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If you are working on a motorcycle with a multi carb and multi-cylinder engine, the carburetors must be properly synchronized for smooth idling and smooth running at different speeds and throttle openings.
Carburetor synchronizing in a multi-carburetor and multi-cylinder motorcycle engine is best done using a carb sync tool.
- Motorcycle Carburetor Synchronization
- What is a carb sync tool?
- How the Tool Works
- How to Sync Your Motorcycle Carbs
- Types of Carb Sync Tools
- Carb Syncs Tool Options
- Carb Tool Buying Tips
- Dos and Don’ts of Carb Synchronization
- Do you have to carb sync a single carb/one cylinder motorcycle?
- When should a carb synchronization be completed?
- What is bench syncing?
- What are some symptoms that indicate the need for carburetor syncing?
- Do I have to sync my carbs after cleaning them?
- What should you do if the carb sync tool is not responding to the adjuster screw?
- Does carb syncing affect the air-fuel mixture of the engine?
- How often should you do a carb sync?
Motorcycle Carburetor Synchronization
There are many benefits when the carburetors of your motorcycle are in sync with each other.
In fact, your bike will not run well and may not start at all if the carbs are out of sync.
A proper carb sync will provide you with:
- Smooth and steady idle.
- Good throttle response at all speeds.
- Lower vibration.
- Better fuel economy.
- Consistent air-fuel mixture in all the cylinders.
What is a carb sync tool?
If you have a motorcycle with a V Twin, parallel-twin, V-double twin, or an inline cylinder (3, 4, or more) engine, a carb sync tool will help you keep your carburetors synchronized, tuned, and in top working condition.
Non-synchronized or out of sync carburetors will experience symptoms including:
- Erratic idling, high idle, or cutting out on idle.
- Excess vibration.
- Bogging issues.
- Excessive RPMs.
- Poor acceleration and throttle response.
How the Tool Works
Carburetors use the vacuum created at the engine (or at the intake manifold of the engine) to draw the air through the air filter. The amount of air drawn depends on the vacuum pressure and the throttle plate opening.
A carb sync tool helps you to balance the vacuum at the engine end of all the individual carburetors (near engine inlet manifold) by adjusting the throttle plates to make them in sync with each other.
In other words, they open at the same times at the rates required to create an equal vacuum in each carb.
Carb syncing ensures the same proportion of air-fuel mixture during idling and hence the same idling speed.
A carb sync tool will have a vacuum gauge or manometer to read the vacuum at the vacuum ports. You’ll need a tool with the same number of gauges as the number of carbs on your bike.
How to Sync Your Motorcycle Carbs
The process for syncing your carbs will vary slightly depending on your bike, the tool you use, and the carburetor set up.
The basic process is this:
- Connect the carb sync tool to the vacuum ports of each carburetor.
- Start the engine.
- Allow the bike to idle.
- Make minor adjustments until the vacuum readings are in sync to one another (or within an acceptable tolerance).
- Repeat at different throttle positions.
If yours is a four-cylinder inline engine, you can connect the hose from gauge -1 of the Carb Sync Tool to the vacuum port of cylinder -1 and so on up to 4.
Sometimes, in a multi-carburetor set up, one carburetor is directly connected to the throttle cable linkage of your motorcycle. On these setups, this carb is called the ‘base’ carburetor.
The vacuum reading of this carburetor is used as a master for setting the throttle plate of other carburetors.
When you run the engine, the gauges measure and read the vacuum for each carburetor/cylinder simultaneously. Depending on your tool, the readings can either be on a set of vacuum gauges or a set of mercury manometers.
If the throttle plates are not in sync, the gauge shows uneven readings for one or more carburetors.
At any given engine speed, the vacuum pressure is directly inverse to the throttle plate opening: the lesser the opening, the higher the vacuum.
At wide open throttle, the airflow is at the maximum and the vacuum is at its lowest.
Vacuum readings are synchronized by using the carb sync adjusting screws.
The location of carb sync adjusting screws can be different for different motorcycles.
For example, inline-four cylinder motorcycle engines typically have carb sync adjusting screws between the adjoining carburetors.
The adjusting screw between carbs 1 & 2 adjusts the throttle plate of carbs 1 and 2, and the adjusting screw between 3 & 4 adjusts the throttle plate of carbs 3 & 4.
The middle screw adjusts the throttle plates of carbs 1 and 2 & 3 and 4.
Refer to your motorcycle owner’s manual to see which order you should sync the carbs in. Doing so will make the job much more efficient.
Types of Carb Sync Tools
There are three main types of Carb Sync tools:
- Vacuum gauges.
- Manometer (mercury (or alternate fluid) column reading).
- Digital Carb Sync Tool.
Carb Sync with Vacuum Gauges
This type of carb sync tool uses vacuum gauges to measure the vacuum and comes with two or four vacuum gauges, thereby allowing you to do Carb Syncing of up to 4 carburetors simultaneously.
If your motorcycle has 6 cylinders, like the 1978 Honda CBX 1000, then you can either do the carb syncing in more than one set up or use two Carb Sync Tools (with 2 and 4 vacuum gauges) to do it simultaneously.
You need to connect each vacuum gauge of the carb sync tool to the vacuum ports of your carburetor/engine manifold (first vacuum gauge to the first carburetor vacuum port, second vacuum gauge to the second…..and so on) and synchronize all the carburetors.
Carb Sync Manometer
This type of tool uses mercury or a patent fluid (since mercury, being poisonous, is banned in many places) manometer to measure the vacuum.
You will be able to synchronize up to 4 carburetors simultaneously. This carb sync tool is generally more accurate than the vacuum gauges.
The unit consists of a metal sheet body with clear graduations incremented every 2 centimeters, and four glass or transparent plastic tubes.
The four transparent tubes may be connected to a common vial of the patent fluid. The top of each transparent tube is closed with caps.
The Carb Sync Kit also consists of line restrictors, adapters of various sizes and lengths (5mm, 6 mm, etc.) to suit different motorcycle carburetors, and an easy to follow instruction manual.
Some manometers have 4 individual transparent tubes having their own fluid and required to be calibrated by connecting all the four manometer tubes to the base carburetor vacuum port.
Digital Carb Sync Tools
Digital Carb Sync Tool or the Digi Sync (as it is known in the market) is the latest tool in the market which gives the vacuum readings by digital display and is more accurate than both the vacuum gauge or the manometer Carb Syncs.
The Digi Sync is designed and made in the USA.
Currently, they have models for simultaneous synchronizing of up to 6 carburetors.
Digi Sync has 4/6 outlets that can be connected to the vacuum ports for balancing the vacuum readings.
Carb Syncs Tool Options
1. ALPHA MOTO Motorcycle Carb Sync Gauges
This tool features 4 precision vacuum gauges mounted on a heavy metal base for benchtop use and each gauge has its own individual damping control and calibration control.
The kit contains the items below stored in a sturdy and compact plastic box:
- Auxiliary gasoline tank of 2 liters capacity, marked with standard and metric measurements, and petcock.
- 4 gauges of 2.75ʺ diameter and 0 to 38 HG gauge reading.
- 4 Rubber hoses 4 mm (ID) * 7 mm (OD) * 75 CM long.
- 4 white plastic valves 5.5 MM (OD).
- 2 each of M5 adapters 102 MM length & 52 MM length.
- 2 each of M6 adapters 102 MM length & 52 MM length.
This kit can be used for the carb syncing of almost all the bikes with carburetors.
Before using the tool, make sure all the gauge readings are showing zero.
If any gauge has an error, open the plastic cap on the dial and use a flat head screwdriver to set the dial to zero.
2. Motion Pro 08-0411 Syncpro Carburetor Tuner
This tool uses manometer gauges to measure the vacuum of up to 4 carburetors simultaneously.
The patented design is spill-proof and can be stored in a horizontal or vertical position.
The kit contains:
- Manometer with 4 tubes and a hook.
- 5 mm adapters.
- One long rubber hose to be cut into 4 numbers of equal length.
- Restrictor for the hoses – x4.
- Calibration tool – x1.
Optional 6 mm adapters can be purchased separately.
Product dimensions: 20ʺ * 8.1ʺ * 1.5ʺ
To use the tool, insert the restrictors on the engine side of the Carb Sync hose.
For calibration of the Carb Sync: Using the calibration tool, connect all the 4 hoses from the manometer to the vacuum port of the base carburetor.
There are 4 screws (one each at the base of each monometer tube), turn them counterclockwise (please refer to the owner’s manual and follow the step-by-step procedure) to allow a maximum vacuum.
Start the engine to idling and calibrate all the four manometer readings to be the same by turning the screws slowly in a clockwise direction.
3.Digi Sync: Digital Carb Synchronizer
Simple, easy to use, digital display, and accurate (you can do carb syncing with a difference of one digit to each other).
Can be used for motorcycle with 2 to 6 carburetors.
More accurate than liquid manometer and vacuum gauge carb syncs with there is no threat of manometer fluid sucked into the engine.
- Battery operation.
- Digital display.
- Displays vacuum and rpm.
- Made in the USA.
- Comes in a compact box.
A reset button is located on the front and a power port on the side. A Velcro attachment on the bottom allows you to stick it at a convenient location.
The bottom of the Digi Sync has rubber feet and some models have a cling option (magnetic) for the bottom which allows it to stick to any metallic body, like a fuel tank.
You need to put on the Digi Sync and allow it to self-calibrate before turning the engine on.
Compact size: 7.1ʺ * 6.1ʺ * 4.3ʺ (1.5 pounds)
The kit contains:
- Digi Sync 6 channels.
- Power cord.
- 9 Volt battery (included).
- Vacuum port plugs.
- 6 hoses of 3 foot length, one end suitable for quick coupling with the Digi Sync hose end.
Carb Tool Buying Tips
If you are a motorcycle enthusiast and like wrenching on your own bikes, the right carb sync tool will be useful addition to your workshop.
If you have a twin carburetor motorcycle now and plan to go for a bigger engine in the future, you have to decide whether you want to buy a carb sync tool for simultaneous syncing of two or four carburetors.
Of course, you may be able to do carb syncing of 4 carburetors in more steps using a two gauges tool, but it will be more time consuming and the result may not be the best.
Secondly, you have to make a decision about the type of carb sync tool to get.
Although the manometer type is more accurate than the vacuum gauge, it does not allow you to check the synchronization at higher engine rpm due to the risk of the manometer fluid getting sucked into the engine.
Then there is the digital carb sync tool which is more accurate than either the manometer type or vacuum gauge type. However, it does cost more.
Dos and Don’ts of Carb Synchronization
Follow these tips to make sure you sync your motorcycle’s carburetors safely and properly.
- Always sync your carbs in an open area to avoid exposure to harmful exhaust.
- Warm-up your motorcycle engine before you start syncing to get better results.
- If your motorcycle engine is air-cooled, either choose a location with good shade and breeze or use a fan to cool the engine. An overheated engine may give erratic results or cause damage.
- When using a manometer type, do carb syncing at idle speed only.
- Do not run your engine in a closed enclosure like your garage.
- Do not start syncing when the engine is cold, the results can be erratic.
- Do not start the job without first ensuring that your motorcycle engine will not overheated.
- If you are using a manometer type tool, do not rev the throttle up or down as your engine may end up sucking the fluid.
1. Perform the Job in a Well Ventilated Area
Always do the carb syncing in a well-ventilated area, preferably outside your garage.
Since the engine is running, you need to protect yourself from the exhaust fumes.
2. Warm Up the Engine
Warm up the motorcycle engine before you start carb syncing.
The sync will not be accurate if it is not done at the operating temperature.
Or, if the engine is cold when you sync the carbs, they may become out of sync once your start to ride and the engine warms up.
3. Finish the Other Steps First
Carb syncing should be the last step.
Ensure all the carburetor work including cleaning, pilot screw setting, bench sync, and air filter cleaning is done first.
4. Always Sync After a Carb Cleaning
Carb syncing is a must if your motorcycle carburetor was dismantled during maintenance and cleaning.
5. Keep your Engine Cool
If your motorcycle engine is air-cooled, have a strong fan to cool the engine while the bike is running stationary.
Carb syncing of an overheated engine will not give good results.
6. Eliminate any Vacuum Leaks First
Check for vacuum leaks in the motorcycle engine manifold or the carburetor engine end.
Fix any leakage before you start carb syncing.
You will not be able to sync if there is a vacuum leak.
7. Use Caution When Working Near Fuel
In many motorcycles, you need to take out seats, side panels, and gas tanks to get to the vacuum ports and carb sync screws.
Be careful when you take out the fuel line.
Gasoline should not drop on the engine when it is hot. Clean up any fuel leaks immediately.
Do you have to carb sync a single carb/one cylinder motorcycle?
No, syncing only needs to be done when your motorcycle has two or more carburetors.
When should a carb synchronization be completed?
You should sync your carbs after the carbs have been cleaned, the air filter has been cleaned, the pilot screws are set, and the carbs a bench synched.
What is bench syncing?
Bench synching is when you take off your carburetors from your motorcycle and do the carb syncing without a carb sync tool.
To do it, you synchronize the throttle opening of all the carbs using a wire or small drill bit.
This will give you a rough baseline to start a more accurate synchronization with the measurement tool when the engine is running.
What are some symptoms that indicate the need for carburetor syncing?
Symptoms can include:
- (When your bike’s idling is erratic and it appears to lose power.
- Abnormal vibration at lower throttle position.
- After accelerating the engine takes more time to come back to idle speed.
- Poor fuel economy and abnormal engine noises.
- Difficult to start the engine.
- Higher exhaust emission.
Do I have to sync my carbs after cleaning them?
Yes, you’ll usually need to sync your carbs after they’ve been cleaned.
Once your take the carbs off the bike and take them apart, they’ll typically become out of sync and will need to be reset once everything is reassembled and installed.
What should you do if the carb sync tool is not responding to the adjuster screw?
If the gauge readings don’t change when you make adjustments, there are often two main possibilities:
- Check the hose connection between the vacuum port of the carburetor and the gauge for any leaks.
- Check for a vacuum leak at the air-cleaner end or the engine end of the carburetor.
Does carb syncing affect the air-fuel mixture of the engine?
No, it does not. Carb syncing ensures the same proportion of air-fuel mixture in each cylinder at the same time. It does not affect the richness or leanness of the mixture.
How often should you do a carb sync?
It typically recommended to do it after about 15,000 to 20,000 miles, or at least once a year, or if the carburetors of your motorcycle are taken apart.
It is best to follow the guidelines found in your owner’s manual.
The goal of carb syncing is to bring all the throttle plates or butterfly valves to work in a coordinated way.
This improves the idle and throttle response of the motorcycle, resulting in a smooth ride and better performance.