New Exhaust lost top speed
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Thread: New Exhaust lost top speed

  1. #1
    Senior Member jannaruto's Avatar
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    New Exhaust lost top speed

    Hey guys,

    As some of you probably know my old discussion on the "starting issue" I was very careful when reading top speed, acceleration etc
    My top speed without pushing the bike to a "death" is always around 120 kmh, always on the same road, completely flat, very few cars

    Now I swapped exhaust
    Actually this was a bit of a project between me and my dad
    We took a KTM trial exhaust, cuttred about 40cm of that, with the opportune modifications inside (shortening all the components, reading, testing bla bla bla)
    I noticed few things
    The exhaust clearly is way bigger, I have a very deep noise, very cool to hear, the acceleration somewhat improved a bit, the 2nd and 3rd gear when I accelerate I can feel that the bike is pushing more, not like cutting off seconds as a supercar, but definetely I can feel the bike pulling my arms
    The only downside is that I lost a lot of top speed
    before I reached stable 120 before releasing the throttle, now with maxed out throttle I can barely reach 105, and the bike seems that won't go over that

    I am reading online that at this point the bike is probably running VERY lean because the exhaust is way bigger than the original
    I should set again the mix fuel air screw? let's say I have now 3,25 (keihin carby) I should go like 1 or 2 turns out more?

    I don't want to rejet the carb so please exclude this option, maybe in the future or if I can find another TW the same as mine 1993 and I will modify that one, but I want my engine and carby as stock as possible, the exhaust is the only real modification which of course is just a swap, I can remove the new and "plug" again the previous one

  2. #2
    Senior Member maddawgj's Avatar
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    Not sure you have a valid option other than rejetting the carb. Your bigger exhaust is allowing more air/fuel out but you have not set up the carb to allow more air/fuel in. Exhaust changes and jetting go hand in hand and both are required to get optimal effects.

    If you think of the engine as a pump you had a pump with a garden house for the intake and a garden hose for the output so you had great pressure and balanced in/out. Leaving that garden hose input and adding a 2” output means you will not be pumping enough to get the same output pressure because your input is now restricted compared to output.

    Jetting is simple and not a permanent change.
    Forever grateful for all your help brother
    2005 TW wrecked in ‘14 and rebuilt in ‘17. ‘06 donor parts and new OEM, Carb jetted & shimmed, ‘04 XT225 gas tank, UNI air filter, Shinko 241 front, modded Krator pegs, IMS folding shifter, JT 14/47 sprockets, huge car horn, tool tube, DR8EIX plug, short rear signals, Cycleracks rear, All Balls steering bearings, Protaper ATV High bars, OEM Kickstarter with more to come...

  3. #3
    Senior Member phelonius's Avatar
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    Just re jet it.
    Phelonius

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  5. #4
    Ken
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    Senior Member Ken's Avatar
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    The jet can be changed without removing the carb.

  6. #5
    Senior Member jannaruto's Avatar
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    Quote Originally Posted by Ken View Post
    The jet can be changed without removing the carb.
    Explain to me brother and which size I should get? Bear in mind my exhaust ia bigger than the DG one for the TW200

  7. #6
    Super Moderator Purple's Avatar
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    Now you’ve done it – you just have to keep trying for that little bit “extra” – lol

    Well – the simple fix is to put the original exhaust back on, which personally I would recommend

    The hard way – is to “re-jet” the carb – this means changing the main jet, not the pilot jet. Once you turn the throttle above 25% to 30%, it’s the main jet that comes into action, not the pilot we were faffing about with earlier – so turning that pilot jet out a few turns more is not going to help you

    You are facing two further complications here (doing it the hard way), the first is that not all jet sizes are created equal. What one manufacturer says is 130 (for instance), is not going to match another manufacturers 130. Yes, “it should” – but “no, it won’t” – we know this from experience

    The second issue is that you have a Keihin carb – superior in many ways, but the knowledge of this board doesn’t cover the Keihin carb. So you have an “odd-ball” carb” – together with the potential for someone supplying the parts for the wrong carb, compounded by the potential for differences in jet sizes for the same size from different manufacturers. All of these things have happened in the past, and will happen in the future – too many “variables”

    Don’t get me wrong, I think the Keihin carb is great, I have one on my ’98 – but it’s best not messed with as there’s little information out there on just how to do it, and where to find genuine Keihin carb jets, rather than an “after-market” part that will just confuse things even more.

    Sometimes, you are better off settling for what “works”, rather than searching for Unicorn droppings in the woods by candle light. My prediction, is that if you pursue this course, you will spend the next six months trying to find the solution, and more than that in in the “parts that didn’t work” bin.

    As you have discovered, there are gains in certain gears, but losses in top speed – and it will always be that way until you modify that exhaust to the point where it most resembles the original TW exhaust, and end up back where you started with close to the original jet

    It’s either that, or the “DG” exhaust, with the TK carb – but again, you’ll be disappointed with performance …..
    (Warning - Forum may contain nuts) ...... Hidden Content

    TW200 - 1998 - Japanese import - 7000 miles on the clock - TW225 Special Edition 2007
    - Hidden Content

  8. #7
    Senior Member jannaruto's Avatar
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    Quote Originally Posted by Purple View Post
    Now you’ve done it – you just have to keep trying for that little bit “extra” – lol

    Well – the simple fix is to put the original exhaust back on, which personally I would recommend

    The hard way – is to “re-jet” the carb – this means changing the main jet, not the pilot jet. Once you turn the throttle above 25% to 30%, it’s the main jet that comes into action, not the pilot we were faffing about with earlier – so turning that pilot jet out a few turns more is not going to help you

    You are facing two further complications here (doing it the hard way), the first is that not all jet sizes are created equal. What one manufacturer says is 130 (for instance), is not going to match another manufacturers 130. Yes, “it should” – but “no, it won’t” – we know this from experience

    The second issue is that you have a Keihin carb – superior in many ways, but the knowledge of this board doesn’t cover the Keihin carb. So you have an “odd-ball” carb” – together with the potential for someone supplying the parts for the wrong carb, compounded by the potential for differences in jet sizes for the same size from different manufacturers. All of these things have happened in the past, and will happen in the future – too many “variables”

    Don’t get me wrong, I think the Keihin carb is great, I have one on my ’98 – but it’s best not messed with as there’s little information out there on just how to do it, and where to find genuine Keihin carb jets, rather than an “after-market” part that will just confuse things even more.

    Sometimes, you are better off settling for what “works”, rather than searching for Unicorn droppings in the woods by candle light. My prediction, is that if you pursue this course, you will spend the next six months trying to find the solution, and more than that in in the “parts that didn’t work” bin.

    As you have discovered, there are gains in certain gears, but losses in top speed – and it will always be that way until you modify that exhaust to the point where it most resembles the original TW exhaust, and end up back where you started with close to the original jet

    It’s either that, or the “DG” exhaust, with the TK carb – but again, you’ll be disappointed with performance …..
    But let's say I would like to try right?
    Which jets I have to purchase and how do I change them without dismounting the whole carby?

  9. #8
    Super Moderator Purple's Avatar
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    I'll try and trace the info tomorrow - this is going to get "complicated" ....
    (Warning - Forum may contain nuts) ...... Hidden Content

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  10. #9
    Senior Member jannaruto's Avatar
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    Quote Originally Posted by Purple View Post
    I'll try and trace the info tomorrow - this is going to get "complicated" ....
    I called a friend of mine (ex KTM driver and seller, regional champion) he asked which cabry I have, I said Keihin, he said, no worries I have jets for it
    Just let me know the measure of the stock keihin if you can Purple, so I can give him the right measure and he will improve adding a bigger one
    littletommy, maddawgj, Ken and 3 others like this.

  11. #10
    Senior Member Badgerflorida's Avatar
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    Quote Originally Posted by maddawgj View Post
    Not sure you have a valid option other than rejetting the carb. Your bigger exhaust is allowing more air/fuel out but you have not set up the carb to allow more air/fuel in. Exhaust changes and jetting go hand in hand and both are required to get optimal effects.

    If you think of the engine as a pump you had a pump with a garden house for the intake and a garden hose for the output so you had great pressure and balanced in/out. Leaving that garden hose input and adding a 2” output means you will not be pumping enough to get the same output pressure because your input is now restricted compared to output.

    Jetting is simple and not a permanent change.
    That’s a great example. Lots of engineering goes into exhausts, even though it’s such a simple principle. That’s why the good ones are expensive. I think Yamaha gave us a pretty good stocker.

    jannaruto - I admire your attempt to wring as much as you can out of your bike.

    And I admire all the guys on here pointing him in the right direction.
    Ken, littletommy, maddawgj and 1 others like this.
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