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Synchronization of Twin SU HS-type Carburettors

Before proceeding to synchronize your carburettors and for best results, some preparation is in order. Your carburettors should be in peak condition with no play in the throttle shaft bushings. If these bushings are worn, synchronization is difficult or impossible to achieve since there will be a vacuum leak past the bushings. Additionally, the jet needles should be identical and correct for the application; adjusted to equal and correct seating in the piston and centered in their jet bores (if not centered, you will feel a bind when moving the piston up and down). If any of these conditions exist, the carbs should be rebuilt or at the very least torn down and thoroughly cleaned and inspected before adjustment.

  • Remove the air cleaner and take the time to clean the area in which you will be working. Check the throttle linkage for free movement and unscrew the throttle adjusting screw on each carb until it is just clear of the throttle lever with the throttle closed. Turn the screw clockwise one full turn on both carbs. Using the elevating pin, raise the piston in each carb and ensure that it falls freely onto the bridge when released.
  • Remove the suction chamber assembly from each carb (Don't mix them up! Do them one at a time if necessary!) and, by turning the jet nut, position the jet tube flush with the bridge in each carb or as high up as it will go. Regardless, adjust the jet tubes of both carbs so that they are the same. Check that the sintered needle guide of each piston is flush with the underside of the piston.
  • Turn the jet nut two turns down. This can be checked by adjusting it so that the top of the jet tube is .125" (1/8- INCH) below the level of the bridge using a vernier or dial calliper.
  • Back the fast-idle adjusting screw well clear of the cam.
  • Reinstall the suction chambers and recheck to ensure that the pistons fall freely onto the bridge (see step 1, above).
  • Check the damper oil level and top up with suction chamber oil (GGL9035) or 20wgt motor oil.

Synchronization

  • Loosen both clamp bolts on the throttle interconnect mechanism. Likewise, loosen both clamp bolts on the cold-start interconnect mechanism. This allows the two carbs to operate individually.
  • Using a "Uni-syn" or other vacuum synchronizing meter, balance the carbs by altering the settings of the throttle screws until the proper idle and balance is set. You can also use the time-honored "HISS"-method: using a length of tubing as a listening device, insert it into the carb throat and listen to the air flow while you adjust the throttle screws. Adjust both screws until you hear both carbs "hiss" at the same pitch and volume (this works better than you might expect, but can make you crazy).
  • After making the above adjustment, set the mixtures by adjusting the jet nut on each carb down/clockwise (rich) or up/counter-clockwise (lean) until you achieve the fastest idle speed. Now turn each nut up/counter-clockwise (lean) until the engine speed just starts to fall off. Then turn each nut very slowly down/clockwise until the maximum speed is re-established.
  • Recheck the idle balance with the synchronizing tool and adjust as required.
  • With the fast-idle cams of both carbs hard against their stops, adjust the cold-start connections so that both cams begin to move simultaneously.
  • Check and adjust for 1/16"free-play in the choke cable before the cable can move the cams.
  • Pull out the choke cable knob until the linkage is just about to move the jet.
  • Using your synchronizing tool, adjust the fast-idle screws to give the correct idle speed.
  • Tighten the clamp bolt on the throttle arm and choke mechanism.
  • Replace air filter assembly.