98 YZ400 Jetting update

8 replies to this topic
  • wilson

Posted May 11, 2000 - 01:06 PM


I changed my air screw to 2 1/4 turns out and went to a #48 pilot jet. No change detected in my hesitation problem on quick throttle bursts. Fuel pump works fine. Any other ideas?

Even with the hesitation, this thing is way easier to ride and way more fun than my '95 KX250 ever was - not to mention my YZ actually starts when I want it to!

Oh yeah, I love pulling up to the gas pump, topping it off and riding all day on a tank of un-premixed gas!

Paul in AZ

  • jbird

Posted May 12, 2000 - 04:37 AM


Just bought a 99 YZ400 last week and have the same problem if throtle is wacked open from low rpm. Asked a buddy who has the same bike and lots of 4-stroke experience what the deal was. He told me the cause was almost like a flooding situation. When throtle is wacked open from low rpm the fuel pump shoots a burst of fuel that is just too much and drowns the bike. Just like if the bike is shut down and you turn the trotle a few times and the bike is flooded. When bike is in higher rpm's this is not a problem.
Don't know if this is fact or fiction but seems to make sense.

  • James_Dean

Posted May 12, 2000 - 04:46 AM


See Clark's comments in the WR400 discussion area under the posting "Hey Clark!!" for his pilot and pilot screw setting. You could try up or down on the needle clip or the DVP needle. Be sure to read Clark's tips on jetting in the tech section under the WR400, it explains needle selections.

  • Tim

Posted May 12, 2000 - 05:17 AM


Hey guys,
I also agree with the "too much gas at one time" theroy. I have messed with my carb a bunch, and have finally admited to myself that this bike is just not going to have the low rpm/ whacked open response as a two stroke. I feel that the reason Yahmaha did not fix this from the factory is because any fiddling with the volume of the pumper, would effect the great response in the upper RPM ranges. A give and take situation, The nature of the beast, or the greater good is what I am writing this off as. In real life riding situations, I have not had any problems with this. You just have to think a little more and make your throttle imputs a little smoother down low. Although I may be wrong and if someone comes up with a fix, please post it!

[This message has been edited by Tim (edited 05-12-2000).]

[This message has been edited by Tim (edited 05-12-2000).]

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  • Guest_Guest_*

Posted May 12, 2000 - 07:53 PM


I think it was posted before by someone else, but my '98 YZ is really picky about old gas.
If gas left in the bike a week or two I really get a hesitation off idle when wicked quick. Seems to go away when fresh fuel used

Ken in OR

  • Matt_Porritt

Posted May 12, 2000 - 10:38 PM


Check this out.
I know of someone who has brought one and they said it was great.. fixed the hicup. http://www.mxsouth.com/perp38lit.htm

**Ride it like you Stole it!**
Matt Porritt
99 YZ400F
Rubber Chicken Racing

  • Clark_Mason

Posted May 13, 2000 - 03:33 PM


The P-38 changes the timing and flow rate of the accelerator pump. It shortens the stroke of the diaphram making the shot shorter. It also has sligtly larger exit orifaces from the pump champer to the exit spigot in the carb throat so the shot is a little stronger but shorter.

I installed one on my WR and with a little fiddeling and adjustment by the Factory R&D guys its working great. If you need more info Call Ken at Factory R&D and get the pitch and technical stuff from the guy who rides YZ 400's and designed this piece.


  • Shawn

Posted May 15, 2000 - 08:03 PM


I found this link to this question on the Dirt rider net forums.
Why does my YZF400 run so lean?
The YZF400 runs lean and hot because it didn't come from the factory jetted correctly. It needs a 50 pilot jet and a 180 main jet to cure the problem. 0 - 4,000 feet.
I haven't tried it but I have only had the bike for a month now.

99YZ400F (him)
00TTR125 (her)

[This message has been edited by Shawn (edited 05-15-2000).]

  • Guest_Guest_*

Posted February 17, 2001 - 01:08 AM


I have a 98 WR 400F which whose carbeurator has had extensive hours on the bench. Although much improved, I am still having terrible trouble with hesitation with a quick throtle blip. When I took apart the accelorator pump assembly I found it to be corroded where gas had evaporated from the pump assembly. The way the fuel pump works on the WR is poorly designed. If you operate the fuel pump with the carb off the bike you see that the pump shoots a stream of raw fuel directly into the intake. The fuel does not have a chance to be atomized or mixed properly with the air. The idea of the P38 seems to be logical because there would be less gas to bogg down the intake. But I have never had a four-stroke that has required a fuel pump and I am not understanding what is creating the need for one. If someone could explain in more detail what the difference between the bottom pieces I would consider buying one.

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