I had a Briggs V-twin last summer on a zero-turn that showed symptoms of BOTH running too lean AND running too rich. It acted too lean when it was cold and just started, but then behaved like it was choking to death after running for a few minutes. Drove me crazy. (There are some that say that ain't much of a drive.)
After tearing apart that split-manifold carb four times, I finally found the problem. It was a tiny shred of black rubber or plastic that had found a home in the seat of the float valve. It was about the size and shape of an eyelash. It was very insistent to stay where it was; I had to dislodge it with a dentist's pick, then clean the seat with a brass wire brush from a gun cleaning kit.
I have not heard from that customer since, which tells me that all is well.
After tearing apart that split-manifold carb four times, I finally found the problem. It was a tiny shred of black rubber or plastic that had found a home in the seat of the float valve. It was about the size and shape of an eyelash. It was very insistent to stay where it was; I had to dislodge it with a dentist's pick, then clean the seat with a brass wire brush from a gun cleaning kit.
I have not heard from that customer since, which tells me that all is well.