912UL Carb issue

[tim_fc3s]

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.

Good to know, I did take a VERY close look at the seat in the carbs while diagnosing this issue and everything seemed to be in good condition. I think we did sort this issue out though, going to the .070 orifice plus getting the carb sync done really helped and I didnt notice any more leaks. I think that with the pressure that the brand new pump putting out at near the top end of the spec and the original lack of a return line and rough engine made the needles unseat.

[tim_fc3s]

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.

The rough idle was for sure a carb sync issue. we checked the weight of the floats, float level (I did have to adjust one a few thousandths and redid the mechanical rigging of the carbs and then we used a high resolution synch tool and dialed in the carbs, way WAY more smooth at idle and across the RPM range. It is really amazing seeing an engine being so sensitive to adjustments, makes me kind of wonder what kind of performance I'm leaving on the table for my cars that get the "Sounds good" treatment haha.

[109JB]

Did you lean the idle mixture. Like I said I had a 80 hp UL no return line and it did the exact same thing on rebuilt carbs. Idle screw had to be turned way in from the setting the book says to start at. I never did put a return line on and it ran just fine after leaning it.

[Av8r3400]

Important tip if you ultrasonic clean the carb bodies:

After cleaning, you *MUST* reapply epoxy to the pressed in balls to re-seal them, there are several I forget exactly how many. If you don't, they will leak air and cause tuning issues.

[tim_fc3s]

Did you lean the idle mixture. Like I said I had a 80 hp UL no return line and it did the exact same thing on rebuilt carbs. Idle screw had to be turned way in from the setting the book says to start at. I never did put a return line on and it ran just fine after leaning it.

No, we decided to just add the return line, seemed easy enough and only took me like an hour at most to install and the mechanic doing the inspection wanted it there.