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

Roger Lee wrote:

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

you are right, I made an assumption. They have no letter and number stamped into them, as the Bing needles have. Therefore my assumption was they are not Bing items, but special design by Rotax.   But all of this is assumptions.  Is there a needle with a longer taper available ? If I had a Bing needle with code it is easy to find the next step, but without any code not so easy

I have some 30 year old carbs that I'm in the process of rebuilding.  I was hoping maybe they used to have numbers but they don't.  The needles look like new since the engine had very few hours, but even under a boom microscope, there's no hint of a number. I did notice that the grooves are slightly different from the current needles. 

They appear to be brass, and could fairly easily be shaped, but it would be hard to make two that were identical without some precision tools. 

Rusty

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

Hi Trond, I never considered changing the size of jets used inside a Bing carb myself and I have no AFR gadgets at hand either. Interesting stuff. But I have come across a description of adaptations and recommendations at some point and I have just found it back now after having seen you latest post.

The folks writing about adaptations of the main jet provide a table with the dimensions „typical altitude“ and „typical temperature“. With a pretty simple calculation they provide a recommendation to change the main jet (which seems dominant in cruise) to the calculated size. Their approach is that it makes sense to change the main jet if one predominately flies in certain conditions that a very different from the standard. In case one usually flies in the Rockies during winter times the Bing benefits from a main jet change compared to a set up for sea level at 20C and vice versa.

Here comes their table. Google translator seems to deliver a helpful translation into English. I wonder if that helps to achieve your goal.

Hi Trond,

hope you find this schematic useful (sorry, it took some time to retrieve it). The way I read it one should not expect a too great effect from changes on the needle jet setting when cruising at let's say 4800 to 5200. As the chart shows main jet is to be looked at the higher the revs.