I have a 2007 kx250f that was rebuilt about 20hrs. ago. I am having problems with it. When idling, if you give it a good bit of gas it bogs down. It also pops when revving. On the low end when easing up through the gears it cuts back pretty bad. I changed the spark plug and that seemed to help a little bit, but after riding for about an hour the plug was fouled out. I have attempted to adjust the fuel screw in some to get make it run more lean. To do this, I have to loosten the screws on both sides of the carb leading to the air boots and twist the carb out in order to reach the screw. When I do this and adjust the screw it seems to run better. However, when I turn the carb back in to its original position and tighten the screws, it will hardly run. I have cleaned the carb and there are no carb leaks. As far as I know, the pilot jet is stock. What could be causing it to run like this? I am thinking it's either the valves not adjusted right or the fuel screw not adjusted right. Could it also be the air filter? It's pretty dirty and greasy. I've been told that there is nothing wrong with the bike and to get a hotter plug. What can I do to get my bike running better? Any help from anyone would be great. Thanks.