To clean the valves you dont have to remove the heads.
To clean properly the exhaust ports,the heads and the cylinders must been removed.
Be careful when removing the valves,the nut on the shaft loosen clockwise.
Also nothing holds the shaft on the cylinder except the seal.It needs just pull.
Also it will helps if you spray on the shaft some antirust or light oil.
Buy a brake cleaner spray if you dont have already and go for it.
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Owner of : Honda NSR250 MC28 95sp colours / Honda NSR250 MC18 R6K SP / Honda RS250 NF5 89 in boxes
/ Honda C50 12V cub / Honda C50 12V super cub Japanese version /Yamaha XT600 89 benetton