Carb Float Bowl Breather/Overflow

HI Sean

Actually the reverse is what happens in respect to high or low pressures.  High pressure over the float = rich running.  Low is the reverse.  The ideal way to measure if you have a rich/lean bias is to check the exhaust system Co levels as shown in the installation manual.  This is the best way to avoid detonation issues also.  The lean area of running in a good balanced system should be in the 4000 RPM range.  At WOT it should actually be a bit on the rich side to avoid detonation.  Backing down on the throttle it should be leaner once you drop into 5500 down to 5000.  

The prop loading can affect your MAP obviously.  

What we want to avoid is high pressure at the carb inlet and not in balance with the vent tubes.  Low pressure on the vent is lean, high is rich.  This however may not be the case if you can balance the carb inlet pressure to the vent tube pressure (pressure over the fuel in the float chamber) 

Cheers

RW -

"Actually the reverse is what happens in respect to high or low pressures.  High pressure over the float = rich running.  Low is the reverse." 

It seems to me that this statement is relative ie high carburettor pressure / relative to float bowl pressure. This is not what I am asking.

When I say HIGH or LOW pressure, this is speculating what may be happening in the airbox, to which the float bowl breather is attached (ie experiencing the same pressure as the carburettor air inlet).

HIGH pressure due to RAM air

LOW pressure due to air filter restricting air flow.

It may be that your answer is - as long as the two pressures (carb inlet/float bowl breather) are the same, the carburettor will function normally, no matter what the pressure is (HIGH or Low).

Hi Sean

Yes you are right, as long as both the bowl and the inlet air are indeed the same air pressure.  This assumes there is no difference.  The item I want to point out is that your prop loading, due to the air density, may be different in that it is ambient pressure and the airbox may be higher or lower than ambient. If this is the case just remove the box for a few flights and see if the condition still persists.  This should tell you if there is any influence of the loading to the RAM or no RAM effect.  In principle the filter in the inlet should be sufficient to stop most RAM air.  (K&N filters do this to a large extent)  Given the extremely fine differences with a CV carburetor balance between the float and ambient pressures it is hard to predict with the airbox.  For sure when the vents are not in the same pressure it has a large effect.  

I am unaware how long your vent lines are to the airbox from your vent connections, they should be equal length.  From what I could see on the pictures they appear to be.  The comment regarding pressure is of course referring to the vent line pressure relative to the airbox or inlet pressures as you note. 

As a last comment I have seen this condition over the years on many carb engine installations, almost always it is a vibration/mount issue.  In some cases this may be under shimmed gearbox or worn gearbox parts inside.  If the gearbox gets into a harmonic range it may induce high frequency vibrations that excite the fuel.  In looking at your mount it looks like you have spent a lot of time on getting it set up for your airframe.  Keep watch on the rubbers and look for any signs of movement out of the ordinary.  We should remember that the intake sockets we fit the carb into is there for this very reason as well, to absorb high resonance in some of the engine running range.  I did not really see how your airbox is attached but assume it is on a rubber hose of some type and has support somewhere to stop the airbox system from direct pressure on to the carb sockets themselves.  Sorry I can't see more, but maybe try it without the box for a bit to see if that is even related.  

Cheers

RW,

Thank you for your extensive advice.

"I did not really see how your airbox is attached but assume it is on a rubber hose of some type and has support somewhere to stop the airbox system from direct pressure on to the carb sockets themselves."

You made a very good point above: The airbox, other than a flexible support near the front of the engine,  relies on the rubber hose connection to each carburettor mouth to remain in place. I agree that this potentially increases the pressure on the carb sockets and may also magnify any resonance type movement, acting on the carburettors.

Today I removed the airbox & replaced it with the standard conical filters on each carburettor mouth.

I am concerned about the float bowl breather tubes/fuel spillage in this configuration.

Dont tell me, I know where they should be located BUT am worried about any significant fuel being ejected (75mm/hr is too much).

My compromise, hopefully temporary, solution is to rout them down to my fuel catch can. This arrangement may cause a pressure  imbalance but will have to do for the test flying.

If I don't get any fuel exiting the breather tubes, I will relocate them to the recommended position, ie  terminating close to the carburettor air/filter inlet.