I have an older YZ250 and the plugs foul very easily... Problem with carb??

I use to put NGK B8ES plugs in my YZ250 but they fouled really easily and were costing my a fortune to ride it. So i found some nice Denso Racing plugs (not sure of model) on ebay, 8 dollars each, got 10 for $16. These help a little but after just 2 hours of riding they will be ok but once the bike is turned off and cooled down it won't start back up until i put a new plug in it. I didnt know if my air/fuel mixture was off a little or my mix is off (i use about 40:1 with synthetic oil and 93 octane gas). Or maybe its as simple as me not turning the bike off right (i dont think that would matter that much tho). To anyone that can help me out i would sure appreciate it!!!