Fuel economy (odd ball)

Do a valve adjustment. Dont forget the intake and exhaust swap at the 3-4 junction. I would not think the difference in Cyl pressure would make it stumble. Check the carb it may be time to rebuild it. DO NOT buy a rebuilt carb have someone rebuild it OR at least keep you old one so you have a backup. Maybe install and extra fuel filter to get you through the winter.
Good Luck,
Frank