A guide for keihin jetting

Written by kelvin hayes
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When working on a Keihin PE, PJ, PWM or PWK carburettor jetting system, it's important to keep in mind that like all other carburettors, jetting should be approached as a dynamic tuning process. Ask yourself questions about how the engine is performing and where it appears to have issues. As you evaluate the engine's performance, decipher how the jetting system needs to be changed so that you can return the engine to appropriate performance levels.

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Things you need

  • Screwdriver

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  1. 1

    Start the engine and allow it to warm up for one to two minutes. Locate the idle screw on the air cleaner side of the carburettor.

  2. 2

    Turn the idle screw ½ turn counterclockwise to briefly raise the RPM by about 500.

  3. 3

    Turn the screw clockwise until it bottoms out, backing it out ¼ turn from that point. Wait 15 seconds.

  4. 4

    Turn the screw ¼ turn counterclockwise. Again, wait 15 seconds between ¼ turn adjustments, allowing the engine to respond to the turn. Continue this process, noting the number of turns made to peak the engine's idling RPM. If the peak RPM occurs between one and two turns, the pilot jet is the proper size. Before one turn or after two turns indicates a larger pilot or smaller pilot jet is needed, respectively.

  5. 5

    Unscrew the four corner screws from the carburettor bowl located on the base of the carb. Pull the float bowl away from the carb and locate the brass pilot jet positioned next to the needle jet, which looks like a needle. Turn it counterclockwise to remove it and increase or decrease the number size by two numbers as necessary. Reinstall the float bowl, tighten the screws and test the engine until the idle screw creates peak RPM between one and two turns from bottoming out.

  1. 1

    Unscrew the hose-clamp attaching the carburettor to the air box and intake manifold. Slide the carb out from between the air box/intake. Unscrew the screws from the top of the carb, where the throttle cable inserts into the carb. Slide the top, throttle cable and carb slide from the tower of the carb, revealing the needle jet within.

  2. 2

    Twist the clip at the base of the needle from its current number towards 5. The number 5 is the richest position and is a safe starting point for the needle adjustment. If the engine runs flat and sputters, which indicates a rich condition within the engine, turn the clip to 4, reinsert the slide, screw the top onto the carb and test the engine at ¾ to full throttle. Turn the clip to a lower setting until the engine revs smoothly and crisply.

  3. 3

    Pull the needle jet out of the carb tower by hand if the clip adjustments fail to properly tune the carb. Locate the three-letter stamped I.D. on the needle. Install a needle with a lower third letter to reduce the needle's diameter and a higher third letter to increase the diameter. For example, if the original jet reads DGK and you swap it for a DGJ, you've effectively installed a smaller diameter needle jet, resulting in a leaner carb tuning. Drop the needle into the tower slot, slip the slide and throttle cable assembly into the tower and tighten the four corner screws to complete the needle adjustment. Test the engine to confirm that it runs smooth and crisp during ¼ and ¾ to full throttle conditions.

  1. 1

    Unscrew the bowl screws and pull it away from the carb. Unscrew the jet placed at the centre of the base of the carb (top of the bowl).

  2. 2

    Note the size stamp and increase or decrease it to lean or richen the carb. Screw the new main jet into its hole, press the fuel bowl to the base of the carb and tighten all corner screws.

  3. 3

    Start the engine and test the throttle from ¾ to full throttle. If the engine sounds crisp and responsive, the main jet is properly set. A flat, muted sound or a high-pitched, tinny sound will indicate the need for a leaner or richer jet, respectively.

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