99 400 not running quite right

13 replies to this topic
  • klxnewguy

Posted December 08, 2007 - 01:27 PM


Yesterday my father purchased a 99 yz 400, the thing is like brand new but it has been sitting for almost 5 years!! Today we took the carb apart and i couldn't believe all the build up etc.. I clean it all out, took out all the jets and the float, needle etc.. and got it all cleaned out. We put in fresh gas, we replaced the spark plug etc.. and it fired up with the choke on. I let it warm up and then went to push the choke in and off it went. I opened the choke again and it fired up and I rode it down the street, while riding I pushed the choke in again and sure enough it died. Any suggestions? With the choke on the bike rips!! and all is great but I know something isn't 100%. Any advice would be GREAT!! Thanks guys.

  • grayracer513

Posted December 08, 2007 - 08:06 PM


There's still something blocking up the carb.

  • klxnewguy

Posted December 08, 2007 - 08:23 PM


Were do you think I should look first. I pulled out the main, pilot and the other one on the other side along with the needle and floats and cleaned them all out. I also pulled the top off when I pulled the needle out. Do you think I should "dip" the whole carb in carb cleaner, then just put on all new seals etc.? What would you do? Thanks for the response!:banghead:

  • grayracer513

Posted December 08, 2007 - 09:23 PM


Did you have have the slide out? If so, you may have made the common mistake of installing the spring plate on the back of the slide inverted. The flat edge with the hole near it goes down.

If that's not it, remove all of the jets and run carb cleaner and compressed air through the passages. If you dip the carb, completely disassemble it first.

  • klxnewguy

Posted December 09, 2007 - 07:11 AM


I didn't pull out the slide, maybe I should have though? I'll pull it all out again and clean it again. I did use a ton of carb cleaner etc.. but I didn't blow air through it and maybe that's the ticket. Thanks, I'll give it a shot and let you know what happens.

  • Thomas6x

Posted December 09, 2007 - 05:35 PM


First thing carb cleaner in a spraycan is throwing your money away. The only thing to use it on is to spray the out side of it.
The only way to do it right is to dip it, and there is no other way.
Old gas turns in to a varnish and is in every hole and passage that had gas in it.

  • redrocket190

Posted December 10, 2007 - 09:47 AM


I agree that a complete clean and re-build of the carb is the way to go, if only for peace of mind. I understand that cracks are a possibility in the throttle slide plate, so you might want to look carefully at that.

Just recently my WR400F would not start and run properly and we found the plastic clip around the main jet tower in the float bowl was worn out and falling down - but only when the carb was put together and you couldn't see it!

I would also mention that if the bike is not warmed up, it will die when you turn the choke off. Leave it idling for a good few minutes until the radiator is nice and warm. I recommend a temp sticker attached to the bottom of the left-hand radiator where you can see it.

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  • adamdf

Posted December 10, 2007 - 11:54 AM


Damn, 99 YZ400s still working? Imagine if there were 1999 CRFs, they definitely wouldn't be. Good luck with the carb, atleast you know the valves are ok!

  • jayh300

Posted December 10, 2007 - 12:36 PM


Damn, 99 YZ400s still working? Imagine if there were 1999 CRFs, they definitely wouldn't be. Good luck with the carb, atleast you know the valves are ok!

yeah... my 99 still runs great, the only reason it had a valve shim change was a hot cams install. It is on break right now as it rider had a boke collar bone, quad ran me off a trail....but the bike is fine.
back to the carb.... pull it completly apart and dip it. Check all the jets and passages to make sure they are all clean. Make sure the fuel screw oring spring and washer are in correct and the pilot jet, as well as the others, is completely clean. don't ask me how I know this.... I think we refer to it as "experience"

check your hot start to make sure it is closing correctly and will open while you are there.


  • klxnewguy

Posted December 10, 2007 - 07:46 PM


If I "dip" it what about all the ruber components? I called and they don't make a kit with just the ruber so I think I would have to buy them one by one. I put the same question in the jetting forum and they told me to pull out the fuel screw and spray through the pilot jet until I get clean carb cleaner coming through? Do you get "dip" at the dirt bike store or do you have to head to a car place? Thanks guys!!

  • jayh300

Posted December 11, 2007 - 05:37 AM


I got mine at a car part store. berryman chem dip in a 1 gallon metal can. it has a basket in it. Get all the rubber parts out. The chem dip will distort the rubber parts. there is no kit for them and i had mine off numerous times and did not need anything extra.

  • klxnewguy

Posted December 11, 2007 - 02:12 PM


Cool, thanks for the info. How long did you soak it for? Are there many ruber parts other than the bowl, needle set?

  • Caseus

Posted December 12, 2007 - 03:15 PM


I just picked up a 98 YZ400F that had the exact same problem you describe.

After throughly cleaning the carb and replacing the Accelerator pump diaphragm it was still doing the same thing. I had already cleaned the carb two times as many people suggested on this very forum, but as I have learned there is something to be said for clarity. The first thing I missed as I learned, was the two air jets hiding under that intake plenum, 2 allen type bolts secure it to the carb. Those little suckers can get goo in them from the airbox so I have discovered. Additionally you may want to closely inspect the rubber boot that secures the carb to the intake on the engine. Mine was cracked and leaking air into the cylinder causing an overly lean (overheating/no idle) condition.

Interestingly enough the above problems made my YZ spit flame out the silencer and backfire like crazy at half throttle. Impressive as the light show was I thought to myself something was terribly wrong. This being my first 4 stroke there was definetly a learning curve but I managed to get it running right in the end.

Hope this helps you track down that no idle problem...

  • evilinsano

Posted December 12, 2007 - 09:55 PM


I don't know what that screw at the bottom of the carb is called but it sounds like that needs adjustment.

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