There are a few things that could be causing your Honda 400ex not to start. Check the basics first, such as the oil level and fuel level. If those are both good, then check the spark plug. If it is dirty or damaged, it will need to be replaced. If the spark plug is good, then check the carburetor. Make sure that it is clean and that all of the jets are clear. If the carburetor is dirty or the jets are clogged, the engine will not start.