my 400ex is cuttin out and back firing constantly at all rpms this is the third time the first 2 times i cleaned the carb with carb cleaner cleaned the air filter and changed the spark plug and the first 2 times it worked good for about 2 hours and i havent done this yet this time but i was just wonderin what it is if im doin the right thing and why it keeps happening again i need quick any help is grateful.

 

sounds like you have the wrong jets in it i would change them to where it ran right looks like your getting 2 much fuel or not enouf one of those take the plastics off and then gas tank where you can get 2 the carb and change them jets youll know when its right but dont put it all back together with out sliding the gas tank back in and the back plastics and ride it around the yard dont bolt nothing back together until its right!!

 

maybe i should add this when i take the spark plug out its a little white i always thought one of the jets maybe gettin a lil clogged from sumthin

 

You are jetted lean. Most likely on the air screw or the pilot go up one size on the pilot jet. Stock is 38 so 40 is the one you'd want.

 

When you take the carb apart...is there alot of crud in there? You might want to add an inline fuel filter, this will help the carb stay cleaner, longer.