My 05 TRX400EX won't start, it turns over but will not start. it makes a fairly loud clicking noise when turning over which seems to be the valves or timing chain. What should I look for/ check? Can incorrect valve or timing adjustment cause this? Any help would be greatly appreciated!

 

Incorrect valve adjustments or timing can cause this, however, it could easily be something else. Have you checked the spark plug and ignition coil? It also could be a dirty carburetor not letting fuel through. The timing chain could have slipped if the it has never been replaced, especially if the atv has a lot of hours on it.

 

Does 400ex have a starter clutch. engine might not be turning over at all. I am not familiar with that engine but most have a 1 way bearing for starting. they make a lot of noise when they go out.