Well if you can put gas in the cylinder and make it run for a second, then it is not getting fuel...as evidenced by a dry plug also.
Pull the air filter off and spray some gas directly in the intake (spray bottle works) while cranking with the choke on...if it fires and runs while you pump fuel then there is something wrong with that new carb...I'd take it back...is it an OEM carb?
Make sure when you are doing this that you have a good fire extinguisher nearby...I've had backfires out the intake go badly! I like a Co2 extinguisher because of no clean up afterward...but a good ole ABC powder extinguisher will still save the day.
Just FYI, with backfires out the intake causing fires, the best thing you can do immediately is to keep cranking the engine...it will suck the fire in and no harm no foul, as long as no external gas soaked items have already been lit up that is.
Make sure the gas is good too...dump any old gas that is in the tank, and make sure any new gas put in the tank is of good quality...I had a Honda 250 that kicked my butt until I tested the fuel by dumping a little bit in the driveway and then lighting it with a torch...it went out as soon as the torch was pulled away...this was new fuel that was supposedly just purchased at a local GasMart! I dumped the fuel and put known good quality fuel in it...problem solved...weeks worth of headache...caused by junk fuel!