Dirtbike mechanics, HELP?!?

ok so I'm no mechanic, and I can't afford one, so I guess I'm gonna have to rely on the internet for help here... after about 30/40 minutes of riding ym rt100 dirtbike, the bike start to bog down, no matter what gear its in. it kinda feels like the engines not catching. then it takes me about 20 times to kickstart it again, then it goes for about 5 minutes, adn the same thing. or else I'm driving it back after it initially shutdown, and the engine keeps catching and shuting down, but I can usually make it about a mile doing this. I cleaned the carb and air filter, changed the fuel(made sure there wasn't any debris in the tank) oil and motor oil, put in a new spark plug(the old one was dirty, worn out and loose), and that about it. does anybody have ANY ideas on what could be up with the bike?