If the problem still persists after you have corrected the fuel lines and re-tuned the needles, then its prolly a pressure problem in the cowl which is fairly common. There are several fixes for this. You can drill a hole in the diaphram cover and solder a brass tube over the hole..then run a piece of fuel line from the tubing into the fuse. If you notice there is a hole on the diaphram plate. if you do the brass tube trick you need to fill that hole with solder..Another fix (less work) is to get a carb plate..I had the same issues with both of my 50's and the carb plate fixed it..here is where I got mine http://www.troybuiltmodels.com/CarbPlate.htm

or here about halfway down the page http://teamsala.tripod.com/teamsala/