Carburetor Synchronization

Each of the engine's cylinders is fed by its own carburetor. It is important that these carburetors be properly balanced. For the engine to perform at its best, each carburetor must operate at the setting that is cooperative with the others. Properly balanced carburetors result in reduced vibrations, enhanced throttle response, more power, higher mileage, and optimum operating temperature. No cylinder should run hotter, cooler, richer, or leaner than another. The synchronization adjustment adjusts the setting of each carburetor's throttle butterfly valve so that the fuel/air mixture passed by each carburetor results in an equal load placed on each of the cylinders.

Carburetor synchronization requires the use of some useful tools. The most important of these is a four channel manometer to measure the vacuum pressure at each of the carbs exhaust ports. The vacuum is a measure of the fuel/air flow. The absolute levels are not important. What is important are the relative values at each of the carbs. They should be as equal as possible. I use a Carbtune II from Morgan Carbtune in Belfast, UK. The cost is about $70. It pays for itself the first time you use it. The columns are calibrated steel rods rather than mercury which makes the meter a little easier to deal with. There is no danger, although there is little chance, of mercury being sucked into the cylinders. Motion Pro also makes excellent four-way mercury based manometers in a range of prices.

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The procedure is:

  • Place Bike on Centerstand
  • Remove the Seat
  • Remove the Fuel Tank Hold Down Bolt
  • Prop Up the Front of the Tank

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  • Remove Four Vacuum Caps

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  • Attach Carburetor Synchronizer Hoses to Carb Vacuum Ports
  • Attach Other End of Hoses to the Corresponding Port on the Manometer which Must Hang Vertically
  • Start Engine and Warm Up for Several Minutes
  • Check, and if necessary, Adjust Engine Idle Speed
  • Verify Carburetor Synchronization from Manometer Readings

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To adjust the synchronization do the following:

  • Synchronize Carb #1 with Carb #2 by turning the synchronizing screw for the carb pair 1 & 2 (left image below) until those two gauges read the same.
  • Synchronize Carb #3 with Carb #4 by turning the synchronizing screw for the carb pair 3 & 4 (middle image below) until those two gauges read the same.
  • Synchronize the carb pair #1 & #2 with the carb pair #3 & #4 by turning the center synchronizing screw (right image below) until all four gauges read the same. This third synchronizing screw balances the two carbs 1&2 with the two carbs 3&4 and must be adjusted last.

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From Mike Smith comes a tip to wrap the shaft of the screwdriver in electrical tape to prevent a possible short circuit with the ignition coil. Thanks Mike.

At each step, rev the engine a few times then allow it to settle back down to idle. If you re-adjust the screw for carbs 1-2 or the screw for 3-4 you have to go back afterwards and readjust the center screw as the last step. The rods should jitter a little bit. If not then the flow in the hoses is too restricted.

The specification for vacuum pressure at idling speed is 22.5 cm, 225 mm, 30 kPa, 8.86 in. of Hg.

The specification between cylinders is 1 cm, 10 mm, 1.33 kPa, 0.39 in. of Hg.

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  • Stop Engine
  • Remove the hoses from the vacuum ports
  • Reinstall the caps on the vacuum ports on each of the carbs. It's easier to leave the hose clamps on the plugs as you do this.
  • Lower Fuel Tank and Reinstall Fuel Tank Hold Down Bolt with Proper Torque Spec, 10 Nm, 7.2 lb-ft.

Another Excellent Article on Carburetor Synchronization can be Found Here

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Last Updated: 03-19-2004

Copyright © 2001-03, Patrick Glenn, All Rights Reserved.
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The information presented here reflects solely my personal experience with my motorcycle and is presented for entertainment purposes only. No information presented here is to be relied upon for issues of rider safety nor to replace the services of a qualified service technician. Any attempts to follow or duplicate any of these procedures are done so completely at your own risk. By reading the information on this site, you agree to assume complete responsibility for any and all actual or consequential damages that may arise from any information presented herein.