I have an 07' 400ex. I just put a k&n filter w/outerwear and a HMF slip on. The dealer I bought the exhaust from gave me a 155 main jet claiming it to be the most commonly used. I have been reading a lot of forums and getting a lot of different feedback. I have been told 170 all the way to 195. Does anyone have any suggestions? I am fairly new to quad riding so I am in the trial and error stage.

 

Start with something that you know is to rich, then work your way down until it just begins to run bad (at full throtle). Then back it up one size richer to be sure you will never run lean.

What did you do with the needle, and fuel screw???

Read about carbs and jetting in the "fuel" section: http://www.dansmc.com/MC_repaircourse.htm

 

I haven't done anything yet. I asked a mechanic at the Honda shop about the needle and pilot, he said I shouldn't have to mess with anything other than the main.

 

Oh yes you'll have to mess with the needle & pilot. 400ex's start hard as it is & it'll only be worse when you lean it out more with the mods.

If my memory serves me correct I had a 45 pilot in my 400ex. I know positively I had a 160 main & the needle raised 1 notch.

 

In the '06 and '07 400EX (maybe previous years also) you cannot change the needle, there are no notches in it.
I put an HMF slip-on on my wifes '06 400EX and went to a 42 pilot and a 152 main. It runs great
I left the stock filter on it and I removed the airbox lid, flipped the two rear clips into the box and put the lid back down, fastening it with the front and side clips.!
I was very surprised at the difference in power!
You can call HMF and they will also recommend jetting for you.

 

I went down to my local Honda shop and the head mechanic said that he had just completed the same set up on his friends bike. (same pipe, k@n filter w/outerwear and no lid) With the airbox lid on a 155 would be fine, but with the lid off 170. So, then I called HMF and the tech told me 152-160 either way. I am not running with the lid because we ride sand only and the Honda guy said that he will almost guarantee the bike will not run good at all with the lid off unless I go 170.....so, that's what I will try. If it's to rich Mr. Honda said he will personally replace the jets his self until it runs great.