Ok. some of you guys may know that i've gotten a new FMF Powercore 4 on my 400ex. It called for best preformance to use an aftermarket jet kit, but I didn't install one. My 400ex is in exellent condition, and i have never had problems due to the engine or other mechanical parts. I have rode it and noticed when i hit my throttle, sometimes it doesn't respond the muffler pops and the engine dies. And at times i pull in the clutch and it dies whenver its rolling. Is this a problem in the jets???