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Discussion Starter · #1 ·
I'm new to the forums and owning a 400ex. I have an '03 400ex that the previous owner had put in a jet kit, T-4 full exhaust and K&N. It is pretty snappy and doesn't seem to have any real issues. I bought it in recently to a local Motorsports shop because it seemed to cut out when running through 1-3 but the mechanics at the shop thought I was just hitting the rev limiter. It was hard for me to tell as it didn't sound like it was winding out but they thought it ran well.

Recently I was cleaning the K&N and noticed that when I was just giving it about 1/3 throttle it kind of coughed a little (could see the carb cough). I put the seat back on and ran it down the street and didn't really notice much. It has a lot of tourqe and seems to have good power with the exception of a little cough and sputter at the 1/3 throttle mark which isn't always.. It's really hard to notice and may not even be an issue.

My question is this - is it possible (I'm sure it is) that I'm running a little lean? I haven't pulled the plug yet as I didn't think it would give me accurate results without running it at the 1/3 mark, having it sputter a tad and shutting it down. Kind of hard to do when it's not consistant. Anyways - any advise/feedback would be most greatly appreciated.
 

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try running some ethanol free 93 octane gas ..i had problem with ethanol gas in my 450r, now ive gotta make a 30 min trip just for some good gas!!!!. but well worth it.
 

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Discussion Starter · #3 ·
Yes sir... I only run non-oxy gas in all my ATV's small engine stuff. I was more worried about the jetting the previous owner did. However the issue is so small and not real noticable unless you really look for it. By default I'm usually either balls out or sitting there for a minute. :)
 

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Most jet kits do not include the slow jet unless it's a rebuild kit. try stepping up one size on the slow jet. From Idle to aout 1/4 throttle or a little better is controlled by the slow jet. also I would pull the needle and check the little "C" clip holding the needle in place if it is bent or loose you will get that hesitation you speak of.
 

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THROW away that K&N filter and get you a Uni filter K&N filters are allrite for the black top or flat track but don't like them for trail or moto x Uni is the way to go.
 
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