dead spot coming off idle 99' YZ400

11 replies to this topic
  • horseman3

Posted April 29, 2006 - 03:08 PM


Anyone have any suggestions for the dead spot im getting coming off idle? Ive seen a few people talking but never really found an answer. Bumping up the pilot jet help maybe? Any help is appreciated.

  • Zique

Posted April 29, 2006 - 03:34 PM


Not exactly sure what 'dead spot' you are talking about. Popping on decel can usually be fixed by a quick adjustment of the air/fuel screw...If that is a no go, then your pilot needs changing.

  • 642MX

Posted April 29, 2006 - 03:54 PM


A fuel screw adjustment could take care of it. If it doesn't, then you may want to clean your carb.

  • Satch0922

Posted April 29, 2006 - 04:16 PM


do a search for "Yamabog"

  • ben_suhard

Posted April 29, 2006 - 04:16 PM


Anyone have any suggestions for the dead spot im getting coming off idle? Ive seen a few people talking but never really found an answer. Bumping up the pilot jet help maybe? Any help is appreciated.

Does it happen when the bike's on a stand and you open the throttle quickly, or does it happen while you're riding? If the jetting is all correct (e.g the fuel screw doesn't need more than 2.5 turns) and if it doesn't happen while you're riding, don't worry about it.
Four-strokes don't create as much vacuum at low revs as a two-stroke so you'll get a bit of a flat spot when you twist the throttle quickly. Some people turn the idle up a bit so that the engine is creating more vacuum at the time the throttle is cracked open.
how to adjust your fuel screw

  • Satch0922

Posted April 29, 2006 - 05:18 PM


here is a start......


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  • horseman3

Posted April 30, 2006 - 06:16 AM


No, it happens when first starting to go , it will like bog then take off, its almost like you have to rev the engine up some before you let the clutch out so it wont do it. Trying to be as specific as possible. Seems like it does it in every gear coming off 1/4 throttle. Mid and high end seems fine.

  • Zique

Posted April 30, 2006 - 07:19 AM


Ahhh...that information definitely helps. Some other guys on here can probably diagnose that in a second. If I had to guess, I would say your pilot is off. If your bike was a 426, I would suggest the BK mod, darn. I think there are still some AP mods you can do, but you'd have to do to a search on that one. Like Satch said, try searching 'yamabog'

Hope that helps.

  • ninthrazor

Posted July 07, 2008 - 05:20 PM


I have a 2000 YZ 426 with the similar problem. I was talking to an amateur racer buddy of mine and he said that the 250, 400, 426, and 450's with a Keihn FCR carb all have the same problem with the "dead spot" off idle to 1/4 throttle. He recommended me, as he did, buy the smallest "slow jet" (which happens to be a 35) and install that. That should help with the problem. he also said if that doesn't fix the problem, many of his racer buddies are just pulling out the slow jet and filling the hole with JB weld. He definately said start with the small slow jet, then move to the JB weld only as a last resort. Hope that helps a little.

  • mjw1

Posted July 07, 2008 - 06:49 PM


i put a power now valve on my carb and got rid of the bog completly.

  • DeeCee

Posted July 07, 2008 - 06:56 PM


I have a '98 400 run standard jetting and have no bog issues at all. I therefore suggest you do not take any drastic measures as suggested by the previous post as this will just increase your problems.

First off I would make sure the carburettor internals are very clean, remove all the jest and blow out all the passages. Also check the diaphragm and operation of the AP.

Then set your fuel screw at 1 and half turns out and see how it runs.

Once you have taken these steps, start looking at changing your pilot jet.

  • 1998YZ400F

Posted July 07, 2008 - 10:25 PM


yeah im with deecee. sounds like the carb needs cleaning. if you dont have any major mods the stock jetting and fuel screw setting should be fine.

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