[Flathead Fever]

With the engine off, look down in the carb and see if the accelerator pump is squirting when you move the linkage. It provides an extra squirt of fuel to prevent a hesitation as the carburetor switches between the different fuel circuits. When coming off of idle or a closed throttle plate like when your shifting, the engine is momentarily starved for fuel as the fuel flow in the carburetor catches up with the air flow. Without that extra squirt from the accelerator pump it creates a lean condition and a large enough lean condition will cause a backfire to come up through carb and spit out a flame. If the carb sits for a long time the accelerator pump's piston will shrink and no longer make a seal or they get cracks. Once that happens it can't push the fuel out. That's where I would check first. Then I would check the timing.