Gearbox Cause of Vibration?

We had a crack in the engine mount and replaced the mount back in July.  Mounting isolators looked beautiful, so we reused them.  They are the new rubber compound per the Service Bulletin.  

I forgot to mention that this same vibration (when power is reduced from 5500 RPM) has happened twice before - two years ago and again about six months ago.  Each time the problem resolved itself.  That's what makes this problem so weird.  Engine has been treated with utmost care.  Engine preheat if temps much lower than 50F.  Soft-start ignitions and warm up at 2500-2800 RPM.  Smooth...

Random thoughts;

No expert at all, however your problem sounds like  resonance ie some part of the aircraft vibrating in sympathy with engine speed.

On the engine mount  & rubbers;

- If possible it may help to get the same design/dimension rubber, in a different durometer (hardness).

- "We had a crack in the engine mount........." - this suggests severe resonance. Your engine mount should not crack, if correctly manufactured & installed.

- Have you checked the engine mount to airframe security? Seems that when you load the engine everything is under tension and no vibration. When you unload, the tension is removed and a loose/soft component is allowing movement.

- Not sure that the appearance of rubber mounts, is a fool proof method of assessing condition. If they are newish (within replacement time/hrs limits) and the correct part number, they should be good assuming no undue stress has been placed on them.

- You have dynamically balanced the prop but have you checked prop tip tracking? 

-"............... well-synched carbs both at idle and off-idle." - While low engine speed is the best for carburettor balancing, it's also important to check balance right up to Max static rpm. If a throttle cable is sticking/delivering unequal movement, it will likly show up as you increase rpm up to Max. 

😈

I did have the parts returned to me where the wear could be seen clearly. I didn't do anything else with them and eventually threw them away.

From what I can remember, I had the symptoms all the time when I throttled back. They didn't resolve until I had the gearbox repaired.

Thanks everyone for valuable input....

This is an update to my original post that started this thread.  I’m still trying to diagnose vibration roughness when throttling down from cruise RPM.  See my post #1 above for full description of the problem.  I spoke with Roger Lee (Rotax Owner’s Forum for 912/914 Technical Questions) and he thought problem is most likely carb synchronization.  Advanced Power Solutions was also consulted and they offered carb synch as more plausible cause Vs. gearbox.  

I’m wondering if anyone has ever hooked up a carb synchronizer in-flight to verify differential manifold vacuum L-R.  I brought vacuum hoses from the intake manifolds into the airplane cabin via the side vent.  Attached photo shows vacuum hose routing before hoses were completely covered with overlay tape.

Carbs were synched nicely at idle and off-idle before first test flight.  The SynchroMate showed a large (4.5 cmHg) difference at full and mid-throttle flight settings.  It took two ground adjustments of the left carb throttle cable housing to get a nearly dead-on synch in flight for mid-throttle and above.  See photo of SynchroMate in-flight at 5500 RPM.  The vibration roughness when reducing throttle from cruise RPM is significantly reduced and not really noticeable now.  EGT’s track nicely in the upper RPM range.  So, I think carb synch is the root cause of the vibration / roughness problem.  To verify accuracy of SynchroMate, I reverse manifold vacuum lines and reading show same but on opposite side of scale.

The problem now is carb sync at lower engine speeds.  With carbs now synched for mid-high RPM, the low speeds show manifold pressure differential at 7 cmHg and engine roughness.  So, it seems I can’t make engine run happily at slow and fast speeds with same throttle cable adjustment.

Next step is probably to rebuild the carbs and then synchronize from bare bones up.  

Has anyone tried a carb synchronizer in-flight to verify differential manifold vacuum L-R?  I’m interested to know if synchronization of both carbs can be attained over full RPM range with a single cable setting…