I just got a 98 KDX 220 also. I started running 32:1 because the manual says to. It's just fine to run 40:1 but I've found out that the 32:1 ratio with crappy oil like YamaLube, the oil I was running, burns dirty and fouls plugs easier. Now I run either 32:1 or 40:1, depending on how much oil I have left :) . The only difference is the oil....try to use synthetic oil only. It burns cleaner, plugs last and its a more protective oil at thinner ratios than pertroleum oils. Good luck!!!
P.S. mix race gas with 92 octane at close to even ratios, i.e. 2 gallons pump to 3 gallons race. Its burns cooler and gives ya an extra pep...:p