Unless the carb is cracked or badly warped,a rebuild should fix it. A carb will make a certain amount of hissing when it's running, so I can't say if what you hear is normal or not.
Yeah, there is a good chance if he doesn't have a brass float the foam float has soaked up fuel and is too heavy. The bad part of them is you can't tell by looking like you can with a brass float. I can't remember if rebuild kits come with floats or not, but I think they do. My experience was if you wanted a vehicle to run real sharp, you needed to rebuild the carb every couple of years.I was using accel pump as an example of possible degradation. I'm leaning more towards degraded float and flooding/run rich causing immediate issue.