The diaphragm on the side is the "air cut-off valve"; it affects idle if it doesn't function properly. Check the float needle seat for damage or built up varnish. Also, you can measure the fuel level in the float bowl with the carb on the bike by attaching a clear tube on the spigot on the bottom of the bowl, holding it up next to the bowl, and opening the drain screw. The fuel will rise in the tube to the same level as inside the bowl.