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Carb'ied bikes and exhaust replacements

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by Soulful Twin, Dec 16, 2009.

  1. G'day all,

    First time posting been lurking for a fair while. it's a great site.

    I bought a '95 Yamaha SRV 250 about 6 months ago. When i got it it had aftermarket 2-into-1 custom staintune pipes. The bike sounded awesome (like a harley just no quite as loud) but it had no low and midrange up until about 6500 rpm (redline is a little over 9000rpm) so i'd nail it in low gears and the bike would get going and after about 6500 rpm it would gain a fair bit of acceleration till i went crashing into the redline.

    Since then i put the stock pipes back on and it has alot more usable power from about 4000rpm or so but runs out of puff at about 8000rpm.

    The bike performs better with the stock pipes. If i put the aftermarket pipe back on and get the carbs tuned properly maybe even get some bigger ones will i be able to keep midrange torque? (which i think is what a v-twin is about)

    I really like the sound of the aftermarket pipes but not enough to sacrifice useable power.

    Any ideas?
  2. Yep you need to get it to a mechanic to be jetted correctly
  3. awesome and harley combined is an oxymoron
  4. or have a crack yourself, save some money and learn something. its not that hard.

    go buy some jets. rip those carbies out.

    rule of thumb if its runs ok on stock. you will probably need to go a little bigger in the main jet for an aftermarket pipe. and maybe 1 up in the needle position.

    google "plug chopping" and carbie jetting and start doing some testing
  5. http://www.factorypro.com/

    Product support/technical support (top of page)

    Motorcycle tuning tech

    CV carb tuning

    High rpm engine

    and slickncghia is right about a main and needle as starting pionts.
  6. So i've gotta rejet the carbs (new main jets?) Then tune the needles for the pipe?

  7. Main governs top end fuel flow. It governs the fuel flow to the needle jet. So you need to set it first (after the float level)

    Then set the needle right for mid range performance.

    Then the idle jet and screw for idle performance and initial throttle responce in the mid range (ie. if it hesitates when you crack the throttle it may not be the needle. I may well be the idle jet)
  8. Its all about balancing..

    The factory setup is balanced since they mass produce it that way. If you open up the exhaust and increase its flow, you move the powerband up the rev range (whilst creating a slightly leaner mixture.
    If you restrict the exhaust more than stock (like some dick did to mine) it lowers the powerband in the rev range, and creates a slightly richer mixture.

    Changing the jets to match (more fuel for open exhaust, less for restricted) will allow you to take advantage of the most power it can make. See previous post by ibast about this.

    Also factory exhausts are usually more restrictive than they need to be due to noise regulations, and if you restrict it more just to lower the power band, you may dramatically lower the maximum power output too.

    On the other side of the engine, if you want to lower the powerband; lengthening the intake and creating a large sealed plennum chamber is a good way to achieve it. On a car this isn't so hard, but with bikes being so small and compact, its not exactly easy.

    You could also try different plugs and (if possible) play with the ignition timing a little.

    And if you wanna throw a couple hundred bucks at it (totally worth it) you can get different camshafts ground for it which will have the biggest effect of all.