Sounds like the engine is not getting enough fuel, or it’s getting too much fuel.
You can just remove the spark plug, and have a good look at it. If the spark plug looks whet just after an attempt to rev the engine, the engine gets flooded with too much fuel.
If the spark plug looks dry, the carburetor is probably clogged, or the you need to have a look at the fuel filter, or look for cracks in the fuel line.