str8 jetting question

6 replies to this topic
  • thumper4life

Posted March 10, 2002 - 07:38 AM


i searched it already, but i would really just like the numbers and specs....finally bk'd an '00 and i would like to know what kind of jetting to put in it. it was having horeible popping on deceleration with a one size up larger main from stock, with an fmf powerbomb, e series silencer with no discs and a uni filter. its going to be 45-65 degrees here for a few more weeks, like 6 weeks. and its 850 feet here. so can someone throw some numbers out here please?

  • skthom2320

Posted March 10, 2002 - 05:23 PM


You probably need a little richer jetting all around due to the FMF pipe as well as the BK mod AND the fact that it is a little cold. The main jet is good but you should richen your fuel screw (turn it OUT) to reduce popping on decel. Go at least one size richer on the pilot jet if you exceed 2.75 turns out on the fuel screw.

That should get you started...

Steve T

[ March 10, 2002: Message edited by: skthom2320 ]

  • tuck

Posted March 10, 2002 - 06:38 PM


With BK mod and Thunder Alley pipe I'm running a 45 pilot,168 main,needle 1 clip richer,fuel screw
2 turns out

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  • Boit

Posted March 10, 2002 - 07:04 PM


Popping on decel is an indication of either a lean pilot circuit or an air leak in the intake tract....or even tight valve(s). If you can't tune out the popping via the fuel screw, go with one step richer on the pilot as mentioned by skthom. The main jet has zero effect on decel popping. Unfortunately, the 426 is prone to small air leaks at certain intake junctures. The main area of probable leaks is the airbox to carb adaptor. This adaptor has pressed-in bushings that sometimes lose their integrity which allows the adaptor to slide over the bushings and create a small gap between the adaptor and carb. The easy fix here is to use washers under the Allen head bolts that mount the adaptor to the carb and use a small amount of sealant such as Yamabond. Make sure the "W" ring is properly installed. In my opinion, many riders overlook the possiblility of air leaks and focus on jetting issues as the culprit.

  • Taffy

Posted March 10, 2002 - 10:05 PM


i can't agree with the above answers. first of all it natural for these bikes to mildly pop on decel. when we dial out the popping completely we are making the engine slightly richer. but this should be seperated from "best performance jetting". i bet that red hot pipe has a lot to do with it! (unburnt gases/secondary burning methinks)

the MJ does have an effect on the popping. i would however say that on the typical MX bike you wouldn't notice it. but trail riding or enduroing in gloop i often just let the revs go from on high down to the lowest revs all in one gear.

if on immediately closing the throttle it back pops gently; it's your MJ/needle clip. upon arriving at the corner and just as you put your foot down (for instance) that's your PC.

we discuss jetting far far mor eover on the WR side and i think you'll find one or ten riders are trying the specs in my signiture and adding 10 MJ for the open pipe (needle down one).

go see!


  • skthom2320

Posted March 11, 2002 - 05:30 AM


Actually, I agree w/ both Boit and Taffy. Boit makes a good point, don't overlook air leaks and/or the possibility of tight valves.

Taffy also makes a good point. My bike runs best when there is very slight popping on decel. You described yours as horrible though.

Try the easy stuff first and let us know how it goes...

Steve T

  • thumper4life

Posted March 11, 2002 - 12:46 PM


will do....that bike is totally apart right now, the subframe and swingarm is off, we're greasin it up and stuff. found the frayed cables everyones been talkin about too, so it'll be about 3 weeks before he rides again.

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