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Jetting Guide

Discussion in 'The Garage' started by gentlemanjim, Dec 30, 2010.

  1. mcrowell75

    mcrowell75 XS400 Enthusiast

    Interesting reading. I'm lost when it comes to this. It really gets old, pulling the carbs on and off, each time knowing that the manifold boots are getting a little more worn down.

    I suppose this is where I'll start once my new mufflers and filters get here.
  2. Travis

    Travis Staff Member Staff Member

    I don't know about that method for getting your starting point. My rule is if you only change one thing, start with new stock jets and work your way up. A lot of times, you don't need to go as big as you might think. If you change the exhaust side and intake side dramatically (free flowing exhaust and pod filters) then start with one size bigger on the pilots and mains and work your way up.
    Kat Majdali likes this.
  3. JamesDavies

    JamesDavies XS400 Member

    where would i be able to buy new jets from ?
  4. gentlemanjim

    gentlemanjim More Wrenchin than Ridin

    Yes as Travis said. All too often we go overboard when just a little change is all that's needed. Get jets from MikesXS.com
  5. Liamthedevastator

    Liamthedevastator It's dingaling'n time

    Important note, the link is now dead. Could someone rehost it?
  6. 16VGTIDave

    16VGTIDave XS400 Guru Top Contributor

    I used to think that guides like this were helpful. Now I know better. In no particular order, this is my guide:

    - Plan on a day to get the carbs jetted correctly. A farmer's 12+ hour day, not a modern "3 hour work day". And start with a full tank of fuel, you'll need it.
    - The bike MUST be fully warmed up. 10 - 15 minutes hard riding at 75F should do it. More if colder. The goal is to get the engine AND OIL to operating temperature.
    - You will need a selection of Main jets, and possibly Pilot jets available. Buy them in pairs. No, you can't drill jets despite what your uncle says.
    - You will need "less traveled" roads that you can perform repeated full throttle runs on. Not necessarily top speed, but certainly full throttle. Consider your neighbours and the local constabulary.
    - Start with clean carbs with stock jets/settings and all the basics taken care of (fresh spark plugs, valves adjusted, etc).
    - Have the bike in it's final state. There is no point in tuning the bike today if you are going to change the mufflers next week.
    - - To be clear - Make NO other changes to the bike during the tuning process. You are either ready to tune or you aren't.
    - Read and follow the steps found here: http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html
    - - Print out that page so you can refer back to it.
    - Take notes! Some thing like this:
    Stock jetting
    Surging at full throttle from 7000 RPM and up.
    - Increase Main jets to 130 - surging at full throttle from 8500 RPM and up.
    - Increase Main jets to 132.5 - pulls well above 7000
    - Increase Main jets to 135 - stumbling from 7000 and up
    - Decrease Main jets to 132.5 - pulls well above 7000
    Stumbling from 5000 to 7000.
    - Lower Needles 1 groove - smooth from 5500 and up.
    Soggy from 3000 - 5000...

    If you think you are good enough to make 2 adjustments at once, or take a shortcut, go for it. You will find out that your mommy was blinded by love when she told you how good you are. Tuning is a process. Either commit to the process, or don't. But don't waste your time.

    Be patient, be safe, and have fun! It is very rewarding to get a bike tuned correctly. So few are these days...
    Kat Majdali and IanW like this.
  7. Liamthedevastator

    Liamthedevastator It's dingaling'n time

    Thanks for the link BBS360.

    An ego only mummy could love:laugh:

    Great post though. That'll help when I get to the tuning part of my build. I like your field notes process, not to mention riding up a straight with your leathers and then making notes about the power seems suuuper badass:cool:
  8. WelderDave

    WelderDave XS400 Addict

    Really liked this thread, good to know. I will be doing my carbs soonish, and this helped me to figure out about how many jet sizes i should order t start off. I have individual uni pod filters and cheapo emgo mufflers so relatively free flowing exhaust. guess I'll buy 4 sizes up to start. Either mikesxs or jetsrus.com

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