A Marvel Schebler carburetor, like any carburetor, might have difficulty starting due to several reasons:

• Fuel delivery issues: If the carburetor is not delivering the right mixture of fuel and air to the engine, it can result in hard starting. This could be due to a clogged fuel filter, a malfunctioning fuel pump, or a stuck needle valve in the carburetor preventing fuel flow.
• Choke problems: The choke is responsible for enriching the fuel mixture during cold starts. If the choke mechanism is malfuntioning or misadjusted, it may not provide the necessary fuel enrichment for starting; leading to difficulty in starting the engine, especially when cold.
• Air / vacuum leaks: Any air leaks in the intake system, such as chracked hoses or gaskets, can disrupt the proper air-fuel mixture. These leaks can lead to a lean air-fuel mixture making it harder for the engine to start. Check for leaks by spraying carb cleaner around the carburetor. Pay special attention to the mounting flange and throttle shaft. If you hear a change in RPM at any point, you’ve found a vacuum leak.
• Dirty or worn components: Over time, carburetor components can become dirty or worn, affecting their functionality. Dirt or debris in the carburetor can block passages, affecting fuel flow, while worn-out parts such as the float or needle valve can lead to improper fuel metering, causing starting difficulties.
• Incorrect adjustment: Incorrect adjustings such as idle speed, mixture screws, or float level can cause starting issues. If the carburetor is not properly tuned for the engine’s requirements, it may not provide optimal air-fuel mixtures.
• Ignition system problems: Sometimes, starting issues are not the carb at all. Faulty spark plugs, ignitiion coil issues, or ignition timing problems can all lead to hard starting.
• Moisture contamination: If water or moisture enters the fuel system, it can disrupt fuel delivery and combustion. This can occur from condensation in the fuel tank or water entering through the air intake.