Dear Forumites
The springs under the diaphragm are supposed to be about 119mm long. Mine where 108mm. I stretched them back to their original length and after that vibration problem never returned. The carb overhaul kits were just installed as a precaution. The EGTs are now well within range and very equal across all cylinders. All spark plugs are now white and I will still try and adjust the main needle accordingly. The Rotax heavy maintenance manual prescribes the clip in slot three on a 912 UL, the local Rotax guys say leave it in slot 2. Mine is still in slot 2. But this would anyway just address any mixture issues in the part-load range. The main jets are 158.
Mike, the wrong rotary valve on the one carburettor definitely admitted some false air into the system but I think the fuel ports on the rotary valve remained closed. Be that as it may, the vibration had disappeared before I discovered the rotary valve mix-up, so that wasn't the man cause of the problem. The engine starts immediately.
Roger, I will replace the wrong rotary valve, but right now I'll wait until my bank balance has improved somewhat.
Blue skies
Arthur
