we have 04 400 will start and idle but when you try to give throttle it will die out.problem started with it running crapy (back firing making alot of noise) if we let sit for awhile it would be good again. now it will only idle.thanks

 

sound's like a dirty carb,dirt/trash and or water in fuel will cause this , to clean properly you'll need to pull carb and give ie a good cleaning ; after this add some Seafoam in wuth your fresh tank of gas ! And add an inline fuel filter between the on fuel line between tank and carb , this will help prevent this from happening again ; for the price of gas now day's it look's like we shouldn't have to worry about these problem's but it happen's ! The Seafoam is a fuel cleaner/stabilizer that will clean out the entire fuel system and also the combustion chamber for carbon deposit build up , cost it $5.99 at O'Reilly auto parts direction are on the can for how to put in fuel ! And depending on the mileage on your quad the valve's might need inspecting and adjusting to spec's , there are other thing's that can cause this but this is the first thing's that come to mind ! williebee

 

A common reason for back firing is water in the gas. I agree with Williebee that the carb needs a good cleaning but before you tear half the machine apart I would try putting some dry gas in as well as carb/injector cleaner in the gas tank. Try spraying some carb cleaner directly in the air intake, also clean the air filter.
If that doesn't work then you have to do as Willibie said and take the carb off and clean it.
A new spark plug wouldn't hurt either.
Start with the simple free/inexpensive stuff first. If that doesn't get it then take it to the next level. Take the carb off and clean it and since you have the tank off and stuff might just as well do the valve adjustment at the same time and pop in a new spark plug. Also at this level dump the old gas out and if you want clean the inside of your gas tank.
If none of this helps then the next step is electrical (ground wire, spark plug wire, battery condition etc) , then mechanical failure (timing chain, magneto, CDI etc).
90% chance it is only bad gas or dirty gas line system, or carb related (needle, jet, float etc)

 

fix for 400 auto - wires that run into rubber grommet on top of case (left side) for magneto were worn through causing short and poor running.bad design atv has only 1000 miles.glad i have grizzly