Bing 64 vent tube discoloration

Hi Alexander,

This is almost certainly fuel being ejected through the vent tube from the float bowl, causing staining.

Although described differently, I had a long discussion, in this Forum, on this topic https://www.rotax-owner.com/en/912-914-technical-questions/9643-carb-float-bowl-breather-overflow

Like your installation I have a non Rotax airbox.

The Rotax Airbox is mounted/supported on the Ring Mount and connected to the Carburettors via a flexible hose/duct.

The Carburettors have flexible/rubber mounting to the Inlet Manifolds, that is there to absorb/minimise vibration and the often violent movement of the engine at shutdown.

The Rotax flexible Airbox/Carburettor duct does not inhibit the function of the Carburettor/Inlet Manifold rubber.

My/your airboxes are solidly mounted AND supported by the carburettors. This significantly reduces the function of the Carb/Manifold rubber connection.

I have modified my Airbox, supporting it off the Carbs and connecting to the carbs with a flexible, thin, silicon, duct (SCAT tube with wire removed).

The carbs can now move independently of the airbox ie the rubber inlet manifold connections can function as designed.

The modification has not entirely stopped the ejection of fuel  but has made a significant difference.

😈

Hello Sean,

Wow, thanks a ton for the point out on your thread.  Great discussion and learning.  A couple observations on mine.  My left vent tube is significantly longer than the one thats discolored.  It’s also routed up and around the throttle cable…where the right one is level directly into the airbox.  Maybe that contributes to it as well.  I’ll likely replace it with a tube of similar length to the other one and attempt to route it in similar fashion…then see what happens.  Good times.  Thanks again!  

V/r

Alex 

Other items to consider,

The tubes should be equal length so they are under the same pressure. Plus this could be something simple like a carb sync or the brass float armature that controls the floats and fuel level is out of adjustment and the fuel level is too high in the bowl. Vibration can cause the bowls to over flow too.

I agree with Rogers other possible contributing factors.

I note that he also included - "Vibration can cause the bowls to over flow too."

After checking all other factors (including sending my new engine carburettors away for inspection) the only remaining factor, was the non Rotax Airbox & its direct attachment to and support by, the carburettors. Once this had been clanged to a more Rotax like configuration, the amount of fuel being expelled fell to a very small amount 😈

Great, thanks Roger.  I’ll definitely make the tubes the same length and do a carb sync.  Very much appreciated.