The easiest way is if You have a lathe or a big drill press is to get a head holding fixture that screws in the spark plug hole and the spin the head and clean it with scotch bright and carb spray. You clean it and give it a good polishing at the same time. If You don’t have a way to spin it then use carb spray and scotch bright. And wear eye protection, Carb spray burns if it gets in Your eyes.

DO NOT spin a head in the drill press. There is a good chance it will come flying out. Those chucks are held in with a taper not good at taking side loads or vibrations of any kind.

You can scrape the carbon off with a small pen knife, just don’t dig in too hard or you’ll scratch the aluminum.

Gif

you can try spraying sea foam into the carb while running the engine. $5 at auto zone.

I would try. Gunk carb cleaner, ( the old dip in the tank type ).

If that fails Glass beading.

Like some one else mentioned Seafoam or Mercury Marine has a spray into intake while

engine is running.  Follow Directions or else.