Carb Float Bowl Breather/Overflow

Hmm! Slow response. Guru's on summer holidays??😈

To simplify the above.

What might be the effect(s) of ram (pressurised) air in the carburettor air intake???

The carb vents are not supposed to be placed in a vacuum or a pressurized position.

OK, have a look.

The blue track is the vent line to the top of the float chamber.  It is primarily a vent line, not an overflow.  The key job for this is to have ambient air over the fuel in the float chamber.  If the pressure is higher than the pressure in the incoming air it will run rich (flood) the float chamber.  

The question is what about ram air.  In principal as long as the incoming air to the carburetor mouth is the same as the vent air over the float chamber it should be fine.  This very fine balance can be disrupted if it is not.

As to your problem of fuel coming out of the vent tube the most plausible cause is fuel foaming due to vibration.  I would look at some way to damp the engine mount with more, or perhaps less, tension on the engine mount rubbers.  Dynamic balance of the propeller may be the solution as well.  This was a major issue for the early Kitfox aircraft with the 912.  The early Kitfox mount required the builder to reverse the intake manifolds from the standard position and exchange them side to side.  This moved the carbs out from the engine centre line and made the very sensitive to vibration within the float bowls.  The result was a lot of fuel venting at given RPMs from the vent lines.  It was ultimately solved when the changed the mounts back to standard position.  

just my view

Cheers

Consider the possibility of an air leak somewhere in the fuel supply line. It may or may not leak fuel! But under suction from the pump, it can still draw in air. Fuel pressure is not a guide because diaphragm style pumps, pump air just fine. In my case, I ended up chasing a fuel problem for about a year before figuring out that the o-ring around the fuel tank selector shaft was leaking air into the system. Because I have the Rotax airbox and the "overflow" lines feed into it, fuel seldom appeared on the ground. But pumping air with the fuel apparently was initially causing the fuel bowls to foam and splash with a significant amount tossed overboard. Plugs showed a very rich mixture as the spill over was sucked back into the engine. The problem got generally worse over time, though as a switched between L-R-off sometimes worse than others. Eventually, the leak got so bad that it was feeding more air than fuel, plugs went to the lean side, engine would "miss" at full power, and finally, at low rpm, the engine wasn't rotating fast enough to supply enough fuel during taxi so the engine would quit.

The fact that your blocked-off fuel pressure built slowly, may be a hint. Air is compressible, fuel is not; it should go quickly to rated pressure.

To check, disconnect the fuel feed line and put the end in a container. Turn on the electric pump. Gas should come out smoothly after a few seconds, not spurting like mine did. Let enough fuel accumulate so the end of the line is under liquid. A few tiny bubbles is okay. Lots, aren't. Move the selector to various positions. One setting may leak more air that the others. Also, I got fooled. My (Limbach) pump is a rebranded one for a car. Most of those, to protect against crashes (in the old days) don't pump if there's no back pressure, like from a broken line. It took sticking a restrictor with a ~2-3 mm hole to get it to behave with an open end.

Roger/RW,

I understand the theory of why & where to locate, the Float Bowl Breather(s)

Do you have any experince/speculation in what happens if high pressure air (RAM) is present in the air box???😈