CB750, XS650, KZ650 Carb Sync & Tuning

CB750, XS650, KZ650 Carb Sync & Tuning

In order to get your old Japanese motorcycle project running well, you’re going to need to spend a little bit of time thoroughly cleaning and syncing the carburetors. With a little patience and the right tools, cleaning and syncing carbs is not a terribly difficult procedure.

However, depending on your motorcycle’s exact set up (i.e. anything other than completely stock – cylinder bore, pistons, exhaust, and airbox vs. pods) you may find that you need to try different jet combinations to get your machine tuned and running properly at all throttle positions.

Materials you’ll need to sync your carbs:

  • Vacuum gauges (with plug hole extenders depending on your motorcycle)
  • Auxiliary fuel tank or long fuel hose.
  • Proper sized wrenches for adjusting locknuts.

While you’re at it, check out info for timing the ignition on these bikes.

Honda CB750 Carb Sync Instructions:

Initial CB750 Carb Sync:

  1. Remove the fuel tank and set up an auxiliary tank or a long fuel line to the carbs.
  2. Remove the air cleaner to confirm proper operation of the choke butterfly valves.
  3. If the clearance between the choke valve and carb body is greater than .5mm, adjust the choke clearance by lengthening or shortening the choke lever adjustment rod.
  4. Adjust the throttle stop screw to line up the T mark with the indicator on the carb body for each carb.
  5. Loosen and adjust the throttle cable lock nut and cable adjuster so that there is about 1 to 2 mm of play in the cable.
  6. Make sure the play in each carbs cable is the same.
  7. Turn each carb air screw until it seats, then back it out 1 full turn.
  8. Reinstall the airbox.

Vacuum sync:

  1. Start the engine and warm it up to operating temperature.
  2. Remove the vacuum plug holes in each carb body and screw in the vacuum gauge adapters.
  3. Set the idling speed to 850 to 950 RPM. Adjust the throttle stop screws in each carbs to reach the proper idling speed and the same vacuum readings for each carb.
  4. Open the throttle and bring the RPMs up to about 3,000.
  5. Watch the vacuum gauges to see that the needles rise to the same position at about the same rate.
  6. Adjust the top cable locknuts to achieve the same vacuum in each carb.
  7. Check the idle again and adjust via the throttle valve lock screws.
  8. The the throttle a few times to confirm vacuum readings.
  9. Turn the wheel to check that the cables are routed and adjusted properly.

Yamaha XS650 Carb Sync:

  1. Remove the vacuum plugs from the carb body and attach the hoses from your vacuum gauge.
  2. Start the motorcycle and bring it up to operating temperature.
  3. Adjust the dampers in each vacuum hose until the needles only move slightly.
  4. Turn the sync crew until the gauges return the same reading.
  5. Once they do, shut of the engine, remove the vacuum hoses and reinstall the plug caps.
  6. Setting the idle speed: Start the engine and bring it up to operating temperature.
  7. Standard idle speed is about 1,200 RPM.
  8. Change the idle speed by turning the throttle stop screw. Turing it in increases RPMs, backing it out decreases engine speed.

Kawasaki KZ650 Carb Sync & Tuning:

Initial KZ650 carb sync:

  1. Turn the idle screw until there is about 1.6 to 2 millimeters between the throttle cable bracket and the pulley stopper.
  2. Push the throttle grip closed, there should be no clearance between the bracket and the stopper.
  3. When the throttle grip is released there should be about 1.6 to 2 mm clearance.
  4. Replace the cable bracket if not.
  5. Remove the carbs from the engine.
  6. Remove the top covers and loosen the throttle slide locknuts.
  7. Turn the idle screw so that there’s a little bit of clearance between the throttle valve and the bottom of the carb.
  8. Turn each valve adjusting screw so that all four clearances between the carb body and the valve are the same. You can place a small piece of wire (.5 to 1 mm) between the valve and carb body to gauge the distance.
  9. Once all clearances are the same, tighten the locknuts, making sure that the clearance hasn’t changed.
  10. Reinstall the top covers.
  11. Open the throttle until the bottom of the lowest carb valve is at the top of the carb bore. Turn the pulley stop screw so it stops at the position.
  12. Turn each carb pilot screw until it seats and back it out ¾ of a turn.
  13. Reinstall the carb onto the engine and properly adjust the throttle cables.

Idle adjustment:

  1. Remove the fuel tank and run a long line or an auxiliary tank.
  2. Start the engine and warm it up to normal operating temperature.
  3. Turn the idle screw until idling speed is between 950 and 1,050 RPM.
  4. Open and close the throttle a few times to check that the idling speed does not change.
  5. Turn the handle bars and monitor idling speed to check if the throttle cables need to be readjusted.

Vacuum sync:

  1. With the fuel tank removed, start the engine and bring the bike up to operating temp.
  2. Remove the rubber caps from the carb boots and attach the hoses from your vacuum gauge.
  3. With the engine idling, close the vacuum gauge pressure dampers until the needle only moves a little bit.
  4. With the engine at idle check that all gauge needles are within about the same position.
  5. If not, remove the carb tops and readjust the throttle valve lock screws until they are. Backing the screw out will decrease the vacuum, tightening it will increase it.
  6. Perform the idle adjustment again.
  7. Twist the throttle to bring RPMs to about 3,000 – check that vacuum readings rise uniformly to the same position.
  8. Continue making fine adjustments of the throttle valve lock screws until they do.

Float Height & Fuel Level for Japanese Motorcycle Carbs

If you’re having trouble syncing and tuning your carbs or you find the your motorcycle is sluggish at certain throttle positions, these general descriptions of your carb’s components may help you troubleshoot the issues.

Floats and Carb bowl – The carb floats control the maximum level of fuel in the bowls. If the float level is set incorrectly it will cause the fuel level to be too high or too low. If it’s too high, you’ll run a rich condition and gas can leak into your cylinders, contaminating your engine oil. If it’s too low, you’ll run a lean condition and potentially burn a hole in your pistons or burn out your valves.

Your specific motorcycle model’s service manual will tell you where to set the float height.

An easy way to check your float height is by attaching a clear tube to the drain holes in each carb. With the bike on its center stand and the fuel petcock running, hold the clear tube next to the carb body and record where the fuel level sits. Adjust as necessary by removing the float bowl, and adjusting the small metal part of the float that sit on the needle. Make tiny adjustments and be careful not to bend the floats.


Throttle Position Troubleshooting

If you floats are set correctly and you find your bike is still having trouble at certain throttle positions, you can try to narrow down the culprit with these general guidelines. The follow carb parts control the fuel/air mixture at roughly the corresponding amount of throttle.

  • Pilot/idle Jet controls fuel/air mixture from idle to ¼ throttle.
  • Throttle valve cutaway controls fuel/air mixture idle to ⅓ throttle.
  • Needle Jet/Jet Needle controls fuel/air mixture about ⅙ to ¾ throttle.
  • Main Jet controls fuel/air mixture ¾ to Wide Open Throttle (WOT).

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