Did you set the idle adjusting needle to the best idle and/or the highest steady vacuum? The needle adjustment is very sensitive, even at 1/16 of a turn, so only turn it a little at a time.
Also a good test - turn the needle in far enough, if everything is working fine, it should shut down the engine.(only ever lightly seat)
What brand & number are the spark plugs?
Here is the Model B spec sheet for 53-55.
http://chevy.tocmp.com/Rochester/9C305p01.htm Notice the std. metering jet was a #58. Should be stamped on yours.
