400EX Running bad

[ouhockeyplayer]

I have a 2004 400EX with a Dubach (Non-spark arrested) slip on, and a K&N air filter. The air box lid is currently on. My problem is that it will not idle at all, and the only way to keep it running when you first start it is to stay on the gas. However, when i do this I notice that the header begins to glow red and it will backfire. I have tried adjusting the idle screw and that does almost nothing to the idle at all. Once I get it running it runs fine while I am riding it in 70 or 80 degree weather. There does not seem to be a bog or any trouble above 1/2 throttle while riding. However, if you get on the throttle quickly from an idle it will bog or stall. The idle is still weak and the idle screw is turned all the way up. I have taken it to a dealership in my area (Michigan) and they rejetted it for me once. I don't remember their jetting changes but it ran fine for alittle while and now I have problems again. I am not too familiar with doing jetting work so I was wondering if anyone can give me some advice as to why my quad is running bad. Also would adjusting the fuel mixture screw help me out in this area? Any help would be very appreciated. Oh I forgot to mention that my spark plug is all black when I pull it out.

[BigBore24]

sounds like you are on the right track with it being a jetting problem. could be that the dealer has jetted it too rich that is why your plug is black. some things you could try without re jetting would be to remove the air box lid or drill holes in it and also to adjust your fuel srew in some (i believe in would lean it). since you say it runs well once you get going it could just be a wrong size pilot jet. you should proly try to find out what size jets the dealership put in.also make sure your air filter isnt clogged

[oleboy87]

now im not the best at jetting a carb but isnt there an idol jet also... that could be why it wont idol that good... and someone please correct me if im wrong...

[CarGuy7a]

Originally posted by oleboy87
now im not the best at jetting a carb but isnt there an idol jet also... that could be why it wont idol that good... and someone please correct me if im wrong...

^^^^^ you got it.

what i'm getting from this problem is that the main jet maybe to rich and the pilot jet too lean.

i also noticed that he said the fuel screw was turned all the way up. did you mean tightened in and bottomed out. if so THERE'S YOUR PROBLEM!!!!!!! that screw is a fuel screw turning it in makes it run lean I.E. making it run lean from idle to 1/4 throttle and that will make those headers glow red.

a correctly adjusted fuel mixture screw should be anywhere from 1 1/2 turns OUT from lightly seated position to 2 1/2 or 3 turns out. if it's the stock pilot jet #38 i would say right around 3 turns out and no more than 3. 3 turns out is the maximum effective adjustment, anything after that is pretty much useless and you run a risk of the adjustment screw falling out.

[QuadManiac]

You are runnig a free'r flow intake and exhaust. This normally leads to lean conditions. Backfiring is a symptom of too lean. BUT, black plug is an indicator of too rich...

I suspect that the jetting the dealer did is too lean at WOT and too rich at idle.

Jet richer, and turn the idle mixture down, IMO.

[ouhockeyplayer]

Thank you everyone for your help. I will try and make these changes as soon as I get a chance. I guess in one area that I am confused is with the idle screw and fuel mixture screw. Are these two the same thing or two different screws? Also where are they located on the carb? The idle adjustment screw is right on the side of the carb that the choke is located? Also let me get this straight. I should lean out the main jet and richen the pilot jet? Once again thanks everyone and hopefully I can get this all straightened out soon. I really don't want to take it to the dealer and spend $80 for them to mess with it for a week again.

[CarGuy7a]

Originally posted by ouhockeyplayer
Thank you everyone for your help. I will try and make these changes as soon as I get a chance. I guess in one area that I am confused is with the idle screw and fuel mixture screw. Are these two the same thing or two different screws? Also where are they located on the carb? The idle adjustment screw is right on the side of the carb that the choke is located? Also let me get this straight. I should lean out the main jet and richen the pilot jet? Once again thanks everyone and hopefully I can get this all straightened out soon. I really don't want to take it to the dealer and spend $80 for them to mess with it for a week again.

your idle speed screw is the big flat slotted screw right next to your choke yes. that's just an idle RPM adjustment.

the mixture screw is located just to the left of the the idle speed screw. you'll need to either take the carb off or loosen the air boot clamps and twist the carb to see it and adjust it. when the carb is in it's normal position you can't get a screw driver in there to adjust it seeing is how it it straight up and down. it's the closest screw towards the engine on the bottom of the carb.

there is a slick tool that i have from Motion Pro to adjust that screw without taking anything apart on the quad. i got mine from www.motosport.com.

also you might look into getting a clymer manual. it's a great investment as well.

hope that helps.

[ouhockeyplayer]

Thank you everyone for your help. I will for sure look into getting a Clymer Manual. Everyone has been very helpful and quick to respond. I will let everyone know the outcome when I get a chance to work on it.