912UL Carb issue

[tim_fc3s]

I'm at a bit of a loss with this thing

[Dusty]

Where on the carbs is the fuel leaking from?

[Dave S]

Tim,

Do you have a check valve installed between the 0.040" orifice and the header tank?

[tim_fc3s]

Where on the carbs is the fuel leaking from?

It comes out of the overflow from the bowl.

[tim_fc3s]

Tim,

Do you have a check valve installed between the 0.040" orifice and the header tank?

I do not, we did a bit more trouble shooting and part of that made me think that I'm going to install one cause I pulled the return line and found that I am not gravity feeding back through it. We upped the orifice to .070 and synced the carbs, seemed to stop coming out of the overflow again, and the engine is running super smooth, just not sure I trust that it won't happen the next time.. it does seem to happen mostly at idle and as soon as some rpms are added it clears up.

[avidflyer]

You do need a check valve in the return line, otherwise you can't totally shut off the fuel. JImChuk

[Av8r3400]

Another common issue is having the float level in the bowls too high.

The Kitfox style mount with the manifolds reversed NEED to have the fuel level in the bowls at a lower point than the book says. If they are too high (or at the spec level), they will blow fuel out the vents.

I have never had an issue with fuel spraying out of either of my model 4s. Having a larger balance tube eliminates the balance issues, and keeping the float levels down a little all but makes this a non-issue.

[Dusty]

Fuel coming out of the vent tube could indicate a major fuel problem as this vent is high on the carb.

It is however unlikely unless the float valve is damaged or has debris restricting its sealing ability. As has been mentioned earlier The carbs are very sensitive to shaking due thier position relative to the centerline of the engine.


Shut off the fuel, carefully remove the float bowl and place on a flat surface so the top is level.
The fuel should be 12 mm down from the edge and the pin on each float should be exactly half submerged.


This has been my go to first step in sorting 912 carbs for years.


Rough running at idle on an engine that hasn't been regularly flown is more often than not, is a restriction in the transition port(below the throttle butterfly)


An ultrasonic clean us best but carb cleaner/ mek and a bristle from a while brush can sometimes work.

[Slyfox]

a couple years ago I had one carb giving me trouble, would flood out while flying and when sitting on the ground. I changed everything on this thing to no avail. What I came up with is that the seat had a crack in it and was opening up. I got another carb and problem solved. Now with that said, I also had another problem with a carb, the needle broke at the clip, this didn't cause a flood out but shut down the whole side of the engine. Had an emergency landing into a field. My caution is to check your carbs every so often and check those pins, if the clip is loose on the pin change it, it's worn and some day you will have it break and shut off the main circuit. I put this as more important than the 5 year rubber replace. take care.

[tim_fc3s]

You do need a check valve in the return line, otherwise you can't totally shut off the fuel. JImChuk

Yeah I noticed that when doing some diagnostics on the .040 orifice I put in. As soon as I disconnected from the bulkhead fitting on the return line it was definitely gravity feeding haha. I will install a check valve for sure.