99 yz400f carb issue?

5 replies to this topic
  • jasonknight

Posted August 02, 2010 - 03:26 PM


I did my best to look through the stickies and searched posts but couldn't find my problem. I apologize if this has been discussed already

1999 stock yz400f, only mod is a different exhaust pipe...it ran great for many years

last year I rode this bike ant it ran like a champ. pulled it out a few months ago and took off for the desert and ran into issues. Bike would start but didn't want to idle. When you came to a stop you had to feather the gas to keep it running. when moving the bike ran fine, just wouldnt' idle. we drained the gas out and put in fresh, that seemed to help a little bit, but the issue persisted. That was back in Feb.

I'm ready to start riding again so I went through my routine. Checked teh valves, intakes were a little out of spec so i reshimmed. Exhaust valve clearances were in spec.

Cleaned the carb with carb cleaner, removed all jets and cleaned with carb cleaner and compressed air.

Got it all put back together and the symptoms changed. Now it will start and idle just fine with the choke on. but even after warming up for a few minutes (i live in San Diego at sea level) the bike will die as soon as you turn the choke off or touch the throttle.

is this a common problem? any quick checks or thoughts? I'm in the garage now taking the carb apart and rechecking everything again and hopin for the best.

Thanks in advance

  • jasonknight

Posted August 02, 2010 - 06:31 PM


got it all back together for a second time....new symptoms

1) hard to start, finally got it to fire with the hot start pulled.
2) let it warm up for 5 minutes, pushed in the choke and it dies immediately.

will idle with the hot start on or off,

it used to die as soon as you hit the throttle, now if you leave the choke on it will rev good, but will still die without choke..

what gives? started with the pilot screw out 1.5 turns like the manual said

  • baxterj787

Posted August 02, 2010 - 07:14 PM


I know you said you cleaned the jets, but did you actually spray out the passages inside the carb body? The pilot jet and the fuel screw each have their own opening in the venturi of the carb. If these passages are not clean and do not allow a healthy jet of carb cleaner through, then you can clean the jets all day long and get nowhere.

Also (and this has been covered elsewhere, but I cannot overemphasize), make SURE you have the tiny flat washer and tiny o-ring present and accounted for (off of the fuel screw) before you spray anything through the fuel screw passage. If that o ring disappears, you will drive yourself nuts trying to tune the carb. I have learned this one the hard way many many years ago.

I am on my second 426, and this is the only bike I have ever owned that seems to like frequent carb cleans and fresh gasoline.

Yours in Sport,

Jason B

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • myoung

Posted August 02, 2010 - 11:03 PM


Did you take the slide out? If you did, did you get the slide plate put back on right side up?

Did you clean out the pilot circuit passages as well as the jets? Did you spray cleaner on everything without removing the rubber seals and o-rings? Did you reassemble the pilot screw washer and o-ring correctly?

This is all typically in the pilot circuit somewhere. If you've exhausted all your options with jets and the pilot screw, it's probably a clogged passage. It's accessible, but not recommended by many to bother with it. I can tell you a clogged passage was my problem and it was easy to fix, but for others it might not be. These carbs are not the best design, later model FCR's are the preferred permanent fix by most around here.

  • jasonknight

Posted August 03, 2010 - 10:29 AM


thanks guys, everything i read points to the pilot jet/orfices. I've sprayed them twice out twice with carb cleaner, ports seem to be wide open...Also...slide never came out, it looked clean. although the needle valve that works with the slide was filthy

i'll clean again and hope for the best....also guess i should try new gas...gas in the tank is only a few months old buy you never know


  • grayracer513

Posted August 03, 2010 - 10:45 AM


Could be worth your while to inspect the vacuum release plate on the engine side of the slide (the same one mentioned earlier) because they have been known to wear, then crack and/or loose a chunk. Makes them behave badly.

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