How to Sync Motorcycle Carbs

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After cleaning, repairing, and reassembling your carburetors, you’ll need to balance and synchronize them. A complete carb synchronization typically starts off the bike with a static carb bench synch followed by a vacuum sync with a carb sync tool after the engine is warmed up.

A carburetor sync is keep to:

  • A smooth idle.
  • Full power out of your engine.
  • Smooth throttle response and acceleration.

Properly synced carbs ensure that each cylinder is drawing the same vacuum pressure at different RPMs (idle and up).

Symptoms of Out-of-Sync Carbs

How to know if your carbs need to be synced:

  • You just cleaned and rebuilt them.
  • Your bike has a rough, sporadic idle.
  • You’re getting poor fuel mileage or notice more emissions from your exhaust.
  • You hear strange engine noises and vibrations.

How to Bench Sync Motorcycle Carbs

Carb Bench Sync

After you’ve cleaned and reassembled your carbs, you can perform a bench sync to help make vacuum syncing the carburetors on the bike much easier.

After you finishing your carburetor bench sync, install the carbs on the motorcycle and continue with the vacuum synchronization and setting the idle speed.

To bench sync your carburetors:

  • Refer to your service manual for exact carburetor adjustments.
  • Find a small wire or drill bit about .5 to 1.5 mm in diameter.
  • Position your carbs so you can reach the throttle slide adjusters and lock nuts.
  • Start with one of the slides and insert your wire or drill bit so that it sits flush with the bottom of the carb throat.
  • Set the throttle slide so that you can barely remove your wire/bit gauge.
  • Repeat for the remaining carbs.
  • Recheck that each slide closes at the same position and that the wire/bit has the same clearance in each carb.
  • Be careful not to scratch or mar the carburetor body or the slides.
  • Your carbs are bench synced!

How to Set your Motorcycle’s Idle

To adjust your motorcycle’s idle, you’ll typically need to adjust the airscrews and the idle adjustment screw(s).

The airscrew is typically set a half turn to one and a half turns from its bottom.

To adjust the engine’s idle speed:

  • Start and warm up the engine.
  • Turn the idle screw until you get the lowest idle within the recommended range.
  • Turn the idle air screw to improve the idle.
  • Adjust both screws to get a smooth idle.

Confirm the proper idle with a vacuum sync and by pulling the spark plugs and inspecting their color.

Balance & Vacuum Synchronize Motorcycle Carbs

Motorcycle Carburetor Synch Tool

To vacuum sync your carburetors:

  • Bring the engine up to operating temperature.
  • At idle, adjust each carb until the vacuum readings are within about 2 cm Hg of one another.
  • Work in pairs, matching the 1 and 2 carbs together, and then the 3 and 4, and then the pairs together.
  • Bring the RPMs up to about 3,000 and make sure the vacuum reading rise at roughly the same rate and stop in the same position.
  • Make adjustments as needed.
  • Recheck the idle sync.

Syncing the carbs and making adjustments will be easiest if you have an auxiliary gas tank. With the tank installed on the bike it can be nearly impossible to make the proper balancing adjustments.

Check your motorcycle shop manual for specific adjustment procedures. You can further tune your bike at all throttle ranges with a plug chop.

For motorcycles with more than 1 carburetor, you’ll need to sync them at idle and at advance RPM to make sure your bike will run properly.

If your carbs are out of balance, the cylinders will be fighting against each other, causing a lot of excess vibration and rough operation.

The best way to sync carbs is either with a vacuum sync or manometer tool. Most carburetors will have hose connections or screw-in fittings near the front of the carb that will allow you to hook up your sync tool.

When syncing carbs, it’s a good idea to point a fan at the engine to help keep it cool. If you find you’re having trouble, take a break and let the engine cool down before you continue with the synchronization. If you’ve changed the intake or exhaust set up on your bike, you may need to rejet your carbs to get them to run properly. 

Carburetor Vacuum Leaks

If your carbs have a vacuum leak or an air leak, you’re not going to be able to properly sync them.

Common symptoms of a vacuum leak include:

  • High idle.
  • Engine won’t start.
  • Racing RPMs.

To diagnose where your vacuum leak is coming from you can spray carb cleaner or starter fluid around the carbs, connections, and manifolds. If you hear a quick fluctuation in RPM after you spray a spot, you’ll have found the leak.

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