Remove the cylinder and use a wooded dowel sharpen to make a scraper. Spray the port witha carbon removal product sold by mercury marine called powertune. Let it sit over night the scrape carbon off.
If it's (the cylinder) off the saw I've found that hot water will soften it and a pan scourer, some will know as a scotchbrite pad, will bring most of it off. If you have to scrap it, I suppose it makes sense that you use either aluminium or something softer. I've used the edge of a steel blade where needed, just be careful.
brake fluid works wonders.....in the old days when ya used to have a problem with carbon build up in the ring groove.....used to pour brake fluid in the spark plug hole, then quickly put plug in, and flip upside down, and roll the crank untill the piston hits the fluid, then leave set for a day. drain and start.......lotta crap will blow out the exaust, and free up rings.........
well the break cleaner worked good with a little help from scotch brite on a plastic stick ,so now the in/outlet are smooth and shiny =)
also dremeld of some edges at the intake so now the fuel should meet less resistans i hope ,,,