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larger front main jets

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    larger front main jets


    i'm trying to understand carb tuning as much as possible in hopes of elevating my mechanical/technical understanding and try to get my bike dialed in and more enjoyable.

    so far, i've dismantled, cleaned, rejetted, adjusted and synched the carbs. it's running pretty good right now. used techniques learned from my previous experience with my '66 bug w/dual kadrons...

    can some explain to me why most people use larger front main jets?

    my google searches show me that other bikes are tuned this way as well. are people using air/fuel ratio instruments?

    thanks in advance!

    edited cause i meant to say "front" not "rear"... he he he

    The reason that tuners often run larger front main jets is because the front cylinder/head runs cooler than the rear cylinder since it is exposed to cooler air.

    The rear cylinder is exposed to the air as it bleeds off the front cylinder which is warmer than the air that was hitting the front cylinder (since it's temp has been elevated by the front cylinder and headers) .

    Since the rear head/carb is exposed to warmer and therefore less dense air (less oxygen) than the front cylinder/head it requires less fuel to maintain the proper air/fuel mixture to acheive effecient combustion than the front carb. This is why it is common to see jet sizes staggered by 1/2 a jet size from front to rear.

    Some would argue that it makes little difference but like indexing your plugs it's a free performance enhancement that poses no negative effects even if its actual benefits can be argued.

    Staggered jet sizes are very common on 2 strokes (a full jet size in common on 250's) but 2 strokes are a different animal entirely.
    Faster than your mother... She's what the pros use.


      Well said, Crash.
      J.D. Hord
      Keeper of Engine Nomenclature, 9th Order


        i see said the blind man!

        thanks guys!


          reading suggested:

          find and read a book about carb tuning by John Passini, mostly about sports cars using Weber carbs, but universal in its wisdom.
          "It's only getting worse."

          MY rides: '97 VFR750, '90 Red Hawk, '88 Blue/Black Hawk, '86 RWB VFR700 (3), '86 Yamaha Radian, '90 VTR250, '89 VTR250 (2), '73 CB125, '66 Yamaha YL-1

          Sold: '86 FJ1200, '92 ZX-7, '90 Radian, '73 CB750, '89 all-white Hawk, '88 blue Hawk, '86 FZ600, '86 Yam Fazer 700 , '89 VTR250, '87 VFR700F2, '86 VFR700F.


            I am fixing to add multiple cylinder datalogging to my dyno, ( up to 4 cylinders O2 and EGT) once I have that operating I am going to find out once and for all about the staggered jetting. I have tested it before, 1 cylinder at a time, but I think that introduces a potential variable in the data. I want to see what is happening in both cylinders at the same exact time.
            J.D. Hord
            Keeper of Engine Nomenclature, 9th Order