912UL CV carbs without airbox-what is the fix for the lean running ?

I had the same situation although I didn't have an AFR meter to use. I did set pressure gauge hoses parallel to the carb vent tubes and went flying with the gauges in the cockpit. The air pressure was all over the place and not the same at both vents. This vent pressure controls the rich / lean of the carbs. Rotax designed the airbox so both vents would "see" the same pressure that the throat of the carburetor sees. I made the following mod and it works great, fuel economy improved and the engine runs smoother.

http://waltsrv12.com/engine/carburetor-vent-line-reposition/

my 2cents

Roger Lee wrote:

Take the AFR out and it won't bother you anymore. LOL

I've never had a plane in the shop or even seen one with an AFR. Unless you have some factory numbers to compare with I don't think I would make too many changes.

yes, might as well remove the thing. I always have AFR meters on bikes and cars until fuelling is right but it tends to become obsessive. For now the engine runs better than it did but it still has lean stuttering on cruise. I noticed it when I got the plane and first thought it was related to propeller so I changed the propeller.   Which did not help obviously. Then I changed the ignition boxes with plug cables. That too did not make any difference except to my bank account.  

On factory numbers; no I don't have factory AFRs at cruise but 15.5 is lean for any engine (but can work with higher ignition in low load settings). Just as 12.5-13 is usually best for best power and 10.5 is filthy rich. AFR numbers are more or less universal 

I believe the needles are Rotax, an not Bing.   I will measure with caliper and micrometer and see if it is close to a Bing needle.  Then go one richer and see what happens. 

Walt wrote:

I had the same situation although I didn't have an AFR meter to use. I did set pressure gauge hoses parallel to the carb vent tubes and went flying with the gauges in the cockpit. The air pressure was all over the place and not the same at both vents. This vent pressure controls the rich / lean of the carbs. Rotax designed the airbox so both vents would "see" the same pressure that the throat of the carburetor sees. I made the following mod and it works great, fuel economy improved and the engine runs smoother.

http://waltsrv12.com/engine/carburetor-vent-line-reposition/

my 2cents

Thanks Walt !   will try that asap. Mine are stuck under the bowl springs as yours.  No room at the back as the left filter is against the bulkhead already but I could try at the throat. I hesitated with that as I thought the KN filter would have minimal pressure drop.   Did you notice any change ?  If the vent hose see reduced pressure the slide&needle will sit lower and engine run leaner. It is too lean already...  btw like your larger balance tube !

anyone who know about the carburettor needle ? They are Rotax pn R961215.  Is there any Bing needle equivalent.   Just if I wanted to look for a needles with a slightly different taper. To get more fuel in the 1/2-3/4 range

Why do you think these needles are made by Rotax and not supplied by Bing?