Why can't my Murray lawn tractor be started Replaced plug fuel line and filter Runs when gas poured in engine but not with key start?

You say it runs when gas is poured in the engine but not with the key start. Two questions: (1) where in the engine are you pouring gas, and (2) how are you starting the engine if not with the key? If you mean you are pouring gas in the carburetor or in the spark plug hole and it starts with the key, but doesn't start if you don't pour gas in, the needle valve on the carburetor may be closed or the float may be stuck in the closed positiotion. You might also want to check the throttle linkage.