Assuming the ignition system is on top condition.

• Is the choke valve closed with the engine cold?
• When cold look in the carburetor, pump the gas once to see if the accelerator pump is squirting gas into the venturi. An extra squirt of gas is necessary to start a cold engine. If no squirt then there is a problem with the accelerator pump circuit. Plugged passage, or perhaps the check ball isn't seating.
• Is there gas in the float bowl?
• Test the fuel pump for close to 4 lbs.
• If the engine is hot then hold the throttle all the way open. There may be too much gas in the intake and it needs extra air.
  • If it starts, then see if gas is dribbling into the intake after turning the engine off. That would flood the intake.