Swap the spark plug leads, If the miss follows the wire, Cut the wire back and retest. Use DMM to check continuity of the boot. Replace Boot if no continuity.
Clean the carbs. A plugged jet can cause issues
Sync the carbs at idle using the idle screws. Then back the throttle adjuster out until the rpm raises and sync the carbs using the adjuster on the top of the carbs. Return throttle adjuster at throttle to just a bit of free play.
Check for leaks around reeds/intake with some carb cleaner, if idle changes, possible leak. Reseal intake.
Check for broken reeds.
Adjust air screws for idle.
Verify exhaust not plugged with a mouse nest, acorns, ect.
Check compression. If below 100 rebuild.