You were right to replace the float acuator needle and seat. That would be the first thing I'd think of when you describe the symptoms you're experiencing. When you say the bike runs rich, is it rich all the time, or only at certain points on the throttle, or under a load?
If all the other jets are correct, start checking the airflow. Is the filter clean, is the exhaust clean and free of water, debris or holes?
If these check out, then it may not be fuel, it could be burning oil from sacked rings, or leaky valves.
As far as the fuel screw goes, I've never been able to make one of those completely kill a little bike when it was warmed up and the idle set high. Try dropping the idle a bit and see if you can kill it then.
You didn't mention what year the XR is, if it's a points model, have the points been changed recently?