Rotax 912 ul, severe vibration
in 912 / 914 Technical Questions
8 years ago
Hello everyone, this is Jorge from Spain.
I have a Kitfox III with a vintage rotax 912 ul from 1998 with 200 hours on it. I bought it almost 2 years ago. This plane was on the hangar for some years without flying.
Let's say the first 5-7 hours of flying were silk smooth until my problems started.
It all begun like a discontinuation on the normal engine running sound which made me suspect of some troubles. It was barely noticeable at first on the air frame and it became worst with the hours until I decided to stop. Now it shakes so bad that my carbs are overflowing, the air filters are falling down and even carburetors are separating from the air intake ... nice uh?
I shakes from idle to full power (the more power the more I think the engine will disintegrate), same on one ignition circuit or the other.
I tried EVERYTHING to fix this engine discriminating the next:
Carburetors. I cleaned them, blow them, checked them like a million times. They were overhauled and I've tried with another 2 different pairs I borrowed with no change on the vibration.
Ignition. All connectors checked and cleaned. New spark plugs, its connectors and ignition cables (making sure you get new copper and its properly screw in). I tried by replacing every single component with parts I borrowed ( stator assembly, trigger coils, ignition modules, coils, etc). Everything checked for the proper gap etc.
Prop. Warp drive tapered 68", all blades checked for pitch, statically balanced. Tried with a different prop having the exact same shake.
Mechanically. Engine was open & checked by a mechanic for compression, crankshaft alignment, camshaft, tappets, chip detector ... all good as per rotax manual!
Installation. New engine rubber mounts, no metal parts making contact, etc. Fuel installation is correct and provides enough flow/pressure to fuel pump (new from last year as well). Engine mount and exhaust checked for cracks. Also tried running it without the exhaust whit no success.
Now ... the air intake manifolds are inverted in this aircraft which magnifies any small vibration.
Also, there is a service bulletin to shorten the metal tubes which goes inside of the engine mount rubbers which will allow me for more compression on the rubbers thus more dampening effect. I am planing to shorten those indeed, but I'm having troubles believing this will fix my problems.
Finally, regarding the gearbox... it was dismantled twice by two different guys. Every component inspected and checked on the bench for compression, but no parts were changed (not the roller bearing nor belleville washers). I heard of other kitfox owners having my same symptoms and getting them fixed by shimming, replacing components, dog hub, main gear etc.
So after all the money I spent and the time I wasted I have all my hopes in that gearbox.
Any thoughts? What am I missing?
Cheers!
Jorge
I have a Kitfox III with a vintage rotax 912 ul from 1998 with 200 hours on it. I bought it almost 2 years ago. This plane was on the hangar for some years without flying.
Let's say the first 5-7 hours of flying were silk smooth until my problems started.
It all begun like a discontinuation on the normal engine running sound which made me suspect of some troubles. It was barely noticeable at first on the air frame and it became worst with the hours until I decided to stop. Now it shakes so bad that my carbs are overflowing, the air filters are falling down and even carburetors are separating from the air intake ... nice uh?
I shakes from idle to full power (the more power the more I think the engine will disintegrate), same on one ignition circuit or the other.
I tried EVERYTHING to fix this engine discriminating the next:
Carburetors. I cleaned them, blow them, checked them like a million times. They were overhauled and I've tried with another 2 different pairs I borrowed with no change on the vibration.
Ignition. All connectors checked and cleaned. New spark plugs, its connectors and ignition cables (making sure you get new copper and its properly screw in). I tried by replacing every single component with parts I borrowed ( stator assembly, trigger coils, ignition modules, coils, etc). Everything checked for the proper gap etc.
Prop. Warp drive tapered 68", all blades checked for pitch, statically balanced. Tried with a different prop having the exact same shake.
Mechanically. Engine was open & checked by a mechanic for compression, crankshaft alignment, camshaft, tappets, chip detector ... all good as per rotax manual!
Installation. New engine rubber mounts, no metal parts making contact, etc. Fuel installation is correct and provides enough flow/pressure to fuel pump (new from last year as well). Engine mount and exhaust checked for cracks. Also tried running it without the exhaust whit no success.
Now ... the air intake manifolds are inverted in this aircraft which magnifies any small vibration.
Also, there is a service bulletin to shorten the metal tubes which goes inside of the engine mount rubbers which will allow me for more compression on the rubbers thus more dampening effect. I am planing to shorten those indeed, but I'm having troubles believing this will fix my problems.
Finally, regarding the gearbox... it was dismantled twice by two different guys. Every component inspected and checked on the bench for compression, but no parts were changed (not the roller bearing nor belleville washers). I heard of other kitfox owners having my same symptoms and getting them fixed by shimming, replacing components, dog hub, main gear etc.
So after all the money I spent and the time I wasted I have all my hopes in that gearbox.
Any thoughts? What am I missing?
Cheers!
Jorge
