I've got a 2002 Honda 400EX, I've always had a problem with it when it goes vertical. I am a wheelie fanatic (and sometimes go up extreamly steep hills) But everytime I get my 400EX vertical, there is gas spewing from the overflow tube that runs to the bottom of the bike, and the engine putters out. This is very frustrating. I've got a 2001 Yamaha WR250, which I can ride a 12oclock wheelie on for a quarter mile, (sometimes even going past 12oclock) and it never has any problems with bogging, putting, or spewing gas from any overflow tubes. Does anyone has any modification ideas which would enable my 400EX to perform vertically the way my WR250 does?  

why dont u just take it to a honda dealer and let them check it out