This might be a dumb question, but I think back 40 years ago to a Yamaha 125 we had on the farm. My Dad put gasohol in it once and it ran OK until it was parked for extended time. It then ran really rich so I took the carburetor apart and found no O-rings. The alcohol completely dissolved all of them (looked like they were never there—not even rubber residue).
Therefore, I am asking did you replace all of the O-rings and any other rubber parts? Look carefully for O-ring grooves on the starter jet, idle jet, needle jet, and main jet or where they screw in. O-ring(s) may have been missing when you first disassembled the carburetors so you may not notice them missing. I am asking this because that Yamaha acted exactly like you describe.
Therefore, I am asking did you replace all of the O-rings and any other rubber parts? Look carefully for O-ring grooves on the starter jet, idle jet, needle jet, and main jet or where they screw in. O-ring(s) may have been missing when you first disassembled the carburetors so you may not notice them missing. I am asking this because that Yamaha acted exactly like you describe.