Piper J3
Well Known Member
Maybe being just a little past the acceptable level would explain the intermittent nature of the problem.
Any progress to report?
I think setting float level on low end of spec is a good experiment. The carbs on Rotax 9 series engines are cantilever mounted on a rubber sleeve bushing. What makes matters worse is the heavy air filter at the un-supported end. A mickey-mouse extension spring attached to the top of carb is used to counter the overhung weight.
I think the fuel inside the float bowl is very un-settled on an aircraft engine and especially so on the Rotax 912. Turbulence can throw the fuel in every direction making float level control challenging. At cruise power setting float level is not so critical because fuel flow is much higher. Engine runs smoother at cruise speeds as well and carbs don?t dance around so much.
Every 912 I have seen does a dance at lower engine speeds. Imagine how floats are trying to set correct level when carbs are jumping around. So, if floats are wiggling around and level is trending high, this is condition for rich mixture and/or float bowl overflow. The condition is worsened because fuel flow for combustion is low at slower engine speeds.