What is the mechanical cause and symptom of a worn needle valve (float needle) tip inside the carburetor bowl?

1. It fails to seal the fuel inlet, causing fuel to flood the carburetor bowl and leak out of the overflow tube.
2. It restricts fuel from entering the carburetor, leading to engine starvation under load.
3. It causes the throttle slide to stick open, risking engine runaway.
4. It allows air to enter the fuel line, causing air locks in the fuel filter.