Main jets affect mixture near wide open throttle.. If you've got problems at part throttle, look elsewhere.
Ignition seems ok.
Part throttle mixture is probably really rich. If the carb has a jet needle, lower it a notch. This will lean mid-throttle mixture. Check the float for a high float level. Check the float needle and see if it's leaking.
Some spitting of fuel back out the intake could mean a couple things. Sometimes it's normal and a matter of intake port timing.
But if the bike is sluggish at low speed there's a good chance the carb is bleeding extra fuel into the intake for some reason. The bike runs like a dog while some of that extra liquid fuel is pushed back out the intake.