I'm not familiar with that tractor but you gotta start somewhere:
You're positive this sudden lean mixture condition started on hitting a
bump? Something simple that could make it go lean after hitting a bump is
an air leak into the induction system; the carburetor and/or manifold.
Check them both for cracks or breaks in a manner which would allow air to
leak into the system. Your "choking" would compensate for the leak and
allow the motor to run better. Your little solenoid valve is most likely a
valve to block fuel from entering the system after shut-down to prevent
"dieseling" or "running-on".
Gotta start somewhere.
Tony
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