Vibration problem

I have developed a problem with vibration on a 912ULS with an Airmaster constant speed prop with WarpDrive blades, installed in a Europa.

Idle is very smooth at ca 1700rpm, operation above about 3500 is fine. Vibration is over a band centred on about 2700. It's not violent but is at a level where I'm dubious about flying until it is sorted. You really notice it when you throttle back for final approach. It seems worse then with engine thoroughly hot than in taxying before take off.

Here's the history, a bit long sorry.

The engine and prop did 47 hours from new and were then in a fire. The prop went back to Airmaster (New Zealand) and was inspected and zeroed with 3 new blades.

The engine went to the approved Rotax service agent in UK at the time, Conrad Beale at ConAir. 2 pistons got new rings because of rust, the carbs were stripped and ultrasonically cleaned and fitted with new diaphragms, the gearbox components were re-assembled into a used casing because the original casing failed hardness test. Other major items were OK - crankcase and cyls passed hardness test. New plugs and fuel pump and various seals etc were fitted. Valves were lapped in. This was in 2012.

The engine and prop were then fitted to another part built Europa that I bought still unfinished in late 2019. It had a few ground runs in that period (gentle ones, plugs were black). I first ran it in May 2020, it seemed fine, and by September test flying was complete and permit issued. Then we had Covid lockdown and a waterlogged airfield so next flight was April 2021. Engine total time is now about 70 hours from new. The vibration has developed over the last 5 hours or so.

I've examined the blades, no damage, and they are in plane and firm in the hub. I've stripped the carbs, all the jets and passages and the bowls were clean. Needles are on position 3. Idle mixture 1.5 turns. The floats are well within weight. Level adjustment is correct. Diaphragms appear good. The chokes work correctly, starting is instant (it's not soft start). I've mechanically and vacuum synched the carbs very carefully, 3 or 4 times. Throttle cables appear to work smoothly. Manifold connection rubbers are good, also balance pipe rubbers. Spark plugs (NGK) all look fine, lower ones a little bit hotter than uppers I would say. The gearbox feels good but I haven't yet checked the free play torque, I haven't got the locking screw. (Is an ordinary conical ended grub screw OK for that or is the special one a hardened point or something?)

The only thing I've found that's not ideal is that I can balance the carbs spot on at idle and above 3000, highest I've gone is 4000. I cannot get rid of a difference - max 2" or so - in the 2000s, the vibration region.

Carburation thus seems the top suspect to me but there are no other adjustments to make! I think a sparks problem is unlikely because of the specific rpm range. Something in the gearbox with a resonance seems an alternative possibility.

All theories and suggestions for diagnosis and remedy will be welcome!

Mike

PS I run it on alcohol free 95RON Mogas.