I was just cruisin' TT and read the the resent post yz400 hp, and read the review of the 98' 400f http://www.motorcycle.com/mo/mcyam/yz400f.html. I found one part that said that racing fuels hurt the carb. I have a 99' 400f and have been running VP 110 leaded for the last year. The bike runs better than new. And I asked the dealer they said they didn't know except that the leaded is good for the top end. Anyone have opinions on this?