Could be 2 things...
Take the sttel cover (4 screws) off...Check the lever in the center, it should be flush with the casting next to it...Put the gasket on first, then the diaphragm...Take the other cover off, put the diaphragm on first, then gasket...
If the ignition is NOT a syncro spark it will kick back when starting, flip the prop harder with a glove...If it IS a syncro spark, the first flip retards the spark..Turn the ignition on and give it a good flip before you choke the carb......Can't kick back when there's no fuel in the cylinder..