You'll need to get some practice before you attempt to silver solder copper to steel. It can be done, is done everyday, but it's not a job for someone without experience. Before you jump into it you might want to check the prices on a roll of silver solder. It might cost you more than the rest of your bike.
When we're talking about 'silver solder' we're not referring to the stuff on the hardware store shelves in the plumbing section. Just because it's silver in color, most solder is, doesn't mean it is the type used to get a good solid solder joint that will be needed to keep your carburetor from vibrating off.
My advice would be to befriend an air conditioning mechanic and have him do the work for you. A/C guys silver solder copper pipe/tubing to steel mufflers, driers and compressors. It requires high heat, preferably oxy/act or a turbo torch. You won't get it to flow with a little butane torch. It also needs a special flux available where silver solder is sold.
Tom