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Jetting D-Base for YZ/WR 250F


Nope it didn't fix it.
I took it back to the shop in traverse city Michigan and they bought and installed a fmf jet kit and told me it was fixed and charged me another $145.00 and now not only does it have a big bog but also has a serious hesitation at mid throttle.
Please help!!

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    • By Chicane
      Hi All,
      I've been recently embarked on jetting my DRZ400E FCR39 Slant equipped bike for optimal performance.
      Especially considering that I had a DRZ400S with a the CV carb before that I swear would outrun my current bike, it was faster, responded better and had way better fuel economy.
      That despite the fact that the E is supposed to have the high compression gasket, better carb and cams.
      I have since made some changes and conducted some tests and frankly need some input from the community and those with more technical knowledge or maybe just practical experience.
      The bike is a 2009 DRZ400E Australia model.
      I have Tumbleton & Twist tachometer / electronic speedo installed.
      Starting point ( should be standard )
      Main jet 165
      Main air jet 200
      Pilot 45
      Pilot air jet 60
      Starter jet 65
      Fuel screw turns unkown
      AP arm linkage worn a little more than the standard 0.6 mm play.
      OBDXP needle on 4th clip
      Snorkel removed
      Restriction between carb/engine removed.
      Throttle stop cut down poorly to only allow about 80% slide opening.

      Staintune full exhaust system ( the header barely fits through the frame, shock canister section )
      1. Checked float level at 9mm ( no adjustment )
      2. Cut throttle stop properly to allow slide to just clear the bore of the carb, adjust throttle cables.
          This should already add some performance!  There was still about 1cm or so slide left in the carb at "WOT"
      3. Did the Eddie Mod on the AP. However I'm not yet convinced this was a good thing.
          Even though I removed the stopper from the AP diaphragm I still get separation from the black linkage and the "timing fork"
          at WOT. Now I have to sacrifice the timing to be later than it should be in order to not bind at WOT.
          The moment you adjust the timing to be correct you get bigger separation at WOT, and if you don't wire the linkage to the
          timing fork then you now have a really really long squirt without the stopper in place.
      4. Purchased/installed JDS0001 James Dean Jetting kit.
          As part of the kit you get 155, 160, 162, 168 main jets, new 45 pilot jet and blue/red needle and a useless fuel screw because
          it won't work on bikes with an electric starter.

      5. The carb was fully cleaned, inspected.
      6. Fuel screw 1 3/4 turns. ( doesn't seem to make any difference to idling )
      I live at about 1000ft but my riding could go from sea level to about 4000ft.

      After reading all I could about tuning carbs and what is established knowledge, I set about making the changes to my carb and may have wandered too deep down the rabbit hole.
      There are a number of things not touched on / explained in forum posts which are
      1. Throttle position vs RPM
          All the talk is about throttle position as if RPM doesn't matter.
          But that cannot be accurate.
          An engine at WOT at 5k rpm has different needs than at 10k.   
          The amount of air needed is different and therefore the amount of fuel.
          The air fuel ratio needed for best performance could also be different at various point across the RPM range.
          One of the few tuning articles around that did talk about this was tuning the Weber carbs on 911's.
          This article also touch on how air is going to behave very differently to liquid and therefore fuel metering is affected and don't behave in a linear fashion regarding air speed / fuel metered.
      2. Lean/rich condition testing on Main jet
          In my quest for determining the main jet for my bike I have questions instead of answers.
          Doing some tests I installed 160, 162, 165 and 168 jets all with the blue JD needle at 4th clip to keep things constant.
           This is done going WOT in 3rd/4th from about 5500 to 10000+.
          160 is rubbish - airbox cover/ no airbox cover.
          162 feels good and smooth across rpm range, with no airbox cover the mid range is stronger and top is almost as strong but seem "rougher"
          165 doesn't feels as good as 162 with cover, with no cover it feels stronger than any 162 config by far in the mid range and equal at or stronger at top.
          168 felt worse than 165/162 in different ways.
           The outcome then was not clear to me. I expected to have find a jet that runs better than any other configuration WITH the cover on. Instead I found a jet that ran better than anything else with the cover OFF. The only conclusion I could come to is that the airbox was actually restricting the bike at mid/high rpm and I should go with a 165 and modify the airbox. Which is what I did.....
           This made me question if this type of testing is valid.
           Removing the aircover make the bike leaner?? How really? The engine will only use the amount of air it needs.
           The only way it will go leaner is if before it was actually being restricted.
           This became quite apparent after modifying my air box at the top in a "reversible" way by making 5 x 2cm holes.
           After doing this, taking off the cover made almost no difference at all anymore because the engine was not being restricted anymore - at least that is my take away.
      3. Airbox modifications
         I also decided I needed to be a bit more data driven in my approach, but it is hard without having access to a test track with very long straights or a dyno. Tuning a liter bike would be impossible because of the speeds involved.
         Especially considering I am now breaking away a bit from the "3x3" mod and established jetting. That however is already the case seeing as the DRZ400E just need the "snorkel" removed and then the hole is actually bigger than "3x3" and every bike will be different, with different exhausts and engine state and cams for all the different variants out there.
         Testing my bike over a set distance and measuring the max speed in 4th gear from 5500rpm going WOT.
         This is from about 75km/h at 5500rpm over a short distance accelerating for about 4-5 seconds almost revving out.
          Standard airbox     - 113km/h, 113km/h, 114km/h
          Standard no cover - 114km/h, 114km/h, 115km/h
          Drilled with cover  - 115km/h, 115km/h, 117km/h
          Drilled no cover     - 116km/h, 116km/h, 118km/h
          Even after drilling the air box, running without the cover still had a higher speed at the end of the run.
          The issue I have is how much the needle could be interfering because of starting at 5500rpm.
          Because the run is so short and not really a max speed, maybe other circuits are having an impact.
          I have not repeated this test with the 162 or 168 main or any other needle.
          I'm willing to bet however that a 162 is going to perform worse than the 165.
          Not sure about 168 but I'm not going to open the airbox any more and will just jet for what I have drilled.
          Also, the AP could be interfering over such short runs...
      3. Airbox mod / Low RPM/Mid RPM power?
         What I have observed is that with the cover off and post drilled airbox the mid range RPM power have really suffered on initial roll-on from a closed, 1/4 throttle position.
         Now I have not yet started adjusting the rest and playing with the needles as I wanted to get the Main jet sorted first, however I am concerned that the needle will not solve the issue and was wondering if there was some method to the madness and that by restricting the airflow of the airbox at WOT/high RPM it actually improves mid range power. This would probably depend on the engine design, cams and exhaust system.
         I suppose I will find out soon enough if I start changing the needle and can't get the bike to behave properly in the mid-range.
         Luckily it will be easy for me to close the 5 holes one at a time until I get the desired airflow if needed.
      More to follow......very happy for some input and thoughts.....

      Another really worthwhile read, very detailed and clear with pictures not found elsewhere like emulsion tube cut away etc.
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