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Definitive AFM adjusting guide (spring tension and bypass screw)

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    Definitive AFM adjusting guide (spring tension and bypass screw)

    The method I used to calibrate spring tension is rudimentary, but it works quite nicely!

    The original AFM on my '93 vert had been altered at some point in its life. This caused the engine to run super rich throughout the RPM band and also made for a poor off idle response. I purchased 2 sealed AFMs ('89 sedan '90 coupe) and took the covers off to measure reference values. Then, I calibrated the original AFM to those values and it now performs exactly as the 2 reference ones! I guess I now have 2 AFMs to get rid of or maybe keep as spares...haha!

    Reference values were averaged between 2 previously unaltered AFMs but both units were close to each value by .01 volts.

    -Start car and get it to operating temperature so that idle is where it should be. 750RPM
    -With the cover off the AFM, place one lead of Multi-Meter on the reference point, and measure DC voltage on points 1 and 2 labeled below. (positive/negative isn't important here)

    point 1 should be between 3.88-3.92 volts
    point 2 should be between 1.12-1.18 volts

    Turn spring tension wheel one or two notches, bleep throttle and let idle settle, recheck values.


    To re-calibrate the bypass screw, there is a small number stamped next to the screw hole. My AFMs had 17.7mm and 17.4mm which were both between 2.5 to 3 screw-turns out. This is a measurement from the top of the adjustment hole down to the top edge of the adjustment screw.

    Use a pair of calipers to measure the depth and adjust accordingly.


    Last edited by salvageland; 02-08-2017, 08:06 PM.
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