On the vibration issue at certain rpm ranges. I have seen way too many throttle setups with a slight bit of friction in the cable system that binds ever so slightly as the throttles open or close which will throw the carbs a little off sync until the cable moves past that point. This usually is only on one cable somewhere int the line and not both. It can be from too tight a cable bend or even a slight kink. I have seen solid metal throttle linkages have play in them that cause the same thing. This is not uncommon and I have had to deal with it many times.
Roger Lee LSRM-A & Rotax Instructor & Rotax IRC Tucson, AZ Ryan Airfield (KRYN) 520-349-7056 Cell
If your floats are OK and you have one carb that you think is over flowing and the engine is running rough at a low RPM, and you have said yourself you think it is the needle valve, then change the needle valve. They don't cost much, you can buy one from almost any motorcycle shop or obviously Rotax. I would suggest you change them both.
Just read through this thread and I have a question, when you reduce throttle and the vibration starts are there any unusual changes to your engine temps. One of my customers with a 912ULS experianced the same condition with a noticable vibration when reducing the throttle from cruise. At the same time the left EGT started going cold and at 3500 was 400 degrees less than the right. The owner documented with photo's taken each 100 rpm from cruise to idle, below 3000 the temps stablized and were normal. When we removed the float bowls each had a low float. When we weighed them the heaviest was 4.2 grams and the lightest 2.7 grams. We installed a new set of floats each weighing 2.7 grams the problem was solved. This was a newer airplane with 120 hours. We let the floats dry for a week and the weight was still the same. Good luck.
James makes a good point. Floats will sometimes develop pinhole leaks that allow fuel to enter the float. A heavy float will cause the fuel level in the float bowl to be too high even if the float valve is working perfectly.
Easy way to check the floats is to carefully remove the bowls and check the fuel level in the bowls. Then with the floats still in place, they should be floating with the float pins just touching the fuel.
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