912UL Engine vibration on throttle back

by Kevin Stewart in 912 / 914 Technical Questions 12 years ago

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Re: 912UL Engine vibration on throttle back

by Scott Buck » 11 years ago

Scott Buck

2 15 0

I have been following this post for a while. I have a very early 912UL with only 150 hours on it, and I have vibration from 2500 on down. I have done all the same things as Kevin- carbs, sync, plugs, prop, rechecked sync, rechecked sync- etc. From what I can see there appears to be wear on the dog gear in the gearbox- and from what I have found out I am not the only one out there with this problem. I currently have a new gearset on backorder from Rotax- when I get the parts I will update everyone on this forum(there is a couple of us on the Kitfox forum that have this issue)

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Re: 912UL Engine vibration on throttle back

by Roger Lee » 11 years ago

Roger Lee

127 5.21k 4.79k

Hi Scott,

Sorry to hear about your gearbox and it certainly looks worn from the picture. This may have happened from more than one cause. I hope you get it taken care of and back in the air soon.

I don't know the history of your engine, but for others and for your new parts here are a few items that can extend gearbox life of a 912 and or the lack of doing some maint. practices can shorten it.
Improper oil, don't use car oils they lack the additives that motorcycle oils have and will cause excessive wear. Changing oil at the proper intervals. I use 25 hrs. for 100LL and 50 hrs for 91 oct. unleaded. I don't use 100 hr. oil changes even if it's in the book. Almost everyone one I know uses the 50 hr. interval for 91 oct. Oil is the life blood of the engine. Disease it and your engine pays later. Keep the carbs synced at every annual or 100 hr. inspection. If you have a slipper clutch make sure the friction torque is up in the proper range. Service the gearbox at the proper hundred hour mark for your engine and fuel use. Check the magnetic plug every oil change to look for any early signs of trouble. Use the proper oil filers. I just came back from Copper State Fly-In and saw to 912's with Tempest oil filters. There is a huge difference in the Rotax oil filter and using something as poor as a Tempest. It will allow unfiltered gritty oil to pass and cause you issues down the road. Moisture in the oil and not getting it up to temp to burn off these impurities. I don't like the Rotax oil filter price any more than anyone else, but I won't sacrifice and expensive engine because of a few bucks. Improper starting and or carb setup that allows kickback. Very cold weather starts that takes many times to start the engine. Warm it up with some sort of engine
pre-heater. Installing the higher torque starter can help. It has a gold colored case verses the older starters black case.It can make starting a little easier and have to crank a little less which translates to less long term wear.

Then once a great while there is some unexplained wear problem that just happens because humans built it or didn't hardened the metals quite right.

The above things that people do to their engine don't cause immediate issues, but like smoking give it a little time and it will cause problems and they usually aren't cheap.It's hard to tell some people that they are harming their engine when it seems to run fine. It's just doing damage slowly so an owner can't see the immediate problem so it's hard to convince some. Doing things the right way up front even though it seems to cost a little more saves tons of money in the long run.

Rob,
Anything you want to add to the list for gearbox care.

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

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Re: 912UL Engine vibration on throttle back

by Rob Seaton » 11 years ago

Rob Seaton

3 1.32k 240

Good sermon Roger. I would like to add:
-Have your prop dynamically balanced. A static balance is not enough.
-The power is in your hands: do not make rapid, back and forth, throttle movements; this is not a chainsaw engine. The prop is a huge, immovable disc and every time you make a power change you put wear on the splines and gears. The true professional pilots of the geared Continental GTSIO-520 have a rule: 1 minute for each 1 inch of manifold pressure. That's slow throttle movement! but the engines loved it. You do not need to be this conservative with the Rotax but it is an example of how you need to be aware of the effects of having so much inertia (prop) attached to your crankshaft.
The Rotax gearbox is wonderful; it allows us to have the optimum piston speed and the most efficient prop speed. Compared to other reduction drives it is incredibly rugged.
Rob

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Re: 912UL Engine vibration on throttle back

by Roger Lee » 11 years ago

Roger Lee

127 5.21k 4.79k

Hi Rob,

A SERMON! :dry:
Oh know you have my Dad rolling in his grave laughing. I always complained about being preached to and now look, I've turned into my Dad.

You probably made his day if he is looking down. :lol: ;)

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

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Re: 912UL Engine vibration on throttle back

by Paul Brydon » 11 years ago

Paul Brydon

3 34 0

Took my gearbox off and had the Rotax Mechanic pull it apart. He said it was 'loose' and he checked everything else (no other problems), re-shimmed it tight and I have put it back on. Flew today and whilst it was a short fly, all seemed in order! No vibe on throttle back, less rattle when started motor and idles much smoother. It may have been the problem all along. Why would a gearbox need re-shimming after so few hrs?

Paul

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912UL Engine vibration on throttle back

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