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I have a 2008 400EX. It ran perfect before. I just installed an HMF performance series slip on, uni filter, K&N air box cover, 165 main jet and (?) which ever pilot jet that was supplied with the HMF kit. It idles ok (when it's warmed up) and full throttle feel great. At slow speeds (1/8 throttle to 1/2 throttle) it feels like the engine putts. I was thinking I have to adjust the needle but I was told on the 08 that the needle is not adjustable. What can it be?
 

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Try putting the stock low speed jet back in. The needle may not be adjustable, but a different needle may be obtained (not through Honda). You may have to take the needle out to find out what the numbers are on it and then check a source like SUDCO.
 

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Yeah, it don't sound like you have it jetted right. Which is cause a poor Air/Fuel ratio. You might also try before you start taking jets out, to see how it runs with the original box lid on (restricting some air) this will change it a little bit even if its jetted right.


Not certified...
Not my job...
Not 100% sure....
Just my two pennies worth...
 

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your pilot shoulda been like a 42, did you adjust your air fuel mix, screw it all the way in till it bottoms out, counting your turns, back it out to about 2 -2.5, keep track of where it was and what it is, see what works, good luck bro
 

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I have an 08 also with an HMF slip on. The needle is not adjustable it has one factory setting. What I did was had a buddy of mine acquire an 02 trx 400ex needle that fit in mine perfectly and was adjustable. I moved it to the third and highest mark with a 168 main jet. I have no trouble in the low end, or with throttle response, so I left the pilot jet alone. I also have a K&N intake and a K&N powerlid, I dont know if you are running a stock airbox with holes, or if you are running open, but I ride mainly desert so I have the intake and powerlid, plus the prechargers. I have never had trouble with popping at 1/4 to half throttle, and it runs smooth all the way through the gears. I just put a Curtis Sparks 6 degree timing advance key on too. This is an easy cheap mod you might want to think about next. If you have an impact wrench the job is a breeze. If you do get the timing advance key, make sure it is set perfect, and the fly wheel is torqued back down to 94 ibs, because it will sheer right off. Good luck brotha, hope that helped.

Just a side note... You can back out the fuel mixture screw a couple of turns, but I'm thinking you have probably already done that, because it is common knowledge.. What you could do is pull your plug, have a new plug standing by they are cheap, pull it and pay close attention to the color, what you are looking for is a nice Milk Chocolate brown. If it is clean you are running too lean, If it is black you are too rich. You can get a couple plugs to do this. What you will want to do is put the engine under strain so do not just prop the tail end up and run through the gears. Get on the bike and cycle through the gears, bring the bike back to the garage and exam the plug, if you have that nice milk chocolate look to it you're in the clear. If not adjust the main jet accordingly, if you are rich go a size smaller and check it again... If you are lean go a size bigger and check it again. As far as the needle get one off of any earlier model year, they are adjustable and they fit.. I kept my pilot stock, and it is fine, but I will leave that up to you. Again good luck bro..

2008 400ex
blingstar iron cross bumper
asv break and clutch levers
spyder grips
Lizard Skinz shock covers
tusk nerf bars
Renthal bars
ITP sand stars
Douglas wheels
HMF performance slip on
Curtis sparks timing key
K&N intake
K&N Powerlid
carb: elevated needle/168 main/stock pilot
 
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