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Tuning Carbs GPX

Discussion in 'Technical and Troubleshooting Torque' started by Zbike, Feb 1, 2008.

  1. Does anyone run a race exhust or any after market one and very non restrictive air filters on there GPX,

    I would like to know how far u have raised the needle and what jets u are using just to start me off tunning in my one,

    Iv googled and check the ex250 site ect just saiz every bike is a beutifull and unique snowflake and is there fore completly diffrent. so it wasnt much help.

    I would do trail and error but i just had the engine rebuilt and need to run it in first so i just need a close to good setting ATM

  2. I've had luck in the past just using the settings that Dynojet do for their
    kits, even though I didn't have the kit.
    I.E. 3rd clip from the top, main jet 098.
    That should get you riding enough to make it to the local dyno centre
    for a proper tuneup.
  3. hmmm ok, my needle doesnt have clips and the dyno jet jets use there own numbering system.

    i might try 3mm up and 110 main jet today.

    not going to a dyno, my mate has a A/F meter ill borrow that
  4. Whoa!!

    Just one washer is usually all that ius needed, placed under the "knob" on top of the jet needle. OD 7 mm, ID 3 mm, usually.

    The OD can be important: the washer or the original knob usually sits in a small recess in the top of the slide. If the washer OD is too great, it will hold the needle up even higher, and the resulting richness might be very annoying!

    Go up one size in mains at a time. Do you know how to tell if it is "right"?


    Trevor G
  5. Well im guessing when i hit a non boggy smooth power curve then its the right setting but if u got any pointers id love to here em,

    Iv read a few guides, but havnt grasped it yet.
  6. The main jet really is on its own over the 3/4 to full throttle position. If this last movement does not give smooth acceleration in lower gears (it's too hard to check in 4th to 6th) then the main might be too small.

    You need to be at least at 2/3 of the rev limit too, when checking it. That doesn't give a lot of time to evaluate. That's why you try the average change listed earlier. Don't jerk the throttle open - open it smoothly and gradually to see the response.

    The needle controls about 1/4 to 3/4, but if the throttle slide or main jet are way out, they will affect the way the needle jet response feels. In other words, make sure the main jet is within a reasonable size of what it should be. Otherwise, if you get the needle right with the main jet "out", once you sort the main jet, you will have to do the needle again.

    The best thing is to ride the bike in its standard form as if you are checking carburation, then make the changes and test again.

    When checking needle jet you need to be in the 1/4 to 3/4 rev range as well. Vary the throttle opening between the 1/4 to 3/4 position to see how the engine responds.

    The make of carb also affects the way you change main jets, although going up one size at a time is the best way. Keihins measure/rate their jets according to diameter. Mikuni rate/measure their jets according to fuel flow. This gives a completely different numbering and "size" relationship between the two.


    Trevor G

    PS If you don't understand something I have written, ask again specifically.

    PPS Could you do me a small favour, please?? :)

    Correct the spelling of "Tuning" in the heading for your original post? Thanks. ;-)

    PPPS Thanks Z :)
  7. ok bit of an update.
    I got some uni filters on it at the moment (the green foam ones) all lubed up ect.
    Using standard jet (105 i believe)
    top end rev range 8-14 is buetifull
    Mid range 2-8 was crap but raised the needle by about 3-4mm and it alot better.

    Howere idle is pretty crap. It needs constant jerking to keep it alive. Or RPM will slowly drop until it dies.

    If i just adjust Idle speed the RPMs stay up for too long.

    Do i need to adjust pilot screws to solve this problem?

    Should i bother gettting a bigger main jet as top range seems good and mid range seems ok I say this because i belive the Uni filters are not much less restrictive than the stock foam?

    BTW thanks trevor for getting me this far.
  8. How on earth did you do that? Shouldn't need any more than 0.5 mm as I said earlier.

    If you have really raised it by that amount it would be running way too rich in the 1/4 to 3/4 range.

    Absolutely! :) That is just what they are there for. But set the needles properly first.

    Then set the idle speed screw so that the engine will continue to run. Adjust one pilot mixture screw at a time for smoothest (fastest) running. Readjust the idle as you go. Then do the idle mixture screw on the other carb.

    What have you done to sync the carbs? If the sync is out that will make setting idle quite difficult as one carb will be doing all the work at idle.

    Have you removed the stock airbox?

    Then you must up the mains, by 2 sizes at least. If they were Keihin you would go from 105 to 115 or maybe even 120. They use smaller sizes but that increment is, I believe, the correct upsize.

    This is what you would normally do...if the mains are too small you will notice that not much happens on the last 1/4 of throttle opening.

    Mikuni use a different system for numbering jets, based on flow rather than diameter. A Mikuni carb expert (the importers are in Qld and advertise in Trader and Just Bikes) will be able to advise you.
  9. probaly because my main is too small. But if i lower the needle i loose bullocks of power mid range.
    i will have to check u may be right i don't remember ever syncing these carbs.
    will post back after synch and play with the pilot will probaly go up a size on the main.
  10. If you have replaced the airbox with pods then the mains could go up 20%.

    Trying to adjust just using the needles only covers part of the range and will overfuel in the mid range.

    Trevor G

    PS It must be a Keihin carb?
  11. ok update
    tried 108 main jet still pretty crap
    when and bought some fine drill bits and made it into a 120 main jet
    its rideable at the moment but revs are to high at idle (needs mixture turned down)
    and theres still a little flat spot 3/4 in the rev range
    still using 3mmm raised needle Ill try droping it 2morow see if it improves anything.
    as it kinda feels a bit underpowered. (maybe im just comming down from the GS)

    Gunna get a air fuel meter as im going to have trouble tunning it anymore just by feel alone.
  12. Trev G, with all due respect, sometimes you are helping these blokes to tune their bikes to a standstill
  13. Lol, I was thinking the same thing. GPXs should be running well, more than that and you should just buy a bigger bike and be done with it.
  14. The mixture is not adjusted to set the idle speed - the idle speed adjuster is meant to be adjusted for that purpose! That's the big screw which simultaneously alters the minimum opening for both carbs.

    What size drill bit did you use?

    You shouldn't need much needle lift, if any, now. Certainly not 3mm!

    You will also need a dyno - there's no way you can meter the exhaust gases while riding - not that I know of. :)

    I think if you follow the tried and true methods suggested earlier you should be able to get it right.

    All the best

  15. If he follows advice he will get it right.

    If he does it his own way he might still get there, but take a lot longer to do it. ;-)


    Trevor G
  16. Not only great minds think alike, you know. ;-)

    It's fun to do simple mods, like add pods. You just have to retune it to suit. That involves changing jets for a start.

    I'd rather teach someone to "tune" a bike than lane-splitting...


    Trevor G

  17. Adding a new pipe or whatever, making it a necessary evil to start fiddling is okay, but good enough is good enough in my books. However, teaching anyone anything gets a big thumbs up from me, and GPXs are a great place to learn how to DIY :)
  18. Just out of curiosity "what do the plugs look like?"

    Black & sooty?
    Lite Tan?

    The answer to your jetting/needle direction could well lie with the answer to the question ?