Jetting help needed

4 replies to this topic
  • 2in2out

Posted September 21, 2009 - 11:15 AM


I need help guys!! I'm an idiot when it comes to carbs, and after much fear and anxiety, I had to open up my carb for adjustment after and exhaust mod, and I need some help.

Here's what I have:

Year: 2002
Model (400 - 426 - 450): 426
Cam Timing (WR/YZ): WR
Main jet: 168
Pilot Jet: 42
PAJ: 75
MAJ: 200
Leak Jet: N/A
Starter Jet: 65
Fuel screw (turns out): White Bros. 2 out (set to original)
Needle Model/Clip position: Stock needle, was in clip 4, tried 3 w/o success
Grey Wire Mod (Y/N): Y
TPS Connected (Y/N): Y
Airbox Snorkel/Lid (on or off): Lid off
AIS Removed (Y/N): N/A
Airfilter Brand: NoToil
Exhaust Brand: Jardine RT4 w/Jardine mid-pipe, FMF Powerbomb SS Header (This was the recent change stimulating this)
Average Altitude: 3500-7000 MSL
Average Temperature: 70
Average Humidity: 7-18%
Degree of Satisfaction (0-10): 8
Additional Mods/Comments:

Prior to addition of Powerbomb header, only symptom at all was intermittent off-idle bog when hot. Otherwise flawless.

Installed Powerbomb header and warm idle at 4000, will not back off with throttle whack. Raise Needle clip to #3, and now bogs at throttle whack, changed to clip #5, no bog, no change in idle.

Help please. :busted: I've read every jetting post since I've joined to try to gain an ounce of an idea, and I can't figure this out, and I need it done soon as I'm trying to sell the bike. Hopefully getting a 2005 WR450 today.

Should I go with a larger PJ? The MJ was fine prior to the change, and I haven't been able to ride yet to test if I need change.

  • mongial

Posted September 21, 2009 - 01:29 PM


Just curious, have you played with the idle screw? I wouldn't think that the power bomb header would make a huge difference in jetting. Sounds like the idle circuit to me (but I am also still confused on the 4 stroke stuff....just started to figure the 2t stuff out when I hopped on the bandwagon.)

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  • 2in2out

Posted September 21, 2009 - 02:16 PM


Idle screw is fine. Actually I usually have it all the way in, or a 1/4 turn out. Only needed to change it when I got above 6000 ft. elevation.

  • William1

Posted September 21, 2009 - 02:35 PM


If you ride with the fuel screw all the way in, your pilot is too big.

Normally, a Hanging idle is one of a few things
1)Too small a pilot or lean due to a stuck Hot Start - Check the hot start, it is not the pilot.
2)Slide release plate in upside down or broken - You can pop the carb off the engine and look in and see easy enough
3)Damaged throttle cable/kinked
4)Blue wire disconnected and idle speed a tad high.

These are 99% of the causes.

  • 2in2out

Posted September 21, 2009 - 04:34 PM


Changed to a 40 PJ. Fixed now. Thanks for the suggestions.


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