I have a 2002 400EX and took it out for the first time in months last weekend. It did ok in the beginning, but it would stall if I let off the gas. Then after riding for about an hour it would stall when I would accelerate. I am a newbie and have no idea what is wrong. It is all stock except for a K&N filter. Do I need to rejet it? Desert season is fast approaching so I need to get it fixed ASAP.