Timing or air leak is the first thing(s) that come to mindYou leave out a lot tho. Things like how it ran before your "overhaul" and what the spark if looks like. Remember you are not talking to someone who is standing next to it. Carburetor is in need of a rebuild?
Yep me too. Got my trusty '03YZ250F for $400 as a non runner. In spec valves & rings no crank slop noted weisco & hot rods mmmkay... eBay'd a stock clutch pack cleaned the carb and ride! Got 20hrs on er so far great bike.
Good tip for cleaning carb jets is guitar strings. Buy a single .009
.013 or .017 and scrub it in there with some chemtool. Amazing the amt of crusty boogers you can push out.