Why did early wartime fighter engines stall when flying upside down?

The float carburetor uses the venturi effect to draw fuel into the engine's intake, and to do that it needs to keep a constant fuel level in the float bowl that preserves the desired air/fuel mixture. Because the float works by gravity, it stops being effective when the plane flies inverted. Upside down, fuel reaches the bowl as fast as the pump can push it in, producing an extremely rich mixture that stalls the engine almost instantly.

When a plane fitted with one of these engines performed a negative-g maneuver, diving hard, fuel was pushed toward the top of the float chamber instead of down toward the engine, causing a loss of power. Fuel flooded the carburetor with an overly rich mixture, and the rich-mixture cutoff shut the engine down completely.

The pressure carburetor solved the problem: it works purely on pressure, so gravity no longer has any effect. For that reason it delivers fuel reliably with the plane in any flight attitude. Since it relies on positively-pressurized fuel, it's actually a form of fuel injection.