I let my lawn tractor sit for about a month without using it.

I went out to start it and it started up, but roughly. But I only let the
engine go for about 15 seconds and then I had to shut down to deal with
something else.

Now the motor won't start.

Battery is charged,

Engine cranks but won't start.

I took out the spark plug and attached it and grounded it to the engine and
I get a spark when I crank the key.

I took off the fuel line and fuel is getting to the engine.

What might be wrong? Suggestions of what to try short of a mechanic?

Alan