I posted this on the Rotax Owner's forum, if anybody on here has ideas I would appreciate it:
I have a vibration issue that started some time ago and has been getting worse. The vibration is not ordinarily present on ground run ups but begins after flying for about 5 minutes (after engine is fully warmed up?) It is very noticeable between 3000 and 5400 rpm and is worse if I yaw the plane back and forth. If I pull power and go below 3000rpm, the vibration stops, even if I maintain airspeed. At around 5400rpm, I can turn the vibration off and on by changing rpm. IOW, at 5390, vibration, at 5420, no vibration.
Gearbox situation: I sent the gearbox out and it was re-shimmed and they adjusted the clutch. No change in vibration issue.
Propeller situation: I have a three blade warp drive that I have used since first flight, it has been checked for consistent pitch and dynamically balanced. I recently installed a Prince propeller, had it dynamically balanced, and have exactly the same vibration problem as with the warp drive.
Engine Maintenance: I do 50 hour oil and filter changes and follow the current Rotax maintenance manual inspection checklist very carefully. At around 100 hours I found a small metal chip on the magnetic plug, nothing but the normal amount of dust since then. Carbs were balanced and checked for float buoyancy at the annual C.I., April, 2015. There are no mods to the engine, I use nothing but the recommended spark plugs, oil, filters.
It was recommended that next time I fly I do a mag check while the engine is vibrating. The motor mounts are tight and do not appear to have any wear, but I'm out of ideas and thinking of changing them.
I push the engine pretty hard but take good care of it. The vibration is getting worse, any ideas would be appreciated.