Problem with starting my honda bike?

Hi. I have a honda cg125. About a month back it started giving problems to start (cold start that is in morning) Before i would just press the start button and it would start flawlessly. Now when i press the start button it starts then cuts out. Start again idles for a while then cuts out. I then have to end up using the choke. The problem is not the battery. Replaced spark plug, brand new ngk iridium. I have absolutely no idea what this problem is. I took it to the honda official service shop, they serviced the carb but i still have the problem. Please help!

Also i hear a whistling sound when i start it i.e is cold start i m talking about. During the day it rides perfect.

Help.

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