Maybe a stupid question, but on my 400EX or 250R, fuel gushes out the drain line when you ride a wheelie, this also happens anytime the quad is in that attitude, i.e., climbing a hill.

Does anyone know why it does this and the correct fix?

 

Adams,
I haven't taken apart a 250R or 400 EX Carburetor.. but I've taken apart a BUNCH of atv carbs... It sounds like the overflow pipe (tube) just doing its job. That small metal tube sticking up vertically in the fuel bowl (the one with the small hole in the top) Its there to dump excess fuel in the bowl. When you lift the front end, all the fuel rushes to the back of the bowl, and, depending on design, the float may open the needle and start letting more fuel in..
As far as a "cure" If your fuel level is adjustable, you might try lowering it some.. but, I would be careful that you don't start starving the engine at high RPMS.
If you aren't able to fix it, and you are getting fuel all over your engine/frame... buy a longer piece of drain hose and route it all the way to the bottom of the frame.
Just Don't smoke and ride wheelies at the same time...