nice and smooth. Just prior to turning base, I throttled back to 3000 rpm and the motor began to shake, just as badly as before. I turned direct for the runway and pulled power to idle knowing I had the runway made already. Smoother, but still rough I landed and taxied to the ramp.


Here are my conclusions: The issue causing the vibrations is still there. (Obviously ) The engine mount modifications helped the problem by not allowing the vibrations to "snowball" out of control during ground testing. As an engineer the wear/spauling I found on the parts in the gearbox, I would not have normally considered very bad. Speaking with several experts I found that this may not be the case. I think that this "damage" to the gearbox parts is not a problem under power, but when "windmilling" or at a very low power setting, it causes this vibration to start.

I have ordered a new gearset and dog hub from Rotax. I will (hopefully) have these parts from Austria by the end of the month. To some additional hope, the new gearset is a different design than the original set I have now. I will install these new parts and report the results then.

If this last step is unsuccessful, I am left with no other option than taking the whole plane (or maybe just the motor) to a close Rotax shop (LEAF) for repair as I am completely out of ideas.


Wish me luck.

[SkyPirate]

good luck Larry ,..hope it cures your problem,..since I now own a 912UL I'm anxious for you ,..I can add a little experience with gears,....when I was racing I ran 6:20 gears in the rear on the short tracks,..this allowed for allot of torgue to be applied to the rear tires,..we are talking allot more HP turning it too,..but every 5 or 6 races we'd have to re adjust the lash ,..the reason, low RPM/ torque chatter in the rear end,..when the gears chatter like that it creates heat,..heat can cause cracks,..and what looks like pitting and blue'd marks on the gears,.just like what your photo shows

[Av8r3400]

The photos don't show it well, but the coloring is not blue, but a copper sort of tone. I don't think they are getting quite hot enough to blue. But, I don't doubt there is some damage there causing some issues. Also, the procedure to assemble these parts calls for nickel anti-seize to be applied to the dog ears and to the spring discs. I don't know if this will make a difference.

The new design gear set does change the angle of the dog ears. This is most likely an improvement on the old design. Call it a revision "B" if you will.

As a side note there are two gear sizes available for the 912. A 22mm wide and 27mm wide set. My motor is of early 90s vintage and the crankshaft can not accept the 27mm set, only the 22mm. My other motor, in the project plane, is from the later 90s and came with the 27mm set (making a simple gearbox swap impossible! ).

I'm still holding out hope for the new design gear set...

[War Eagle]

Okay.


Wish me luck.

Oh man! It hurts me to look at this.

Nothing from Rotax is cheap so I hope this fixes the problem and that you don't have to mortgage the house.

By all means: Good Luck.

[Danzer1]

Same can happen with rapid power adjustments or operating to long at to low an rpm - can cause similar issues as overspeeding as the engine is lugging at low rpm.

While you've got it apart, waiting for the parts, I'd suggest checking the crankshaft for true and runout - the specs are in the heavy manual you downloaded. If out of range you likely have an internal issue that means sending it out.

If withing tolerance, reassemble, being mindful of the clearances in the manual and I would then would check the prop hub for true and runout.

If the crankshaft is okay, and the prop hub isn't, your problem would likely still lie in the gearbox. If both are okay, you could have (or had) a prop issue.

You mentioned balancing with a DynaVibe - were there any unusual readings? How far out was it before adjustment? and how close did it actually balance?

I don't necessarily agree that the problem lies in the windmilling, as most vibration/rough issues are more noticable at low rpm - that doesn't mean they aren't there at higher rpm, their just harder to detect. Also the spalling is not consistent around the gears, on all the flats and all the valleys, which suggests something is out of alignment, balance or true.

The issue could have been caused by the out of spec spring discs and/or an out of balance prop and now replacing the gear set might be the last step in fixing the issue (hopefully). I don't think the problem is with the gear set itself, but rather is the result of what the problem was or is.

But if either the crankshaft or prop hub are out of spec - I wouldn't bother gong any further until the cause is found and resolved.

Good Luck,
Greg

[kmach]

Hi all,

Av8r3400 ,did you measure the friction torque before and after the gearbox rebuild ?

I am very interested in the before and after numbers .

[Av8r3400]

I do not have a clutch in my gearbox, so there is no friction torque measurement.

[kmach]

I missed that , looking at your signature its says both engines 80 hp.

[Av8r3400]

Gearbox parts arrived today.

High of -5º tomorrow and not much better for the next few days. I won't be putting this project together for a bit…