So it is the carburetor body you have been trying to solder, not the brass jet. The carburetor body is a casting of either aluminium or zinc, with aluminium being more likely. You cannot solder aluminium. I'm aware of the stunt with the alleged aluminium solder and the guy with the butane torch. I'm told that stuff is more than 80% zinc, alloyed with a small percentage of aluminium, and sometimes you can get it to bond a bit to extremely thin aluminium sheet (I've seen a guy demonstrate it at a flea market stall). I've read reports from people who claim to have succeeded in using it to repair small defects in aluminium castings, but most of the experts dispute that it has actually happened. So, it may be that you could use that stuff more or less successfully. Of course the small package that they sell would cost over $20 from memory, and a healthy second hand carburetor should cost less than that.
As long as you are having fun with this, I guess you aren't doing any harm, but in the end I think the solution is to get a usable carburetor from a junked engine.
