I would do a compression test. That'll tell you real quick whether or not you have a stuck valve. And before you assume the valve is stuck you should consider the decompressor (holds right exhaust valve open slightly while starting). If it is stuck you can dislodge it by quickly turning the crankshaft back and forth through the access hole in the left cover. You don't need a large stroke because the starter clutch could get messed up.

 

i am sure that it is a stuck valve because its pulling alot of air in through the exhaust. is the 400ex a overhead or underhead cam?

I dont think its the decompressor because it holds it slightly open and it would start if it was only the decomp

 

That's where the cam is, the cover containing the rocker assembly bolts to the top of the head. You have to be at TDC on the compression stroke to to remove the cover properly. You can open the valve adjustment holes to make sure there is a little play in the rockers before you attempt to remove the cover.