I have a yamaha 20hp also, I swtiched to star-tron a little while after buying it and it seemed to do a better job IMO. Not to say seafam didn't work, but if you buy it make sure it is rated for gas with ethanol. then just add it to the gas as directed.
Bud how is it running? If you used star-tron, stabil, or any other fuel treatment you should be ok. Try it out on the house first to see how it runs. Put fresh gas in your tank, get rid of the old stuff at the dump/lawn mower/etc., and put some fuel treatment in. You should be good to go, if there are problems the carb may need to be cleaned.
Might want to try this. I run it all the time in my motors. From Merc 250 XS to 25 hp Yamaha.
1oz. Marine Stabil takes care of stabilizing fuel and ethanol
1oz Seafoam takes care of injectors
1oz. Merc Quickleen takes care of rings and carbon
per 6 gallons of fuel
Doesn't cost that much and takes care of issues related to fuel. Will notice idling and runout improvement. Much less carbon in prop.