my 2004 400ex wont start and i dont know why. the starter is brand new and it turns over but it acts like its not turning any gears or anything. does anyone know what could be the problem?  

Check your starter clutch. It sounds like its not engaging when the starter is kicking in. It is what mates the starter to the crank. I replaced the one on my raptor, it may not be exactly the same, but mine was not very difficult, other than having the right gear pullers and tools,etc. Also check that the starter is kicking the gear out and not just spinning inside the housing.  

I just recently had this same problem..i bought a new starter and it turned over but did not engage.Took it and had it replaced.started fine after that... ended up i got a bum 1..