I tore apart my 400ex to replace the valves because the guy i got it from didn't do the timing right. So i got two more sets of valves on and set the timing right by turning it clockwise then it wouldn't start so i found out that it was suppose to be turned counterclockwise i took it back apart then saw that the two intake vales were sticking out then i looked at timing chain and found out that the tensioner was all the way out. So i replace the valves turned it counterclockwise to the mark put all lobes down and tried again and didn't work. I took the rocker head off and spun flywheel counterclockwise to the t mark and lobes were up.

I think that the timing chain is stretched but not sure.

 

Its common for the cam chain to stretch. When you replace it inspect the guides. If they look worn replace it.