Why is my moped running sluggish?

hi I have a 50cc 4 stroke moped and the last few days is has been going slow and spluttering when ever i pull the throttle back and i struggle to maintain a constant speed. its really slow, iv checked my spark plug and the oil, it was going loads better today when i had my exhaust welded back together but it was backfiring a bit. now its running crap but its not back firing, any ideas what it could be? thanks