Need help with a jetting question

4 replies to this topic
  • Thinblueline

Posted November 08, 2009 - 05:33 PM


Hey guys im sorry to ask a jetting question, however I have been searching for hours and can not find an answer. Heres my set up:

99 YZ400F
Power Bomb header, with stock exhaust after it.
top of air box removed with UNI filter
50 tooth rear sprocket

I just bought the bike and trying to get her dialed in. I rode today for a few hours and she liked to stall or bog unless I was really getting on the throttle. So it was useless from 1/4 throttle and less, it would just stall. Otherwise she pulls like a monster and runs great. just my first 1/4-1/2 blows. I need to know if I should be closer to the stock jetting with these minor mods. This is whats in the carb now

fuel screw 2 turns out.

The bike stalls / bogs if I crack the throttle from idle while in neutral. I apologize again if this has been covered, but I searched and searched and could not find an answer. Should I just go back to stock settings? Thanks in advance guys!!

  • Thinblueline

Posted November 08, 2009 - 05:45 PM


OK Guys im guessing the only difference is the main jet from stock settings. And my problem doesnt seem to be in the main jet range in reference to the throttle position. So im guessing theres another issue im gonna have to look into. Maybe the taffy mod will help it out. Moderators please close this thread. thanks

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  • grayracer513

Posted November 08, 2009 - 08:47 PM


The taffy mod is only an adjustment to the accelerator pump, and will likely not help your problem as you describe it.

The first thing to look at with a YZ400 behaving this way would be the spring plate on the engine side of the carb slide. These have a tendency to crack or break as they age and wear, and can cause trouble (typically described as running rich, stuttering, running poorly until the throttle is opened beyond 1/4-1/3 or so). The next thing would be to verify that there is a pilot screw actually in the pilot screw bore, as they do sometimes fall out. From that point, go over things carefully, being sure that everything is clean and open and assembled correctly.

  • Thinblueline

Posted November 10, 2009 - 06:06 PM


Well I got it all figured out today. The bike is an animal now! The spring behind the black AP arm was weak and not moving the AP rod enough to produce a squirt. I was able to in essence wind the spring up and have it tucked under the silver arm which comes down and moves the black arm. This fixed it perfect!! better then wiring it up. Now I just have to do the taffy mod and im set!

  • tek_01

Posted November 10, 2009 - 09:45 PM


good to hear you got it worked out! i ordered a stiffer spring for mine from merge racing

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