Have you checked to make sure the choke is working properly? When I got my bike, I thought the choke was okay but I later found out that although I feel like I am pulling on the choke lever, I am merely pulling the slack on the cable without actually opening the choke valves. I had to relube the lever so that pulling the lever takes the slack on the cable and opens the choke on each carburetor.

If the choke is working properly, have you tried opening the throttle a bit while starting, even with the choke on? If this works, the idle speed will need to be adjusted.