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

Peter thanks. That is a good plot but I believe it applies for 'normal' slide carburettors and less relevant for vacuum carburettors.  I already have 12.5 at full gas under load (climb) så main jet is not too small

Excellent, in that case you do indeed have a needle jet job to finish. Sure we can draw some conclusions for standard Bing 64 set ups. Please keep us updated!

Roger Lee wrote:

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

Got info from Bing. With the 64/32/417 type carburettors I have there is only one needle for it, the #961-215 from Bing, not Rotax as I wrongly assumed. They suggest to look elsewhere but I have good compression, great leakdown, new plugs and injection and new carb sockets and o-rings on manifold. Also new floatrs and diaphragms.The only I did not check is the butterfly/throttle valve seals. Never done WD40 spraying behind a prop and how to do that ? 

The plot was taken of a Bing 64-32 carb document - the type of carb usually used on a Rotax 912.

Trond Reinertsen wrote:

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

Yesterday I stuck the vent tubes into the KN filter bases. And yes it seemed to calm down the fluctuations in mixture, but unfortunately made the engine run even leaner. As expected as the pressure inside the air filters will be slightly lower.   I went for a test flight and it was running very lean, with AFRs up to 16 at cruise and misfires. Pulled choke and made it back.   Changed to larger needle jet 2.76 (richer than the 2.74 I had, and stock is 2.72 btw) and went for a new test.   The slightly larger needle jet made AFRs drop to 15.5 and noticeably better running than with 2.74.. But anything above 15 is too lean. Priority 1 is richen up cruise settings.   When I first took ownership I rebuilt the carbs and set all as stock settings but it was not happy and engine was running lean all over except on full throttle climb.  Now it is only cruise I can't get nailed.   Once I see below 15 AFR I will follow advice and remove the AFR meter, but until then it has me worried

It seems to me that sticking the vent tubes into the KN filter would slightly richen the mixture because the lower pressure in the float bowl would give a somewhat higher fuel level in the bowl. A higher fuel level should richen the mixture. Where am I off?