Double check the connections on all your lines to the carb. Perhaps you have a vacuum line disconnected somewhere. When you replaced the needle, did you "clip the tit" on the yellow plastic retaining piece or (quite possibly) was the tit already clipped? When reseating the top cover and the rubber diaphragm, are you sure you got the diaphragm and cover properly aligned? The rubber diaphragm top edge has a loop that must fit into a certain depression in the carb body. The metal cover has a small notch which must go over this loop. With the air filter out and the airbox cover off, reach into the carb throat with a long screwdriver and see if the metal slide holding the needle will move up and down. If you did not clip the tit, it is possible that the needle is not free to float (held only by the small spring pressure), is cockeyed and stiff, and can bind in the jet hole into which it fits.