Did you rebuild carb? Do you know if it was jetted when u got it? If the lid is removed it would need to be rejetted. If your not sure try puting the lid back on the air box to see if will run. If u dont have the lid take the carb apart and c what size jets are in it. It should have a 130 stock with the lid off should be around 150 155so i need help i bought a 2003 kfx400 and it runs fine but when i push on the gas about three quarters of the way down it starts cutting out and back firing i have cleaned the carb and it has been rebuilt about three weeks ago it has a k and n filter no lid on the air filter box and i have no clue what i am doing anyone want to help plz really want to enjoy it it is my first quad!!!