The KDX's I've owned want NO throttle when starting. My drill is turn the gas and choke on. Lay the bike over on its left side for a few seconds. This gets gas dripping out the overflow tube and helps prime the carb. Kick the engine over gently a few times to further start drawing some nice explsove fuel vapors into the cylinder (but don't open the throttle).
Raise the kick lever to the top, and slowly move it throug the stroke to find top dead center (the motor will get harder to move as it gets near the top of the compression stroke. Go a little past TDC. By doing this, the motor has a chance to build up as much speed as possible before the next compression stroke.
Raise the lever and kick it as hard as you can. DO NOT give it throttle. In fact, put your right hand on the cross bar so you don't inadvertantly twist the throttle. If you have a fresh plug and good gas and the bike is still difficult to start, it could well be a worn top end, as mentioned above.
Another possibility on older KDX's is the stator degrades and you have a weak spark. The problem gets worse and worse until it's impossible to cold start with the kicker. The stator can be replaced or rewound.
Do you know if the previous owner was having starting difficulty?
