12 replies to this topic
  • brownz71

Posted May 28, 2015 - 09:51 PM


Hi All,


Last week i picked up a 98 Yzf 400. Ive gone through the whole bike and replaced what was needed. Air filter, plug, checked valve specs, and cleaned everything really well. Tried to fire it up and it started for about 3 seconds then dies. I figured i needed to clean the carb, so tonight i did so. Turned out the carb was fairly dirty inside. I pulled everything out of the carb and blasted every hole with carb cleaner. Triple checked that everything was clean and free from debri. The carb is the Keihin FCR flat slide. Hooked everything back up and its still doing the same thing. Seems to me that its wanted more fuel? How do i fix this? Any ideas? Here is a link to what its doing.






Edited by brownz71, May 28, 2015 - 10:01 PM.

  • OldOkie

Posted May 28, 2015 - 09:54 PM


Good chance its the pilot jet from everything I've read. I just picked up a 98 400 on Monday and same thing. Cleaned the carb + new pilot and main jets and solved the problem. First kick starts when cold.

  • brownz71

Posted May 28, 2015 - 10:00 PM


Ive seen a lot of threads and talk about the pilot jet! I made sure i cleaned that one out especially. I might just have to order all new jets just to rule them out. Curious where did you get your jets from?

  • OldOkie

Posted May 28, 2015 - 10:38 PM


Local dealer had them in stock. $16 for both

  • grayracer513

Posted May 29, 2015 - 09:34 AM


Read: http://www.thumperta...o/#entry6879695


Also, check the vacuum release plate on the back (engine side) of the slide.  Be sure it's installed with the "square" side down (not the round side), and that it isn't cracked or missing a corner.  Pull the seal out and let it dry overnight so that it shrinks back into a more normal shape, then inspect it for tears, etc.  If it seems OK, put it back.


If you just can't ever seem to get it right, there is one thing left to try.  First, order the center body gasket from JD Jetting.  They are, as far as I know, the only source of it.  This is the gasket that goes between the upper body and the lower body/upper float chamber.  DO NOT REMOVE the four screws that hold these two parts together until you have the new gasket in hand, because you can't put it back together without one.  You may find some gunk built up in there, or evidence of an internal leak.

  • brownz71

Posted May 29, 2015 - 05:18 PM




So i came home today and tried to fiddle around and further my knowledge on the issue. I got it to kick over with one kick with the choke open. It then ran for 3 seconds and died. I took off the carb and adjusted my fuel/air screw( the one on the bottom of the carb next to the drain on the bowl. I set it all the way in then 2 turns out. Kicked it over again and it idles a lot better. Only it will idle forever with the choke open and if i slowly give it gas it backfires and dies. And also wont idle at all with the choke in the off position. Next Suggestions please?

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  • brownz71

Posted May 29, 2015 - 07:47 PM


Could this be an injector pump problem?

  • OldOkie

Posted May 29, 2015 - 08:04 PM


That's easy to see, pull the airbox boot and twist the throttle. If pump is working then a small stream will shoot through the now open slide towards the intake

  • grayracer513

Posted May 30, 2015 - 06:40 AM


You mean accelerator pump.  No. It's a blocked fuel passage, probably in the pilot circuit.

  • brownz71

Posted June 08, 2015 - 10:54 AM


Turns out I had defective pilot jet. All is fixed and I was able to turn it over and ride! Also any ideas on how to get more response at the lower end? Seems a little boggy just starting off.

  • OldOkie

Posted June 08, 2015 - 12:00 PM


Exhaust and rejet put mine where I want it on stock sprocket sizes. I ride in hayfields and a big wooded section. Glad you got it running man.

  • brownz71

Posted June 08, 2015 - 12:12 PM


Thanks Olkie! It has a full pro circuit exhaust t4 I think on it..... What do you mean by a rejet? Bring the main down a bit?

  • OldOkie

Posted June 08, 2015 - 02:03 PM


Tuning the jetting sizes to match added flow of the pipes.

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