Riding lawn mover will not start replaced the carb

As a very minimum, you need to do the following checks:

1. Check that you're getting a spark to the spark plug.
a. Remove lead to the spark plug and hold it near the plug when cranking the engine.
If there's no spark, you magneto has failed or is not working properly
b. If you have a spark, remove and replace the spark plug.

2. Remove the spark plug and put a 1/2 tsp of gasoline in the chamber.
Replace the spark plug and crank the engine. If it starts and runs for a few seconds,
you are not getting fuel through you new carburetor to the combustion chamber.

3. Did you leave fuel in the gas tank over the winter? If you did, your fuel system is
probably gummed-up. When you put your mower up for the winter, minimize the fuel
in the gas tank and allow the engine to run until it has no more fuel

4. If you have the mechanical skills, remove and check the float valve.
a. Put a clamp the fuel hose.
b. Look for gumming and make sure the float valve is functioning properly.