Results 1 to 6 of 6

Thread: Anyone dyno different size carbs on the same motor?

  1. #1
    Racer supernutt's Avatar
    Join Date
    Jul 2013
    Location
    Michigan
    Posts
    312
    Post Thanks / Like
    Mentioned
    3 Post(s)
    Feedback Score
    0

    Anyone dyno different size carbs on the same motor?

    Just wondering if anyone has any dyno charts of like a 38pwk vs a 41pwk on the same motor. I have a midrange ported cylinder and I put a 38pwk on it but the carb must have been messed up. Could never get it to clean out no matter what I did to it. Put a 41mm ESR bored carb on it and it woke it right up. Runs really good up top for a stock cylinder. But I am wondering what I am giving up with that big of a carb. Thanks.
    86r 310 pro-x C-Leigh ported TRX5 CR ignition HLS shocks 38pwk
    97 Laeger XC narrow CR500 link +4 t-pin
    89 trx250r PEP ported ct mid pipe

  2. #2
    Racer
    Join Date
    Mar 2014
    Location
    Austin TX
    Posts
    338
    Post Thanks / Like
    Mentioned
    2 Post(s)
    Feedback Score
    0
    You are giving up bottom end. If you are riding wfo, then it doesn't matter.
    A 38pwk should be around 175 main, 52 pilot. I run a ddj needle in the 4th from top clip position, #7 slide. That is sea level, air box lid off, around 70'F ambient.

  3. #3
    Rider
    Join Date
    Sep 2014
    Location
    West Coast
    Posts
    83
    Post Thanks / Like
    Mentioned
    1 Post(s)
    Feedback Score
    0
    Every mtr is diff. If mtr needs bigger carb it needs bigger carb. I've seen bigger carb have better power than a smaller carb

  4. #4
    Pro-X
    Join Date
    Oct 2013
    Location
    Phoenix Arizona
    Posts
    626
    Post Thanks / Like
    Mentioned
    4 Post(s)
    Feedback Score
    0
    An accurate comparison between 2 carbs of the same size or different sizes cannot be made if both carbs are not working or jetted so that they are both metering fuel accurately at all throttle positions.

    A 39 pwk is not too large for a mildly ported 250. I do not have time to search through 100,000 plus dyno runs to find some dyno runs to post to show the behavior of small carbs vs larger carbs. On a well developed 250cc to 350cc Honda ATC/TRX engine package where the pipe, ports, reed, intake system (intake tube and air filter ) were developed to complement each other and using an exhaust system that is in tune between 6000 and 8500 RPM, you will see the following power characteristics when comparing a 34mm carb and a 39mm carb:

    1. Starting will be the same.
    2. Idle will be the same.
    3. The 34mm carb will make slightly more power in the 3000 to 4500 RPM.
    4. The same power in the 4500 to 5500 RPM range.
    5. The same power from 5500 RPM to the RPM where the 34m carb will not flow the amount of air the engine package is needing.
    6 At the RPM where the 34 carb is too small, the power curve of the 39 carb will continue to climb to a much higher peak power while the 34mm carb will climb more slowly to it's peak power
    6. The peak power will occur at approximately the same RPM for both carbs, but the 39 carb will not drop off as rapidly after it's peak as compared to how quickly the 34 carb drops off after it peaks.
    7. The RPM where the engine makes peak power is influenced more by the pipe than by the size of the carb.


    On my good running 246cc to 265cc engines I run 41.5mm carbs. These engines will make more power from about 6000 RPM and up, than the same engine with a smaller carb. If you are running in the 3000 to 4500 RPM and need more power, down shift one or two gears and you will have a lot more power than the 1 or 2 HP difference that the 34mm carb will provide over 41.5mm carb in the 3000 to 4500 RPM range.

  5. #5
    Racer supernutt's Avatar
    Join Date
    Jul 2013
    Location
    Michigan
    Posts
    312
    Post Thanks / Like
    Mentioned
    3 Post(s)
    Feedback Score
    0
    Thank you Jerry for the detailed explanation. I understand and from the way it sounds it probably isn't worth putting a smaller carb on this motor since I already own the 41mm. This motor is mostly used for aggressive trail riding and I really don't have any issues riding it the way it is.
    86r 310 pro-x C-Leigh ported TRX5 CR ignition HLS shocks 38pwk
    97 Laeger XC narrow CR500 link +4 t-pin
    89 trx250r PEP ported ct mid pipe

  6. #6
    Rider
    Join Date
    May 2014
    Location
    New York City
    Posts
    83
    Post Thanks / Like
    Mentioned
    2 Post(s)
    Feedback Score
    0
    Great info Jerry, thank you for the detailed explanation.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •