Calling James Dean

3 replies to this topic
  • F-Pilot

Posted April 02, 2001 - 08:12 PM


About the acc. pump mod,
Seems it will disable any squirt at 1/2 and up throttle, hence the larger main?
Is the needle going richer or leaner? I read it to be leaner. I may be wrong.
Fuel screw should be 1 1/2 to 2 out?
Any other tips?
I ride tracks at about 1000 to 1800 ft elev.
Temp is about 70-90 now.
Bike is a '00 with a Pro-Meg (open) and an 8oz flywheel weight (for the occasional trail ride).

  • James_Dean

Posted April 03, 2001 - 08:08 AM



Do you have stock jetting? The settings depend on what you have. BK mentioned richer(?) clip and #168 main. He was also using a #45 pilot in the YZ426.

In general, things can be richened to get more consistent power. There is an interesting post on the YZ/WR250F forum on Fonseca's jetting by SUnruh about a 250F setup.

The pump mod is adjustable, so set the working stroke at about 1-1.5mm stroke. Feel the light freeplay movement in the pivot linkage with your finger. The delay is adjustable too, and this is normally about 3/4-1 turn (in) from where the rod touches the pump diaphram, less delay gets fuel sooner. Too little delay and the pump will not refill if you enter a corner with the throttle cracked, making a small amount of delay better.


[This message has been edited by James Dean (edited 04-03-2001).]

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • F-Pilot

Posted April 03, 2001 - 02:44 PM


Ok, I reread the mod post and it is richer on the needle. I was running stock jetting but recently went to a 45 pilot. Unless you have a better suggestion I am copying the jetting BK recommended with the mod. Will see how it works this weekend.
Thanks for the help!

  • James_Dean

Posted April 04, 2001 - 07:23 AM



The jetting on a stock '01YZ426 includes a needle OBEJP which is a little richer just off idle than the '00. You won't be matching the BK jetting exactly but will be close.

I have another setting which gets more involved on the WR forum, under the title "To the honorable DR J, Professor Taff and Aussie Andy" by splint. This uses many different components to squeeze more out than even BK has been getting (IMHO). Look for - The "works"


Related Content


100 hrs on 2014 yz450f, shim valves or replace them? by ttr230rider6

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • Hot  79 replies

Is it worth it? (Cam and High compression piston) by macgi77

Dirt Bike   Make / Model Specific   Honda   CRF 150/230 F/L
  • Hot  36 replies

First Hare scramble tips by dhend8

Dirt Bike   General Dirt Bike Forums   General Dirt Bike Discussion
  • Hot  33 replies

Thinking about Yamaha 250... by Arctic Pride

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  491 replies

yz400f throttle bog by yz400fmaniac

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • * * * * *
  • Hot  28 replies

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.