high frequency vibration 912 UL above 5200 RPM

[Wheels]

I had a high frequency vibration on my 912UL. I throttled back to 5000 rpm and it went away. going to full power brings it back. I checked the carb balance and did a full mechanical and pneumatic. Kitten smooth at idle and on the ground at WOT.

reaching 5300 and higher I get the vibe back. I had a new propeller still in the box and for giggles, I installed it. Better performance and the vibration is back. The engine has 400 hours on it, so I"m not thinking it needs new rubber yet, but ... maybe.

anyone else see anything like this in the rotax 912ul?
Its a Kitfox IV and I have about 1000 hours on it.
thanks.
Wheels

[Dave S]

Wheels,

Any luck on your vibration diagnosis yet?

Just a few thoughts, since nobody has responded yet. Diagnosis without seeing the engine has it's limitations so grain of salt on this.

Since the vibration is about the same with both props, it is hard to think that the problem would be caused by prop imbalance.

Carb balance can affect low rpm but not normally high power settings.

Engine mount bolts? Both to the case and to the rubber isolators - correct torque. I dont think the ones to the case have any safety wires or cotter pins.

I have seen cases in engines (not necessarily airplane engines) where a high power setting when the engine is at maximum cylinder filling, a miss can develope as a high density charge, contrary to what a person may normally think, makes lighting the fire more difficult. An undiscovered spark plug with a high resistance value can potentially mess up the ignition cycle.

Making sure the module grounds are intact, particularily if a bracket is used as a single ground versus having the module grounds go to separate points on the manifold. Anything with the secondary ignition wires or plug caps might be worth checking.

Trying to figure out if the vibration is caused by fuel, ignition or mechanical issues might take a little digging.

One thing on the mechanical side a person might consider (although I would rather not) is a valve train/valve spring issue which prevents a valve from closing completely at high RPM. Might check the Rotax service issue documents for your particular engine serial number on that.

[bogmonster]

On our Zenith 701 with a Rotax 912ULS, we forgot to turn the fuel valve on when taxiing and ran the engine until it starved out. Turned the fuel on, restarted and taxied to where we were going. Later, when attempting to depart, we had a perfect run up but when trying to takeoff, the engine got really rough over 5200 rpm. Long story short, we found what appeared to be grass seed stuck in the main jet on one of the carbs. At lower power settings, it allowed enough gas to go by but not enough for full power.

Mike

[Wheels]

OK, I am making progress I think. I decided to pull the gearbox and send it in for an inspection and repair if necessary. While it was in work, there was some damage discovered although it was minor. The two Belville washers were flat and there was fretting on the step washers. There was some chatter marks on a spline for the drive shaft And all of these indicate movement in the gearbox. I had the gearbox rebuilt and I had a slipper clutch installed even though it’s for an 80 hp engine. This was a recommendation to also dampen vibration. Let’s see if it works. I have reinstalled the gearbox tonight and will try to get a flight test in the next week if the weather cooperates. Here in North Idaho , in the winter, it can be wild.

[Av8r3400]

I put a clutch in my gearbox on Hal's recommendation. It did help with vibrations.

[Wheels]

I put a clutch in my gearbox on Hal's recommendation. It did help with vibrations.

I got the propellor on tonight and did a few clean up items.
I"ll finish safety wire tomorrow and if its flyable weather, I'll see if we have a flyable airplane.
stay tuned. and thanks for all the replies. I"m happy you guys all read my worries and set me straight.