Alright, here is what has happened. I didnt ride my fourwheeler for about three weeks and I got on it about 2 weeks ago and it fired up and i let it warm up about 5 minutes and then took off acrossed the backyard. It runs fine until you get the rpms up to about 5 grand, then it bogs down and starts birpin and fartin. I knew I was running it a little bit rich cause I have gone through some plugs, and they are always charred black. So I thought that i would open up the carb and change it out. I went in there and dropped it from a 170 to a 160 and it helped out a little. It made it start bogging down a little closer up top, i would say more like 6500 rpm now. My neighbor said I should play with the airscrew a little and I did. I turned it about 2 times counter-clockwise (checking about every 1/4 turn) and that made it worse. Then I went about 5 turns the other way and it made it a little better. Have any of you ever had this problem, or know what is going on? Im not familier with carbs a whole lot. Thanks a bunch in advance, I wanna ride!

 

Well from a 170 to a 160 is a drastic change. Thats enough of a difference to make it lean from being rich. You should by all the jets between that, 162, 165, and 168 and start with the 168 and work your way down. Also where is the clip located on the needle? That might be too rich also. Needs a lot of experimenting to get a carb perfect usually.

 

Well, we did try a 165, did the same thing as the 170, only a little better, and the 155 was terrible. so we went with a 160. Im pretty sure we have the jet right. I have no clue what you are talking about on the needle thing. Can you explain it a little better? Thanks

 

Do a plug chop to make sure you're running the correct main jet. Find a long road and go up and down it a few times at wide open throttle. Then, while going wide open, quickly pull in the clutch and hit the kill switch. Check the plug. You'll want a tan color.

 

To check the needle take the top cover of the carb off on the 400ex your gonna have to remove to little screws at the bottom of the slide thats where your needle is but before you do that instead of doing that chop test try pulling your choke out just a little bit at a time and running it if it gets better you know your running lean and you either need to go up on the main or raise your needle I cant see how your ex is rich most I have seen including mine are lean from the factory and you might want to set your fuel screw to 2 turns out from lightly seated dont screw it in more than that cause then you could be screwed

 

What did you have it at before you went to the 170?

I'd try going down on the main jet a bit more. I'm runnning a 152 main with a Curtis Sparks pipe, a UNI filter, and a 416 10.8-1 piston. Unless your air filter fell off or something I can't think that your running lean on a 160 main with the mods you have listed. Take it down to a 155 or 152 and see what happens.

Dennis

 

You guys are correct on the over jetting thing, still black. I bought it used with the dynojet kit, and with a 170 jet already installed. It worked fine last summer and started acting up in the cold weather. Shouldnt you run a larger jet in the winter? Im gonna go play with the jetting a little more. I'll let you know how it turns out.