Carb Float Bowl Breather/Overflow

Sean sorry, should have included this clip from the old SI on the needle valve.  A lot of the motorcycle parts are not correctly matched for the Rotax high compression and you can get a bounce if using too light a sealing needle on the seat.  

Cheers

Thanks RW,

That SI is dated 2004 my engine is a late 2019, so that fact and that the carby's have not long come back from the Australian Rotax distributor, Bert Flood Imports, should mean that I have the latest valves - but I will check, best to be sure.

Thanks again.

My Son, an enthusiastic dirt bike rider/home mechanic, advises me that his carburettor used to eject fuel quite often.

He insist this is normal and the result of the float bowl being tilted (TO/CO), such that the float comes away from the valve, allowing extra fuel into the chamber, which then exits the breather tube, when the bowl returns to level.

His fix - extend the breather line into a loop, such that the top of the loop is slightly above the float bowl - he is urging me to try this - your comments?

HI Sean,

Happy New year.

It isn't normal for fuel to flow out the breather tube. The most common things that cause this are;

1. the fuel level in the bowl is too high and the brass float armature needs adjustment

2. the floats are heavy allowing to high a fuel level in the bowl

3. rough running engine causing too much vibration and it keeps the floats from doing their job of regulating the fuel

4. Severe bumpy weather in flight

5. the needle valve and or needle seat is worn and not controlling the fuel flow very well

There is no reason to put some different type of vent tube. Putting a different type of tubing setup is a bandaid. Just fix the cause and all's well. Most of the Rotax flying around the world don't have issues so you would need to find the cause. None of my clients have any fuel venting issues. 

Hi Roger, and a  Happy New Year to you,

It isn't normal for fuel to flow out the breather tube. The most common things that cause this are;

1. the fuel level in the bowl is too high and the brass float armature needs adjustment - (read back through thread) - tried this no good. Further the carbys have been to the Australian distributor (Bert Flood Imports) been checked/ set & returned.

2. the floats are heavy allowing to high a fuel level in the bowl - (as above) replaced new floats, with newer - no improvement

3. rough running engine causing too much vibration and it keeps the floats from doing their job of regulating the fuel - engine start & runs beautifully. Couldn't be happier. In its short operational life, the carbys have been balanced very carefully, no less than  twice.

4. Severe bumpy weather in flight - I don't fly, when the forecast is for any turbulence, not fun. However in the Sonex short, 12 month life, most flights would have some turbulence.

5. the needle valve and or needle seat is worn and not controlling the fuel flow very well - (as above). Carbys sent back to dealership. Seats found to be "porous" and replaced - assume needle valves okay but will chick spring colour at next opportunity.

There is no reason to put some different type of vent tube. Putting a different type of tubing setup is a bandaid. Just fix the cause and all's well. Most of the Rotax flying around the world don't have issues so you would need to find the cause. None of my clients have any fuel venting issues. 

Band aid or not, have tried all of the conventual fixes & still getting about 25-30mm fuel / hr in catch can, so now trying the "left of field" fixes (as long as no mechanical/safety risk).

As for non of your clients having a venting issue - have asked this before - do any have a catch can/system on their fuel vent(s) to actually check for fuel?  With no way of collecting small amounts of ejected fuel, it is quite possible/likely even,  that any evidence of fuel would be dissipated by air blast/engine heat - your clients would not know., ergo they would not be talking to you about it.

This is my temporary catch can:

Sorry system won't accept my photo.