This may sound stupid, but is the fuel line connected to the right port on the carb? I learned this the hard way. When I changed a carb once the mechanic put the fuel line on a vaccuum port because that is where the fuel line was originally. So then, everytime I would try to start it, it would catch on fire.

If you are all done laughing at me, it was my first "hot rod" and I dutifully learn from my mistakes.