Randakk's Cycle Shakk, LLC
Randall Washington, aka "Randakk"
President and Chief Executive Rider
Chapel Hill, NC USA

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Carb Synchronization Procedure

You might consider this item...synchronization is shown: Randakk's GL1000 Carb Rebuild Video

Proper carb synchronization is the key to a sweet running bike. When carb sync is out of whack, you can have poor idle, poor off-idle response, "hanging" throttle and lazy return to idle. You can even suffer from fouled plugs if one or more throttle butterflies are opening early, thereby partially activating the carb's progression circuitry.

Very small adjustments will have a major impact in idle smoothness!

I use a mercury manometer device (mercury "sticks") rather than vacuum gauges as I feel this method is inherently more reliable and accurate. Using a 4-gang set of vacuum gauges to synchronize is fine, so long as they are cross-checked and calibrated to each other. It is important that they be accurately matched to each other or your efforts will be wasted. The procedure detailed below is essentially the same for "sticks" or gauges.

Caution: If you choose to use mercury "sticks," be aware that mercury is an extremely toxic substance. Follow the instructions provided with your synchronization kit to the letter with regard to mercury hazards. If you are not confident in your ability to perform this work accurately and safely, have a professional motorcycle mechanic do this work for you.

  1. Imperative - all other tune-up issues must be in order before you synchronize.

    Note: You should do a valve adjust and compression test (in that order!) before doing ANY ignition or carb tuning. GL1000s will run poorly when valve lash is not set exactly to spec. Fortunately, valve lash stays "in spec" for a long time once properly set.

    Note: Valve timing on a GL1000 is critical. Verify that your cambelts are installed correctly. The "match marks" on the flywheel and the cam pulleys must align perfectly. The GL1000 is an interference design engine. If your belts are off by 1 tooth, your engine will run like crap. If your belts are off by 2 or more teeth, serious engine damage will occur immediately.

    Note: Keep in mind that all ignition issues must be perfect before carb work is undertaken. This includes coils, plug wires, plug caps, spark plugs, ballast resistor, breaker points, condenser, ignition advance mechanism, ignition switch, kill switch, main fuse, battery and all associated wiring harness components.

    Note: Ignition performance and timing is critical on GL1000s! Ignition problems often mimic carb problems to an untrained ear. Click here for helpful information on setting points and ignition timing properly.

  2. Imperative - Your bike must be outside for this procedure to avoid fire hazard.
  3. Be sure you an adequate supply of FRESH FUEL in the tank (at least 2 gallons) so you won't have to use the "Reserve" setting during initial startup
  4. Your bike must be fully warmed up before you begin. A for a 10-15 minute ride around the neighborhood should suffice.
  5. Bike should be parked on a level driveway using the centerstand.
  6. Place a large fan in front of the bike blowing on "High" toward the bike. This prevents overheating. Also, your goal is to complete this synchronization expeditiously enough so that the bike's radiator fan does not come on. It's not a tragedy if it does, but it introduces an unnecessary variable by reducing the idle speed somewhat.
  7. Connect the vacuum gauges or mercury "sticks" according to the manufacturers instructions. I use a simple set of 4 mercury "sticks" as these are inexpensive an quite accurate. Be very careful handling the "sticks" as mercury is very toxic. Also, make sure you install the flow restrictors into the tubing per the instructions which come with your "sticks."
  8. You will be attaching the 4 hoses to 5 mm brass ports you install into the sides of the intakes after removing the small screws. Hang the sticks from the handlebar so they are secure, vertical and easy to read. You may have to extend 2 of the hoses to reach the far side of the bike. Don't allow any kinks in the hoses.
  9. Mark sure the fuel petcock is turned to "ON."
  10. Start the engine. DO NOT REV THE ENGINE ABOVE 3500 rpms... THERE IS A DANGER OF SUCKING MERCURY INTO THE ENGINE IF YOU DO. IF THIS HAPPENS, DISCONTINUE THE SYNCHRONIZATION PROCESS AND RIDE THE BIKE FOR ONE MILE OR MORE TO PURGE THE MERCURY FROM THE ENGINE. NO DAMAGE TO THE ENGINE WILL OCCUR IF THIS PURGE PROCEDURE IS FOLLOWED.
  11. Adjust idle speed (if necessary) to 1000 rpms using curb idle screw.
  12. Check gauges or "sticks." According to the factory specs, all 4 cylinders should be within 50 mm Hg (2 inches Hg) of each other. Actually, a variance of 2 inches is fairly out of whack in my opinion. I aim for a max difference of 1/2" between the cylinders...1/4" is better. A low reading means that cylinder's throttle plate is too "open." A high reading means that cylinder's throttle plate is too "closed."
  13. If the readings are out of spec, adjust as follows using the 3 sets of adjusters:

    Adjuster A - between carb #1 and carb #3 on the right side (balances carb #1 and #3 to each other)

    Adjuster B - between carb #2 and carb #4 on the left side (balances carb #2 and #4 to each other)

    Adjuster C - the one to the right of carb #4 (left rear carb...this screw balances left pair to right pair).

  14. Note: I use a "combo" tool which allows unlocking of the locknut, adjustment of the balancing screw and re-locking the lock-nut in one step. This tool is available from Motion Pro. It's almost as easy to use an 8 mm wrench and plain flat screwdriver.

    15. "Combo' sync tool (top) or flat screwdriver and 8 mm wrench
  15. Loosen the locknut at Adjuster A. Turn the screw to balance carb #1 and carb #3 on the right side. Adjustment is correct when both cylinders show approximately the same reading. Carefully, tighten the locknut while holding the screw stationary. Don't over-tighten! Re-check readings on gauges or "sticks." Repeat as necessary if the readings have drifted.
  16. Loosen the locknut at Adjuster B . Turn the screw to balance carb #2 and carb #4 on the left side. Adjustment is correct when both cylinders show approximately the same reading. Carefully, tighten the locknut while holding the screw stationary. Don't over-tighten! Recheck readings on gauges or "sticks." Repeat as necessary if the readings have drifted.
  17. Loosen the locknut at Adjuster C . Turn the screw to balance left carb pair with right carb pair. Adjustment is correct when both pairs show approximately the same reading. Carefully, tighten the locknut while holding the screw stationary. Don't over-tighten! Recheck readings on gauges or "sticks." All 4 cylinders should now show the same reading. Repeat as necessary if the readings have drifted.
  18. Note: Don't get too anal here. Perfection is not possible. I strive for a tolerance of 1/4 - 1/2" Hg between all 4 cylinders. You can "chase" this all day and not get much closer due to the inherent "slop" in the linkage design.
  19. Check idle speed and re-adjust if necessary to 1000 rpms.
  20. I like to do one additional check - SLOWLY raise the engine speed to 3000 rpms and observe the gauge readings. The readings should still be closely grouped. If not, you may have carb, ignition, compression or vacuum problems. Mis-matched main jets or jet needles can cause this problem. Other causes are dirty air filters, worn carburetor bodies or throttle slides and restrictions in the exhaust system. Can also be simple mechanical problems ("slop") with the linkage. In any case, if the readings fall badly out of spec at 3000 rpms but are good at idle, you may wish to investigate the cause. Some folks recommend synchronizing at 3000 rpms for better cruise performance but I do not. It's hard to live with a poor idle!
  21. Remove the gauges or "sticks."
  22. Replace the small screws in the intakes which were removed in step #8.
  23. Road Test.
  24. Recheck for leaks.
  25. Miller Time!

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Intellectual Property Notice: All material appearing in this website is the property of Randall Washington, and is protected under United States and international copyright laws. The photographs, text and other content may not be copied, reproduced, distributed, stored, or manipulated in any manner without the express written permission of Randall Washington. For more information on the use of material from this website, click here.

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