Carb Float Bowl Breather/Overflow

Hi RW,

Yes they are Hutchinson/ Barry 22001 series mounts. Mount range is from - 11 to -15 (11 being the softest). I have the catalogue, with all the graphs. Have not had a detailed read but noted that the mounting plate thickness can have a significant effect on mount performance. My plate is about 9.75mm

Sean Griffin wrote:

Hi Roger,

"..........just polish the needle valve seat"

I think you may have given detailed instructions how to do the above - please direct me to your instructions.

An additional symptome:

I installed the longer & looped, breather tubes. Result - no fuel in catch-can.(I agree - addressing the symptom not the problem)

After a 1.5 hr flight, I removed cowling - right side breather tube, fuel bubbling part way up, the carby side of the tube falling back, before reaching the apex of the loop and repeating many times. Looked to be fuel "boiling" in the float bowl. I have an aluminium radiant head shield, that covers the full length of the carby (inlet manifold to air box attach points) underside and a heat shield on the exhaust manifold below. A light touch of the float bowl found it to be too hot to to keep my finger in position.

The left carby did not have this symptom.

"I installed the longer & looped, breather tubes. Result - no fuel in catch-can.(I agree - addressing the symptom not the problem)"

Referring to the above. I would be careful of using a loop in the breather tubes, if fuel gets trapped in the loop you won't have atmospheric pressure to the float bowl which will put your mixture out.

One thing I have been wondering with a tail dragger using carburetors: Is it likely that the angle the tail dragger sits at would cause the float level to fall and then allow too much fuel in the bowl for once it is in level attitude again? This doesn't seem to be a problem with mine but then again, I don't have a spill tray or catch can to notice.

Hi Murray,

"Referring to the above. I would be careful of using a loop in the breather tubes, if fuel gets trapped in the loop you won't have atmospheric pressure to the float bowl which will put your mixture out."

I refer you to the video below:

https://www.rotax-owner.com/videos-topmenu/rotax-minute/454-carb-vent-lines

Take special note of the vent/breather tube routing, from carburettor to air box, as shown in the video.

Interesting: The presenter calls the breather tubes "overflow lines" - even the experts get it wrong sometimes. Sure they will act as an overflow, should the situation demand but their primary purpose is to normalise air pressure between carburettor inlet and float bowl chamber.

Further to my comment above:

So far the installation of a looped  tube, just to the height of the carburettor "Chamber Top" as per the engine in the Rotax video, seems to have done the trick - no further fuel leaking.

I speculate (despite the wisdom from Roger et al):

That Rotax 9 naturally aspirated engines, that have the carburettor float bowl short (or long) breather, exiting directly to atmosphere, are having some degree of fuel leakage (more so if a "taildragger"). They don't notice because the small amount of fuel is quickly dispersed/evaporates in the hot air blast. If they were to fit a catch can(s) they may see a small quantity of fuel that has been expelled.

All is not well.

Despite the optimism of the last post, my carburettors are still venting fuel.

I have just clicked over Hobbs 100 hrs of engine running. This problem has persisted - every "fix" seems to have an, initial, beneficial effect but with time seems to fail, at least in part.

In the 2.9 Hobbs hrs since 100 hr service, my float bowl breather catch can has collected 150ml of fuel - this is a  massive increase on the previous tolerable 10 ml /hr average.

 # Could the flight profile be a contributing factor?:

In the  2.9 hrs (above) I did one flight of 2 hrs (2 engine starts);

About 75 minutes at 4500 ft, 5000 rpm,130 knots indicated, ground speed 120 knots, OAT 7C, in lumpy conditions.

Return flight about 50 minutes. Continuous climb (smooth air), from 900 ft, to 9500 ft at 110 knots / 5300 rpm / WOT / 1000 ft /min (about 9 minutes). Leveled off into smooth air, 9500 ft, 124 knots indicated, ground speed 167 knots, OAT -1C. Descent (15 minutes?), due to CTA steps, down to 3500 ft, at about 140 - 150 knots indicated, -300-500 ft/min.

Engine ran very well throughout flight.

This was the first time this aircraft had flown above 6500 ft and for an extended period, the longest continuous climb and descent (at high forward speed).

 # Air Box 

Could my (non Rotax) air box be delivering ram air? The airbox has a cold air delivery duct from the cowling front air inlet, directly behind the prop..

What might the effect of ram air be on float chamber operation?

 # Float Valve Operation

This time, I tested the effectiveness of the float valves by:

(Engine off)

Blocking the fuel return line.

Switching on the Boost Pump - over the next few minutes the fuel pressure rose to 8.8 psi.(normal is 4.5-5 psi)

No sign of float bowl overflowing. Float valves are working effectively.