912 ULS Rough Running Low EGT - Solved!

Les

I believe that a blocked bowl vent tube would create a lean condition. The moment fuel started being pulled towards the venturi, pressure would drop in the float bowl and restrict fuel flow – like putting your finger over a straw allows you to lift water from a glass. Incoming fuel pressure may balance that a bit so it's hard to know precisely what would occur, probalby very erratic operation. I think the only way the vent tube could cause a rich condition is if it was positioned to get higher air pressure during flight, thus maintaining that higher pressure in the float bowl.

Jeff,

I think maybe the fuel pressure would keep the bowl air pressure up, since the float valve is supposed to immediately let fuel in, but it is a moot point. I reamed out the vent tube with soft things this morning, and flew it with no difference.  Again using the choke proved it was rich.  Again all ground operations, plus a full power runup on the ground show equal EGT's.

Taking the carb off this coming week.

A common symptom of your engine running lean is that your spark plugs are clean or turn white. Spark plugs are generally a brown or grey color, which is caused by normal ignition. However, your spark plugs will become either too clean (evidenced by a lack of normal color) or white when your engine is running significantly lean, as the mixture is too lean, then the exhaust temperatures will be cooler as well.your case shows rich tendency problem at carburetor 

We finally fixed this problem today by carefully tieing up all the control cables, especially the right carburetor choke cable.  The right side on this plane sees blast air that the left does not, and the loop in the choke cable did appear to move the control arm when it was flexed.  We feel it must have been vibrating and or distorting in flight, causing the starting carb/choke to enrich the mixture on that side of the engine.  Amazing.

?  Have a safe flight ✈️❤️✈???