Popping through the carburetor might indicate a lean condition. Things to check include:

• Fuel pump pressure too low.
• Kinked, or collapsed fuel line. Be sure all rubber hose is injector ready. Ethanol can eat the inside of rubber. Any black particles in the float bowl would be an indicator of the Nitrophyl float getting eaten, or the rubber fuel line collapsing inside.
• Float level
• Float drop not enough to allow the needle to pull out of the seat.
• Fuel filter plugged. Look for in-line as well as filter screens in the fuel inlet and possibly connected to the needle & seat.

Make sure the choke valve can open & close. If the valve is stuck closed, then the engine won’t get enough air to start.