Another 912 carb venting cause

During the past month I had been struggling with fuel venting overboard and thought I had it fixed and over with, but it suddenly returned on one carb. While investigating I noticed that the float valve arms on that carb were bent in towards the center of the carb. In all my previous messing with the float arms I must have inadvertently squeezed them inward to the point where they were no longer consistently riding on both float side pins. A simple act of straightening the arms out solved the issue with no more venting. Add that one to the list of things to check when investigating the accursed 912 carb fuel venting.

Thank you, these venting issues are difficult. It seems with the newer fuel pumps the slightest issue with either rough running or float adjustment and they start to overboard fuel.

Yes - I was told by a Rotax mechanic that the newer 912 fuel pumps tend to output a slightly higher fuel pressure than the older ones. This works against the float valve's ability to readily close off the fuel flow and thus aggravates any latent tendency to overflow fuel out the vents. These Bing carbs seem to be more sensitive to all the variables that can cause overflow than they should be. Of course, if there are multiple variables that are slightly off that aggravates this issue all the more. I found that one has to consider all the possible root causes, and there are many possibilities, and meticulously go through each one and eliminate them one at a time until the issue is resolved. In my case I had multiple issues that were contributing factors to fuel venting and it took time to resolve them all. Although it was highly frustrating at times, it was an educational journey in troubleshooting and I did get to know this engine and its fuel system much better in the process.

These bing carbs were designed for gravity feed on a BMW motor cycle. That way they would not see much fuel pressure. The higher pressure of the newer style rotax fuel pump is one more reason to put a return line back to the header tank. JImChuk