Ok, this is very strange. My '04 250 ex will start, run, and drive with the gas "on" as long as i keep giving it gas. As soon as I stop, it will die. However, if I have the gas on reserve, it will idle just fine. It seems like I just need more gas in it or something but i know thats not the case. Idk, I'm stumped. Anyone have a clue what it is?

 

Sounds to me like some debris has accumulated in your fuel tank switch valve and it's somewhat restricting fuel flow.
Try this:
Pull the line off your tank and blow some air back up into the tank while switching back and forth from reserve to on position. Make sure you have your gas cap off if you use compressed air so you don't damage the tank.

If it is dirty and has debris at the outlet, it should free it up for a short time until the crud re-settles back into the outlet. Since you say it changes when you switch it to reserve, that tells me the problem is at the fuel shut off valve on your tank.

You'll probably need to remove the valve and thoroughly flush your tank and valve assembly and possibly replace the screen. Good luck

 

Its not a tank flow problem, and I base that on the fact that he says it runs and drives fine, just wont idle.
Im not really sure the connection between the non idling and fuel **** position.
Those units were very lean from the factory on the idle circuit, and required the slide needle to be move up one position ( the clip gets moved dowm one position)to idle properly.
Sounds like a carb adjustment will do the trick.

 

The float level can affect how it runs also.

 

The best I can recall... those have an all plastic float thats non adjustable.
If I had it on my lift, I would first check and adjust the slide needle, then do the idle drop procedure.