Bing carburetor throttle arm angle

I am by no means am expert but I have synced my carbs before and gave the process some though, so here's my opinion:

Mechanical sync process in MML tells you to check that vales open and close at the same time. This means the lineal run of the two carb arms needs to be the same. Some difference in angle is not a problem: the Bowden cable adjustment on the mechanical sync step is precisely to correct that.

By first fixing the idle screw relative to the totally closed valve position of each carb, and then fixing the bowden on the totally open position of both carbs, you sync the valve position to me mechanically equal on both carbs. This will hold as long as both have the same angular run from fully open to fully closed, which is always true (unless the valves themselves have been forced, and fixing that would require a carb repair).

Hope it helps!

Dear Alexander,

BING caburators are not hight precisition instruments but they are still precise enough that i can say there is something wrong. 3° or 4mm is to much deviation. First find out when the butterfly is really closed. Really closed is not the position when the arm stopped. Closed is when the butterly is really closed. For to check this, screw out  the idle adjustment screw and check the butterly in closed position. On both carburators the butterly have to close without any gap. Maybe remove the carburators for checking this to have a look through the carburator. In my oppinion the butterfly is not perfect mounted at the axis so one butterfly closed not in the right way and stopped wiht a gap. Its only my oppinion because for me this is the only reason why the arms are not in the same position. To loosen the arms makes no sence because there is no possibility to adjust it (if axis and arm are origin). Axis and arm are matched witch an rectangle hole so the position of the arm and the axis of the butterfly is fixed. 

If the butterlys are really closed syncron than we can discuss further of the angle of the arms. 

Why the angle position of the arms are not unimportant while a wire pull linear is for interest but first check why the arms are not syncron. The difference can make an effect, but by checking the pneutatic adjustment you can see the effect ist minor.

Greetings

Thanks for the suggestions. I might release both throttle arms from any interference from the idle stop screws and cables, remove the air filters, and make sure that the butterfly valves open and close fully and symmetrically, including their respective control arms. I've read the carb synching instructions in the Rotax LM manual and from elsewhere (e.g., this forum) but don't remember seeing anything about doing such a direct and visual inspection of the throttle parts, inside and out.  Ideally and normally, one shouldn't need to do that for carb synching.  But it sounds like a worthwhile thing for me to do to see if there's any mechanical mismatch between the two sets of valves and control arms.

Dear Alexander,

you are right, inspection of the butterfly valves are not part of a synchronisation. And it is no need to do this for further synchronisations if you checked it once a time, movement of the butterly at the axis is in practice nearly unpossible.

You wrote your arms are not in the same position at closed butterflies. If they closed (the valve cannot further closed) and the armes are so different then it could be that one of the butterfly are not right mounted at the axis. Normally this could only happend if there is a missmounting from the factory or someone had disassembled the carburator totally, included the butterfly from the axis (this is not easy because the screws of the butterly are squeezed after installing). If the mounting of the butterfly at the axis is not perfect than it seems the butterly is closed but there are gaps between the carburator housing and the butterly.

This could be, but it is very seldom and unlikely. For me your post show me that you synchronizised your first time or this is the first synchronisation at this carburators. Once check the closed butterflies and if they really closed on both carburators then synchronizies and don't worry about it anymore. 

Greetings

Hello

First you use the 914 shematic with 115% that is for the turbo action, their is no angle captor for the ULS. Second your measurement is not correct, how can you be sure of the 3% you say without a center axel? And you cant find this center axel if you don’t loose the screw that you ask if it’s the solution to adjust. Third carburetors are industrial products that can have deviations it’s one of the reasons why you must make a vacuum adjustment. Five with carburetor adjusted to the nominals positions if you follow the ALL that steps you make a perfect synchronizing that corret the « 3% »

 1 cables positions are identical

 2 with the throttle command opening one carb to 0,8mm

 3 adjust the second one at the same position  AFTER that yours carbs are mecanickely adjusted

 4 connect you vacuum tool on the manifolds

 5 unplug and close the compensation tube between the manifolds 

 6 start the engine (dont look at the idle)

 7 reach 3000rpm

 8 Adjust vacuum with the throttle cables (ONLY) accelerate up an down 3 or 4 time for control

 9 Plug back the compensation tube

 10 adjust the vacuum with the butterfly screws (only) ((screw driver))

 11 adjust the idle (1400rpm) 

 12 secure the cables

To finish don’t make brain knots and fly!

Just working for nearly 50years of internal combustion engines…

Bye

Kristof

www.hceaero.org