I just bought an '05 400EX and the darn thing is a pain to start. And after it does start it won't idle on its own for a few minutes. It needs constant help to stay running via the throttle. I am positive this a lean condition that can be remedied with the idle mixture screw. I'm sure it is set lean at the factory for emmissions reasons. I was all set to do this until I read a post here about "blowing it up". I also read that someone suggested a larger pilot jet. The idle mixture screw, if I'm not mistaken, is the fine tuning of the pilot jet, for lack of a better definition. If someone changes the exhaust they would definitely have to change this setting to allow the bike to start, as I have on other bikes, mostly street bikes. Since mine has the stock exhaust I'm wondering what Honda could have been thinking to have this set so lean. I'm going to purchase a service manual, either Honda's. Or a Clymer or Hayne's, whatevers available from Part's Unlimited. Anybody else have more on this or suggestions?

 

I have a clymer book, and it doesn't say a whole lot about the fuel mixture setting. I have been itching to set the mixture screw, but have also been scared of blowing it up. I am begining to wonder if my machine is also too lean...I washed it the other day at dark, started it up, and held the throttle about 1/4 of the way open only to notice that my pipe began to start glowing after about a minute or two at a constant speed (I have another post going on this topic). Does your ATV header pipe glow when you do this at dark? All I know is that I am beginning to lose confidence like you can't imagine in Honda's factory carb settings.

 

could be trash in the seat for the float causing it to drip fuel and flood but you should have problems when engine is hot too.
just my .02
a properly adjusted and running engine should never have oil coving the plug
keep an eye on it

 

Is your 400 completely stock? If the plug is black, is it wet or is it dry? If it is black and dry your too rich. The air/fuel mixture screw should be around 2 turns out from bottom. If your running an exhasut system run a 40-42 pilot,150-152 main jet,needle in #3 clip, air/fuel screw 2-2.5 turns out. If your headers are glowing red adjust your air mixture screw out until it duls some. They will always be somewhat red at night. If your running an aftermarket CDI, alot of manufacturers CDI 's throw the ignition curve out and make these very hard to start when it is cold. Hope this helps....It really sound like your pilot jet is too big or air/fuel mixture screw are adjusted wrong.

 

Could be idle speed set too low, or idle circuit too lean. Being lean can cause problems, but blowing a motor, is way out there. Youll have lots of drivability problems before blowing anything up.
Heres a good article on carb adjusting, and tells the symptoms of lean and rich running. The only thing youre gonna hurt by being too rich, is wasting gas.
Have you checked the valve clearance? If they are too tight, itll be hard to start.
jetting

 

When you say idle circuit, I guess thats what I'm refering to when I say idle mixture screw. It must be set lean from the factory regardless of what year the 400 is. I've heard 400EX's have always been hard to start from anybody thats had one. I just took mine out for awhile tonight, its snowing like a son of a gun and pretty damn cold in northern Illinois. It started ok, but won't idle at all on its own. I had full choke on and had to keep it running with the throttle for 2 to 3 minutes, after that it was fine. I always thought you should be able to start it with full choke and then go to 1/2 choke to warm it up, without any throttle.

 

Hondabuster, I just checked out the article from you link and I have to say that it seems to be one of the best I've seen on carb adjustments. Thanks.

 

Thats what I thought ,too, when I first saw it.
Welcome to the forums.

 

hey i have a 400ex 2001 with a 460 big bore stroker kit stage two jet ket k.n filter lid cut stage 2 cam white brother head pip and muffler bike work great a pulls hard but every 4 tanks of gas or so ill go to ride it and it wont start plug is dead and the old one is black like soot im perty sure it is running to rich but the bike is not slugish and has good power but do see a lil bit of black smoke come out the muffler at quick snaps of the gas i was wondering what i could do to fix this the needel c clip is on the 3 one and the air/fuel screw is about 2 1/2 turns out from lighty seated .. any help would be great ...