Carb Float Bowl Breather/Overflow

Refreshing this conversation, as I am still getting a small amount of fuel in my carby "catch can's" despite the new "R" floats.

When I first started this topic I asked if there is a "normal" discharge from the carburettor float bowls - dont think anyone addressed this part of my question.

It seems to me that some fuel discharge might be expected, given the vibrations on engine start up/shut down,bumpy grass strips, hard landings, take-off, steep turns, turbulence, etc. If I am correct, then what amount might be considered "normal/acceptable"????????

Hi Sean,

You shouldn't have any fuel discharge. The common causes are heavy floats, poor carb sync, the brass float armature out of measurement or the rubber bowl gasket not 100% seated in its grove. Can be old style bowls gaskets too. A really bad gearbox or way out of balance prop can cause this vibration. All my clients have dry drip trays. If the engine shakes bad enough to dump fuel out the vent then you need to find out why it's shaking that much. 

Hi Roger,

As mentioned earlier - this small but consistent discharge came to my notice about 3 hrs into flight testing. There was no sign during ground running and taxying.

Hope that I can rule out floats - just purchased/installed the latest varient.

Carby's were "synced" just about 20 hrs ago - worth a check I suppose. EGTs even at all stages.

Float armature - would hope that this would not be an issue for new carby's but I guess that if all else good, will have to check this out.

Float bowl gaskets are the "Viton" type - Access very tight but I take my time to locate/seat them, so should be okay.

Gearbox as new as the engine (now 21 hrs) - would hope that this would not be an issue.

Prop balanced by supplier Airmaster and seems (from cockpit) to be exceptionally smooth running.

I don't have drip trays - instead have airbox, home made copy of Rotax. Has a lowest point  drain on each side, with a collection tube to small individual catch cans.

I don't have bad vibrations at all BUT Sonex engine bed rail style adapter for Rotax, is pretty soft and I think that start/shut down may produce significant (whipping) movement.

In the faint hope it might help - have just fitted new K&N air filter - yet to fly.

Roger,

"You shouldn't have any fuel discharge"

It strikes me that, with out having some way of catching any fuel spillage (as I have) any small spill may not be noticed. Engine heat would flash vaporise the fuel and air blast carry vapours "overboard".

What say you?

Hi Sean,

I agree. Everyone should have a heat shield / fuel drip tray between the carb bowl and the exhaust pipes. This helps protect the bowl somewhat from a little heat (it still gets pretty warm) and keeps fuel from dripping down and a very hot exhaust pipe.

All my clients have fuel heat shields / drip trays so fuel is easy to see. I do get guys in here that have fuel in their drip trays, but I can usually fix that by either replacing the rubber gasket or just repositioning it.

All most all my clients have dry fuel drip trays.