When you checked the floats, did you remove them from the carb completely? The reason I ask is the float valve stopper has a rubber tip that over time becomes grooved sitting against the seat letting fuel leak past. After as many years it's been around it probably needs to be replaced.

A new needle wouldn't be a bad idea also, as they do wear down. Chasing problems it's best to start with parts that will give consistent/reliable results.

No I didn't remove the floats completely. Is there a way to fix this problem without replacing them? 

I did however do a carb rebuild and put in a new needle and stuff like that.