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Main Jets vs. Slide needle position

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    Main Jets vs. Slide needle position

    DOes increasing the size of the mains have the same net effect as raising the slide needle position?

    #2
    No. There is some overlap in the transition but you can often run a bike without the mains fitted as long as you don't go too deep in to the needle circuit.
    79 GS1000S
    79 GS1000S (another one)
    80 GSX750
    80 GS550
    80 CB650 cafe racer
    75 PC50 - the one with OHV and pedals...
    75 TS100 - being ridden (suicidally) by my father

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      #3
      Ah very interesting. Thank you

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        #4
        The needle position affects the air/fuel ratios in the 1/4 to 3/4 throttle positions.

        The advantage of dynoing your bike is that you get good feedback from the data. When they do your test for maximum HP and torque, it's always done in 4th gear, at WOT, with a 20% load on. Because of WOT from 2500 rpm, the needle position has limited/no effect on the AFR during the engines climb to its peak. It will have more effect on the lower end rpm range on CV carbs though.

        By analysing my 850's data, I found that the main jet was providing an AFR of 13.2 -1 to 13.4 -1 from 7200 to 9,500 rpm and then richened slightly to 12.8 -1, up to 10,500 rpm. This is slightly lean, so a next sized up main was called for. I deliberately went up 2 sizes and dropped 2 HP off the max, but the AFR map was identical, be it in a more richer range than above.

        You can do runs that allow AFR's at certain rpms too, but that doesn't tell you what part of the needle shape is affecting the recorded AFR.

        Plug chops at marked throttle positions will deliver the most accurate data for your fine tuning. Learn to read your plugs and enjoy the tuning challenge.
        Last edited by 49er; 07-17-2009, 09:20 PM. Reason: Added CV sentence.
        :) The road to hell is paved with good intentions......................................

        GS 850GN JE 894 10.5-1 pistons, Barnett Clutch, C-W 4-1, B-B MPD Ignition, Progressive suspension, Sport Demons. Sold
        GS 850GT JE 1023 11-1 pistons. Sold
        GS1150ES3 stock, V&H 4-1. Sold
        GS1100GD, future resto project. Sold

        http://i155.photobucket.com/albums/s...s/P1000001.jpg
        http://i155.photobucket.com/albums/s...s/P1000581.jpg

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