Need help with 400ex

Im a bit of a newbie but im fairly mechanically inclined so bear with me! I recently picked up this bike its a 2000 400ex. It has a twist throttle and K&N filter. Other than that it is stock. Previous owner says it burns a little oil (rings?). Never been rebuilt. The day i got it it wouldn't idle. Still doesn't. I rode it all day with no idle but other than that it ran great. It was smoking a little though. White smoke that i only noticed at start up. When i downshifted it would backfire sometimes. At the end of the day it died. It would start but still no idle and it bogs down bad in the middle of the throttle and will die on the top of the throttle. I figured maybe it fouled a plug. When i checked the plug (today) it was black. Put in a new plug and it starts easier but still same issues. I took off the carb. I checked everything out. The main jet is stock 148 and clean as a whistle. The flaots are clean and move freely. The pilot jet was stock 38 and clogged to hell. I changed it out with a new 40. The carb looks almost immaculately clean besides the dirty pilot jet. put it back together and it still wont idle and still bogs out. Oh and when i was in there i noticed the idle screw wasn't even close to touching the throttle. I turned it in till it was just touching. Once it was back together I tried turning the screw in with no difference all the way in. So thats about it. I would like to know whats wrong. I have a Honda Service manual. I'm gonna try and do any work myself. My guess is needs new top end. Thanks in advance for all your help!!!!

Oh and also by the looks of the side cases it hasn't had that hard of a life. The whole bike looks to be in good condition overall.

Id say valves need to be reseated thats what happened to friends the valves were blowing white smoke but it also sounds like you need jetting if the bogging didn,t go away when cleaned like you said it could be rings that wouldn,t be that hard to do Id do a 416/426 bore on it why its apart then cams aswell.

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Originally posted by RATPACK Z400
Id say valves need to be reseated thats what happened to friends the valves were blowing white smoke but it also sounds like you need jetting if the bogging didn,t go away when cleaned like you said it could be rings that wouldn,t be that hard to do Id do a 416/426 bore on it why its apart then cams aswell.

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I didn't clean the carb at all. I was going too but it looked real nice except for the clogged pilot jet. Its not rideable at all like this and the spark plug gets wet (maybe oil because its brownish?) when i start it and rev it. Would valves make it run that bad? if so:D . I'm not gonna bore it unless i have too. I dont race or anything just like to trial ride, climb some hills and rip through creeks! The power is fine for me.

also why wouldn't it idle? i'd think the 40 pilot would be fine with just a K&N

I would still pull the bowl off the carb. Pull the jets and spray it all down with carb cleaner. Make sure your needle is seating properly too. Even look into a 450r carb. Makes a world of diff.

Check around the carb for vacumn leaks. Spray some carb cleaner around the boot, also, try taking the lid off the air cleaner, see how it runs. I wouldn't tear into it until you know all your issues.


1 Carb should be clean and operate correctly.

2 Air cleaner should be clean.

3 No vaccumn leaks

4 Check compression, leak down test too.

5 Clean oil and right weight for conditions.

6 Valves should be checked for clearance since the bike is new to you.


Treat the bike like everything is wrong and check off everything that is right. Either that or you'll find yourself getting towed out by a friend or stuck somewhere. After you figure everything out then figure what you want to spend or Need to do. Quads are more fun when you ride them then wrenching them. My quads needed a rebuild due to rings but she still runs fine, the smoke comes out the tail and I don't smell it!!! Plus, my buddies got a Suzuki ltz and my 400ex will pull right beside him!

Good luck and hope this helps, Matt

Great thanks! I'll go over the checklist. I gotta get a compression tester. One thing i do wanna add is it was running well the day i rode it. Oil is good and clean runnin 10w-40 i heated my garage up while working on it so it was warm-ish. I am not interested in buying a carb for it unless i would have too. thanks to all!!!!