Generally, the butterfly plates in the top of a carb are the air-restrictors for the choke system. If it's cold then these should be nearly closed. Come winter you may have to adjust this..

http://www.explainthatstuff.com/how-carburetors-work.html
http://www.thecarburetorshop.com/Automaticchokes.htm

"Next thing to look into is this rattling sound coming from the carb whenever it's started, and that's whether it's cold or warm. I think it's related to those butterfly plates on the top, and the linkage to them on the side of the carb. From what I can tell, they look to be connected to some kind of "cam" gear looking thing on the side, with an adjustable screw that looks like it's supposed to touch it. Well, in it's current setting, that screw isn't coming anywhere near touching the cam, so I think those butterfly plates are pretty much wide open at start up. "