Would a cracked fuel cap cause a lawn mower to stall?
IO doubt it. A fuel tank must have an opening to air so a vacumm will not be created as fuel is drawn from the tank. A crack should increase the ease of air entering the tank. However, if the crack affected the original outlet to air and plugged that outlet that could cause stallng. The other thing that could happen is that the cracked fuel cap is allowing dirt to enter the fuel tank. Dirt could be filling the filter in the fuel line and if insufficient gasoline is going through the filter, that could also cause stalling. Sorry this is not a yes or no answwer. But both possibilites are possible. Frankly, don't think you should use anything with a cracked cap. Fuel can spill ignite and cause a fire. That could soon exceed the cost of a new cap.