Have a honda HS520 snowblower and it dose not through the snow as it use to be.The engine stats well but when it gets into little bit of heavy now it stops.I checked the spark plug its was black I cleaned it up but nothing changed its seems like the impeller is not spinning fast to through the snow out properly.

Check any belts for lost tension.

Some times cleaning plugs doesn't work.
I would replace the spark plug.
And check your RPMS make sure its running at proper speed..