Just put the new plug in, cleaned the air filter and fired it up. Ran and idled great. One week later, it won't idle at all. Have played with the idle screw all the way in and out and in between - with no luck at all. The bike actually runs great but it just won't idle. Thanks, in advance. Brendan

 

same thing goin on with my 03 400ex wont idle and it bogs out alot

 

The no idle situation, is very common, and is almost always a plugged up pilot jet, which is also called the idle jet. Drop the float bowl and remove the jet, and make sure when youre done, you can see though it ..like looking down a straw. The deposits may be stubborn if theyve been in there along time.
When ever youre done riding the quad...shut the gas valve on the tank to off, and run the motor until it dies. This will leave the float bowl empty, and the jet wont plug up again.

 

I would check your carb nd clean it out. Sounds like the carb needs a cleaning. My friends 02 400ex had the same problem and that was the solution.

 

ya i cleaned my carb out but it still runs like hell

 

Tell us more.
Is it totally stock? Air box lid is on? Baffle in?
Have you tried adjusting the mixture screw?
Where is the bog..right off of idle?

 

Ya, we need many more details than it idles poor. Tell us the history of the bike and all mods including what jet sizes. Did you take out the pilot jet inside the carb when cleaning it and make sure the hole looked like a perfectly full moon?

 

ok bike is completley stock exept for dg nerf bars and bumper theres alot of back fire from the muffler im the second owner of this 2003 honda 400ex ran great when i first got it but then i did some crazy mudding in upstate new york with it for a week straight i also flipped it a couple times nothin real bad and then it ran like hell then i came back home changed spark plugs cleaned carb and air filter messed with throttle cable i dont know wats goin on i was thinkin just bringin it in this week if i dont find out wat is wrong with it

 

Most likely there is still a blockage in the carb. There are lots of tiny pasageways, and if one is plugged, itll run crappy.
Did you remove all the jets, and blow out each pasageway? Then check each jet?

 

you would think there would be but i cleaned it realy good its spotless