i ran my 400ex with air box lid off,it ran terible please help

 

... put the lid back on ...

 

you definitely want to jet your atv when you remove the air box. You are in essence increasing the airflow, so you more than likly running very lean (very dangerous). Normally, if you incease airflow, you have to compensate and add fuel as well. Until you get a jet kit, put the lid back on.

 

Was told that when I ride at Glamis (below sea level) to make my 400EX run richer. Some people either remove their air box covers or cut them. I simply rejetted mine to a fatter jet, it ran fine.