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Help Rejetting- where to start?

Discussion in 'Kawasaki 250cc In-line 4's' started by DamnitLaverty, Aug 27, 2018.

  1. DamnitLaverty

    DamnitLaverty Doing things the hard way since '78!

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    Hey all,

    So I did a thing the other day. Decided to repurpose a Ducati exhaust I had kicking around and put it on the ZXR, along with a wideband O2 sensor bung.

    Between my open bottom airbox, and this exhaust, its gonna necessitate rejetting.

    ... but I dunno where to start. I'm roughly at sea level, so I don't have to sweat altitude tuning, yet.

    if I was to buy a bunch of jets, can someone with an exhaust recommend me where to start? I would hate to buy like 135s, and realize I need 165s or something. Like I said, I'm building in a wideband, so I am expecting to do some tuning, but my primary motivation is to not end up buying a million jets I'll use only once. :)

    Any benefit to keeping the slight offset in jetting to the middle two carbs?
     
  2. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    I put a new free flowing muffler on my FZR and i don't think i'll need to rejet
    You might need to rejet if you are changing the air box to something that flow's more with the new muffler., but then you might need some extra pilot jet's too.

    I would be keeping the leaner jet's on the middle 2 cylinder's as they generally run a touch hotter due to not getting as much air flow to them as the outer 2 cylinder's
    And if you find you only need to go up 1 jet size you can reuse the 2 jet's from the outer carby's in the 2 middle carby's
     
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  3. DamnitLaverty

    DamnitLaverty Doing things the hard way since '78!

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    My airbox has the bottom cut out of it- so stock filter and top, but more air able to get in.
    I will likely be 3d printing a frame to fit a single filter up top in the near future- think pods, but a single filter instead of 4.
     
  4. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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  5. DamnitLaverty

    DamnitLaverty Doing things the hard way since '78!

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    That's the kindest "search, ya noob" I ever got. :) Thanks!
     
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  6. DamnitLaverty

    DamnitLaverty Doing things the hard way since '78!

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  7. DamnitLaverty

    DamnitLaverty Doing things the hard way since '78!

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    Oh yeah, I've read this article, yyyyears ago.

    in short, don't screw with it, the factory knows better than you.

    .... but on this re-read, I got "exhaust shouldn't hurt." and "leave the airbox top, you numpty".
     
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  8. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    If you are building in the wideband sensor bung.. get your hands on a readout and see where it is running now.. to be honest though.. best way is to run it on a Dyno.. you might be able to get the bottom end OK but mid range (under load) and at the top end is where you will quickly kill an engine if you are a long way out. the cost of some dyno pulls would be well worth it.
     
  9. DamnitLaverty

    DamnitLaverty Doing things the hard way since '78!

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    I have a display. I drafted and 3D printed a GoPro mount for it so I could swap it between bikes easily. It’ll get mounted on the bike for sure- but I was hoping for jetting help to get me in the ballpark before I have to pay by the hour to a tuner.
     
  10. Andych

    Andych Moderator Staff Member Premium Member Contributing Member Dirty Wheel Club

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    Probably still best to see where you are with the current jetting and work backwards from there.. or is it forwards..
     
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  11. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    I would fit the readout and run it and see what you get first. If the numbers are way off then getting some jets and playing yourself will save some dollars at a dyno shop.
     
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  12. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    I'd start with an unmodified airbox... everyone thinks they know better than the manufacturer of their bike.

    You can even find them fairly cheaply from Japan, before shipping that is.

    https://buyee.jp/item/yahoo/auction/n229123812
     
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  13. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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  14. kiffsta

    kiffsta Senior Member

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    +1 to sorting the airbox out
     
  15. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Re main jets. I start rich, which generally they all are as standard, and go smaller until it just goes off song at peak rpm under load. Then go back up one size. This is the opposite of what the masses do, which is to keep fitting bigger jets until their head explodes. If your bike was magically flowing more air over the emulsion tube then it would automatically pull more fuel. A ZXR400 uses 98 main jets https://www.megazip.net/zapchasti-d...574/zx400-l9-805778/carburetor-parts-14360925 so what does that tell you? A different exhaust does not magically equate to the need for bigger main jets.

    From Mikuni HSR manual:
    Roll-off Method: This method is a good way to get the main jet either correct or within one size of correct. It is based on the fact that that as the throttle is closed, the air/fuel mixture richens momentarily. This normal enrichening can be used as a diagnostic tool. The test is started with the engine running at an rpm high enough to ensure that it is “on the cam.” Open the throttle fully and let the engine pull for several seconds. Then, quickly close the throttle to about the 7/8ths position. If the engine seems to gain power, the main jet is too small (lean). Fit a larger jet. If the engine hesitates as the throttle is rolled off, the main jet is too large. Fit a smaller one. When the main jet is correct, the engine will continue to run smoothly and evenly as the throttle is closed. Note that a main jet that is far too rich or lean may cause the engine to misfire at full throttle

    From Dell'Orto Manual:
    The correct main jet size should be selected by running on the road, preferably by first starting with an over-large size jet and gradually reducing it. At full throttle, turn the starting device (choke) on, thus further enriching the mixture and, if this produces a worsening in engine running ie. it reduces engine rpm, it is advisable to reduce the main jet size until you finally get satisfactory operation. Other signs revealing the main jet is too big are a very dark exhaust pipe, dark exhaust gases and damp spark plugs and an improvement in engine running when the fuel supply is temporarily shut off. In a case where too small a main jet has been fitted at first, and the running with the choke on makes a noticeable improvement, you should increase the main jet size until the conditions mentioned above occur.

    I always use my method but I stuck these in anyway. The Dell'Orto method would be pretty rough depending on how much fuel would be released by opening the choke. The key point is that you start rich and keep going down. It doesn't matter what size you think the bike should run, only what works.

     
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  16. DamnitLaverty

    DamnitLaverty Doing things the hard way since '78!

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    I still have an unmodified air-box. I cut up a spare off an A that somehow ended up in Texas on eBay. :) I couldn't get it to go over 12k RPM (check my post history) with fresh rebuilt carbs, new filter, etc.

    Over the next two weeks, it gets a valve adjustment, the vac pump reinstalled, and I concede the airbox point and will likely go back to stock there- I just really wanted to ride it, and it sounded AWESOME with half an air-box (y'all saw my Redwood video a few months back in the what'd you do today thready- you know how awesome it sounded).

    Thanks guys, next time I find myself in your neighborhoods, drinks are on me. Y'all are a shining beacon of sanity walking me back from the precipice of screwing it all up. :)
     
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