If you are using 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.
II. What is a Carb sync tool
If you are owning a motorcycle with a V Twin, parallel-twin, V-double twin, or an inline multiple cylinder (3, 4, or more) engine, the Carb Sync tool will help you keep your carburetors synchronized, healthy, and in top working condition.
Disclaimer: some of the links on this page might be affiliate links. When you buy through links on our site, we may earn an affiliate commission. Click here to learn more.
Non-synchronized or out of sync carburetor engines will have erratic idling, vibration, and other issues.
How the Carb Sync tool helps you?
You are aware that the carburetor uses the vacuum created at its engine end (or at the intake manifold of the engine) to draw the air through the air filter and the amount of air drawn depends on the vacuum and the throttle plate opening.
The Carb Sync tool helps you to synchronize or 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.
Carb Syncing ensures the same proportion of air-fuel mixture during idling and hence the same idling speed (same range of rpm).
The Carb Sync tool will have a vacuum gauge or manometer to read the vacuum at the vacuum ports and the number of gauges is 2 or 4.
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.
In a multi-carburetor set up, one carburetor is directly connected to the throttle cable linkage of your motorcycle, and 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 (idling), the gauges measure and read the vacuum for each carburetor (cylinder) simultaneously and the readings can either be on a set of vacuum gauges or a set of mercury manometer.
If the throttle plates are not equally set, the gauge shows uneven readings for one or more carburetors. At any given engine speed, the vacuum is directly proportional to the throttle plate opening; the lesser the opening, the higher the vacuum.
Vacuum readings are synchronized by using the carb sync adjusting screws. The location of carb sync adjusting screws can be different for different motorcycles. Inline-four cylinder motorcycle engines 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.
III. Types of Carb Sync Tool
There are three types of Carb Sync tools (i) with vacuum gauges (ii) with manometer (mercury (or alternate fluid) column reading) (iii) 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, 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 with Mercury or liquid manometer:
This type of Carb Sync 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 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, adopters 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 Tool:
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 either the vacuum gauge or the manometer Carb Syncs.
The Digi Sync is designed and made in the USA.
Presently, 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.
IV. Selected carb Syncs.
1. ALPHA MOTO Motorcycle Carb Sync Gauges with Auxiliary Gasoline Tank Combo for CB KZ GS XS 550 650 750 850 900 1100. Brand-ALPHA MOTO.
This Carb Sync 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:
|1||Auxiliary gasoline tank of 2 liters capacity, marked with standard and metric measurements, and petcock.|
|2||4 gauges of 2.75ʺ diameter and 0 to 38 HG gauge reading.|
|3||4 Rubber hoses 4 mm (ID) * 7 mm (OD) * 75 CM long.|
|4||4 white plastic valves 5.5 MM (OD).|
|5||2 number each of M5 adapters 102 MM length & 52 MM length.|
|6||2 number each of M6 adapters 102 MM length & 52 MM length.|
This Carb Sync Kit can be used for the carb syncing of almost all the bikes with carburetors.
Before using the Carb Sync 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.
This Carb Sync 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:
|1||Manometer with 4 tubes and a hook.|
|2||5 mm adapters|
|3||One long rubber hose to be cut into 4 numbers of equal length.|
|4||Restrictor for the hoses – 4 numbers.|
|5||Calibration tool – 1 number|
Optional 6 mm adapters can be purchased separately.
Product dimensions: 20ʺ * 8.1ʺ * 1.5ʺ
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.
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 Carb Syncs and gauge Carb Syncs and there is no threat of manometer fluid sucked into the engine.
Battery operated, digital display, displays vacuum and rpm. Made in the USA. Comes in a compact box.
A reset button on the front and a power port on the side. A Velcro on the bottom to stick it at a suitable location.
The bottom of the Digi Sync has rubber feet and some model has 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:
|1||One number Digi Sync 6 channels.|
|2||Power cord -1 number.|
|3||9 Volts battery (included).|
|4||Vacuum port plugs – 6 numbers.|
|5||6 hoses of 3 feet length, one end suitable for quick coupling with the Digi Sync hose end.|
V. Buying Tips
If you are a motorcycle enthusiast and intend to ride it for many years, plan your Carb Sync tool very carefully since most Carb Sync tools serve you for a long period.
If you are having a twin carburetor motorcycle now and plan to go for a higher range (4 cylinders) 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 setups using a two gauges Carb Sync 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. Although the manometer type is more accurate than the vacuum gauge type, it does not allow you to check the synchronization at higher engine rpm due to the fear of the manometer fluid getting sucked into the engine.
Then there is the Digi Sync tool which is more accurate than either the manometer type or vacuum gauge type. However, it costs more.
VI. Dos and Don’ts about Carb Syncs
|1||Always do Carb Syncing in an open area to avoid exposure to harmful exhaust.||Do not do Carb Syncing in a closed enclosure like your garage.|
|2||Warm-up your motorcycle engine before you start Carb Syncing to get better results.||Do not start Carb Syncing when the engine is cold, the results can be erratic.|
|3||If your motorcycle engine is air-cooled, either choose a location with good shade and breeze for Carb Syncing or use a fan to cool the engine. Carb Syncing of an overheated engine may give erratic results.||Do not go for Carb Syncing without ensuring that your motorcycle engine is not overheated.|
|4||When using a monometer type Carb Sync tool, do carb syncing at idle speed only.||If you are using a manometer type Carb Sync, do not rev the accelerator more or suddenly throttle up or down as your engine may end up sucking the fluid.|
VII. FAQ (Frequently Asked Questions)
|1||Can I do Carb Sync for my single carb motorcycle?|
|No, Carb Syncing can be done when your motorcycle has two or more carburetors.|
|2||If my motorcycle is under overhauling, when should I do Carb Syncing?|
|You can do Carb Syncing only after the complete work on the carburetor including all the pilot screw adjustments and air filter cleaning.|
|3||What are the symptoms that indicate the need for Carb Syncing?|
|The symptoms can be – (i) When your bike’s idling is erratic and it appears to lose power (ii) Abnormal engine vibration at lower throttle position which may become negligible at higher throttle openings (iii) If your motorcycle engine, after accelerating (revving), takes more time to come back to idle speed (iv) Poor fuel economy and abnormal engine noises (v) Difficult to start and (vi) Higher exhaust emission.|
|4||What does Carb Syncing do? What is the function of Carb Sync?|
|Doing Carb Syncing for your motorcycle ensures that each cylinder draws the same amount of vacuum through its carburetor and all the individual cylinders have the same idling rpm.
Using the Carb Sync tool, the vacuum at each carburetor (or inlet manifold) is synchronized by adjusting the throttle plate.
|5||Do I have to Sync my carbs after cleaning the carburetors?|
|No, not necessary if you have not disturbed the carb sync screws between the carburetors.|
|6||At what stage should I do Carb Syncing?|
|You should do Carb Syncing after completing all the carburetor work including pilot screw adjustments and the engine should be at its operating temperature.|
|7||My carburetor is not responding to the carb sync adjuster screw, the gauge reading does not change.|
|There are two possibilities- (i) Check the hose connection between the vacuum port of the carburetor and the Carb Sync gauge for any vacuum leakage and make corrections (ii) Alternately, there can be a vacuum leak at the air-cleaner end or the engine end of the carburetor, check and correct it.|
|8||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 and does not affect the richness or leanness of the mixture.|
|9||What is the normal gap between two Carb Syncing for a motorcycle?|
|It may be ideal to do it after a run of 15,000 to 20,000 miles, or more than once a year, or if the carburetors of your motorcycle are taken apart. It is best to follow the guidelines of your Owner’s manual.|
|10||What do I benefit from Carb Syncing?|
|There are many benefits when the carburetors of your motorcycle are in sync with each other such as (i) smooth and study idle, (ii) good throttle response at all speeds, (iii) low vibration, (iv) better fuel economy, and (v) consistent air-fuel mixture in all the cylinders, etc.|
|11||What is bench syncing of carbs?|
|When you take out your carburetor set from your motorcycle and do the carb syncing on a workbench (without a Carb Sync tool), it is called bench syncing. In this process, you synchronize the throttle opening of all the carbs using a hardwire or drill bit of less than 1 mm diameter size.|
VIII. Carb Sync Tips
- Always do the carb syncing in a well-ventilated area, preferably outside your garage. Since you do carb syncing when the engine is running, safeguard yourself from the exhaust fumes.
- Warm up the motorcycle engine before you start carb syncing. The carb syncing will not be accurate if it is not done at the operating temperature.
- Carb syncing should be the last operation, hence ensure all the carburetor work including screws setting and air filter cleaning is done beforehand.
- Carb syncing is a must if your motorcycle carburetor was dismantled during the maintenance.
- If your motorcycle engine is air-cooled, have a strong fan to cool the engine. Carb syncing of an overheated engine will not give the required outcome.
- Check for vacuum leaks in the motorcycle engine manifold or the carburetor engine end and stop the leakage before you start carb syncing. You will not be able to do carb syncing if there is a vacuum leak.
- In many motorcycles, you need to take out seats, side panels, and gasoline tanks to approach the vacuum ports and carb sync screws. Be careful when you take out the fuel pipe. Gasoline drops should not fall on the engine (which is hot). Place a rag under the fuel cock.
The aim of Carb Syncing is to bring all the throttle plates or butterfly valves to work in a synchronized or coordinated way to improve the smooth idling of the motorcycle, thus resulting in a smooth ride.
The carburetor is the heart of your motorcycle. If you do any mistake while working on it, it may result in a mishap or an accident (sometimes even fatal).
Hence, unless you have previous experience, do not attempt to work on your motorcycle carburetors. Discuss with the authorized technician for your motorcycle and seek their guidance before putting your hands on the carburetor.