Synchronizing the intakes of a multi-cylinder motorcycle is one of the most important maintenance tasks that need to be done on your bike. Keeping your carbs or throttle bodies balanced means your bike will run smoother, be more responsive to the throttle, and can even help fuel mileage. Many people with fuel-injected bikes don’t realize that throttle bodies need adjustments just as often as carburetors do.

Balancing your carbs or throttle bodies (we will just refer to them as carbs from now on) means that you use a tool to make sure that all the carbs open at the same time and at the same rate. Any engine that has more than one carb will need to have them balanced regularly. This is generally done with some type of vacuum manometer, which measures the intake vacuum of each cylinder. If one carb is misadjusted and further open (or closed) than the others, the load on the cylinders will be unbalanced, and engine performance will suffer. The carb that is further open will cause a lower intake vacuum on that cylinder, which is why measuring intake vacuum is the most common way of synchronizing your carbs.

There are as many different procedures for synchronizing carbs as there are different kinds of motorcycles. Inline two, three and four cylinders use similar methods, but v-fours, v-twins, and opposed twins, fours and sixes all have different methods to balancing their carburetors.

Just to give you an idea about the process, we will take a typical inline four-cylinder engine and go through the most common process. Every bike is different though, and so this process may not apply to your bike. Always refer to the service manual for your motorcycle to get exact procedures. While properly synchronizing your carbs can be a satisfying job, and make your bike run at it’s best, if you get it wrong and make it worse, your bike will run terribly, if at all. Make sure you are familiar with the requirements of your particular bike and understand what needs to be done. This is not a job for a first time mechanic!

The critical tool that you need for a carb synch is a vacuum manometer to measure each cylinder and be able to compare them. Vacuum manometers are commonly called carb tuners. While there are many types of tools for this job, be it mercury, gravity, digital, analog or others, the best tool for the job is, of course, the Motion Pro SyncPro (p/n08-0411). The SyncPro is an affordable tool that does not use dangerous and toxic mercury, is easy to read, and easy to set up and use. Using the SyncPro takes a bit of learning, and would be too lengthy to go into here, but there are instructions included with the tool, and they are also posted on our website at http://www.motionpro.com/images/documents/syncpro_tech.pdf

This article is meant to give you an idea of the process of synchronizing carbs, so that you can judge for yourself if this is a job you are able to tackle. One other special tool that may be necessary is an auxiliary fuel tank (p/n 08-0189 or p/n 08-0032), which is necessary if you have to completely remove the OE fuel tank. Many modern bikes will just hinge the tank up and out of the way, but some older bikes need a way to get fuel to the carbs with the stock tank removed. Motion Pro makes two versions; you can check them out here:

http://www.motionpro.com/motorcycle/tools/view/auxiliary_tank/
http://www.motionpro.com/motorcycle/tools/view/deluxe_auxiliary_tank_for_
fuel_injected_systems/

First thing you need to do is get to the carbs you need to work on. Lucky BMW and Moto Guzzi owners have them hanging out in the breeze, but with the inline-four we are working on, they are a bit more hidden away. Generally, you will need to remove the seat and the fuel tank, and possibly the air box. This is where your service manual comes in handy, so that you know exactly what to remove to do the job on your bike. Once you can get at your carb you will find that somewhere along the intake manifolds or at the bottom of the carb are spigots for each manifold that you will connect the carb tuner to. Some bikes will just have a threaded port with a screw plugging it that you will have to remove and thread in an adapter to connect the carb tuner. The first thing to do before anything is connected is to warm up the engine so that it is fully up to temperature. This is necessary because all the components need to be at operating temperature to be fully functioning. Testing a cold engine will likely produce inaccurate results. Also, many motorcycles run rather rich when cold, and the excess fuel can contaminate a carb tuner. Once the engine is warm, it’s time to connect the SyncPro. On the four-cylinder engine we are using for this example you should connect the first channel of the carb tuner to the first cylinder, typically the one on the left side of the bike. Then you need to connect the second channel to the next cylinder, which is #2, and so forth, until all four channels are connected to all 4 cylinders.

Okay, we are pretty close to starting the bike, and seeing how far out of sync your carbs are. But before that, there are a few things you need to know as a user. One, all carb sync work should take place at idle speed. Your service manual may ask you to “blip” the throttle to double check the sync, and it is very important to be careful here. When the throttle opens and then closes rapidly, the engine vacuum will spike, and can cause the fluid to be pulled out of the SyncPro. Anytime you raise the idle, be very careful to close the throttle slowly, so as not to exceed the vacuum limit of the tool. Every bike is different�"a big 1200cc engine will develop higher vacuum than a small 250cc engine. Take your time, and pay attention to how your bike reacts. Also, make sure you are in a well-ventilated area. You will be running your bike for some time while you are making adjustments, and the last thing you need is to asphyxiate yourself with exhaust gases.

Let’s fire it up! With the bike running, you will be able to see on the carb tuner each channel of fluid rise in response to the engine vacuum. If you are lucky, they are all perfectly in line, and your job is over. Congrats, have refreshment, and feel good know that your carbs are perfectly balanced. See you later!

What? The columns of the tuner are not even? What do you mean they are all completely different? Oh well, we have some more work to do.

Most inline four carbs are adjusted the same. The number 1 and 2 carbs are linked together, and the 3 and 4 carbs are linked as well. Then the two pairs are linked together. The process of balancing works like this. First, the number 1 and 2 carbs are balanced. There is an adjuster screw between the two carbs that will change the throttle butterfly position. By turning the screw in and out, you can adjust number 1 and 2 so that their tuner channels are at the same height, or pretty close. It is not critical that they are EXACTLY the same, having them within a millimeter or so is acceptable. Be aware that if you push on the adjuster with your screwdriver, you can affect the reading, so always remove the screwdriver from the adjuster screw, and then check the reading. Once those two are the same, then you move to numbers 3 and 4, and perform the same adjustments with the screw that is between those two. At this point, number 1 and 2 should be the same, and 3 and 4 also, but the two pairs may be different. There is a final screw between number 2 and 3 that balances the two pairs together. During the balancing process, you may experience the idle rising or dropping, as the carbs are opened more or less. Use the idle adjuster screw during all the adjustments to keep it in the normal range. Once all the columns of the tuner are the same, you are almost done. Remember what I said about raising the idle and being careful about the vacuum? This is where you get to practice. Open the throttle a slight amount, and then close it again. Now double-check your balance. There may be a slight change as the linkage settles itself. Make any adjustments necessary to the carbs, and then blip the throttle again slightly. It may take one or two more adjustments until they remain balanced.

There you go. Shut off the bike, remove the SyncPro, cap off or plug the vacuum connectors, and put the bike back together. Head out for a ride, and revel in how smooth and responsive your bike is with all the carbs working in blissful harmony.

For more information about all of Motion Pro's products and applications, look us up on the web at http://www.motionpro.com

About Motion Pro, Inc. - Motion Pro was founded in 1984 by Chris Carter. The company designs and manufactures quality control cables, throttles, levers, tools and accessories for the powersports market. Motion Pro products are available through all major distributors.