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How to Sync Carbs on a Ninja ZX7R

Dan has always done his own auto repairs whether on motorcycles, boats, cars, or even motorhomes.

Carburetor Synchronization Is Essential

Any time there are multiple carbs on an engine, syncing those carbs becomes a part of the maintenance schedule to be performed on a regular basis, just as changing oil is. While this job requires a little more effort and tools than a simple oil change, it is well within the capabilities of an average home mechanic who does his/her own bike maintenance.

Without proper carburetor synchronization, a motorcycle (or any engine with multiple carbs) will not perform optimally. Different cylinders will receive different amounts of fuel/air mixture and perform differently; this will result in an overall degradation of performance and could, in severe cases, damage the engine.

While this article is dedicated to syncing the carburetors on a Kawasaki Ninja ZX7R (and all the detailed photos are of that model), the basic procedure will be the same for any bike. The location of adjustment screws and vacuum ports will be different and it would require a little more knowledge of your particular motorcycle, but the article can still be used for general information on the procedure.

Syncro kit used in this project.  Not shown are 4 pieces of rubber tubing vacuum lines.

Syncro kit used in this project. Not shown are 4 pieces of rubber tubing vacuum lines.

Necessary Tools

  • Carburetor or vacuum synchronizer tool
  • Set of metric sockets
  • A fairly long shaft Phillips screwdriver
  • A pair of needle-nose pliers

The primary tool that will usually not be found in a homeowner's tool kit will be a carburetor or vacuum synchronizer tool. There are several types available, with the most popular being either a set of manometer tubes with associated vacuum lines or a manifold of several mechanical vacuum gauges. The mechanical type is probably easier to use for most people as it will work in any orientation (upside down, maybe) and is a little more rugged. But either will do the job.

The biggest key is to make sure that there is at least one gauge for each carburetor. More won't hurt (except that it drives the cost up), but each carb must have its own gauge. Some kits contain extra tips for a wide variety of applications and these tips can be very useful. The ZX7R is one of the bikes that will require one of these extra tips, as shown in the following paragraphs.

These are not inexpensive tools, but at the same time, the cost of one could be saved the first time it's used. Shop costs for the same job are not cheap, either. The kit used for this project is one of the better ones available and can be used for a wide variety of bikes. There is also a variety of options available from Amazon; this is by no means all that they have to offer, but it will hopefully give an idea of what is available in the way of carburetor synchronizers.

Preparing the Bike

In order to attach the synchronizing tool as well as make any adjustments, the carburetors must be exposed. In addition, a fuel supply must be made available as the bike must be operating when the procedure is done. The pictures below show the location of the various bolts and hoses. You may click each thumbnail to view each photo.

1. Start by removing the gas tank. Pop both the rear and front seats off. There will be two bolts exposed that, along with two more on the front of the tank, hold the gas tank in place. Remove those bolts and set them aside. To provide the necessary fuel in this example, the area behind the tank was covered with a protective cloth and the tank carefully rotated completely around and laid on that area without disconnecting the fuel lines. This is not a stable position. Strap the tank firmly into place with bungee cords or something similar.

2. With the tank removed, the air cleaner housing is exposed. Remove the bolts that hold the cover in place and lift the cover off. There are two hoses that fit into the front of the housing and two in the rear. Disconnect these hoses from the air cleaner housing. Inside the lower housing are three more bolts that attach the housing to the engine. Remove these bolts as well and set them aside. At this point, the lower half of the housing can be worked off with a little patience and care and set aside.

With the carburetors now exposed, the next step is to attach the tool and test and adjust the carburetors.

Using the Carb Sync Tool

1. Under the carburetors, and fairly difficult to reach, is a set of vacuum lines: one each to three of the carburetors. The number 2 carb, counting from the left, does not have a vacuum line to it. Remove these vacuum lines, leaving the hose fitting in place on the engine. A bolt fills the hole on #2 carb where the line would go. Remove this bolt and lightly screw in a hose fitting of the appropriate size. If your sync kit does not come with extra fittings for this purpose, you will have to purchase one separately.

2. Push on a vacuum line to each fitting and attach the other end of the hose to the sync tool. It is advisable to attach the lines to the tool in the same order that they are on the carbs. It will make it easier to see which carb needs adjustment.

3. Start the engine and let it run until it is at operating temperature. Adjust the idle to factory settings: the Ninja ZX7R should be set to 1100 RPM plus or minus 50 RPM. The idle adjustment thumbscrew is located between the fairing and the engine on the left side of the engine, towards the rear of the engine, and is easily adjusted by hand.

4. There are three carb adjustment screws: one between carb #1 and carb #2 that syncs these two together, and one between carb #3 and carb #4 that syncs these two together. These two screws are both on the top. The third adjustment screw is underneath the carbs, between carb #2 and carb #3, and is the final adjustment to match all 4 carbs together.

5. Read the amount of vacuum on each carburetor on the synchronization tool. If the needle on a mechanical gauge is very jumpy, there should be an adjustment on the gauge assembly to limit the amount of air that can enter the gauge. Adjusting this will limit the jumpiness of the needle and make the readings much more accurate. The readings should all be within 2" of each other. For instance, they might range between 21 and 23 or between 24 and 26. If they are not, the carbs must be adjusted.

Adjusting Carbs

1. To adjust the carbs, gently turn the first adjusting screw (between carbs #1 and #2) and watch the gauges for carbs #1 and #2. Be aware that simply pressing on the screw will affect the reading. You will have to turn the screw slightly and remove the screwdriver each time before reading the gauge.

Secondly, adjust the far right screw located on top between carbs #3 and #4 while watching those two gauges. The final step is to adjust the center screw to match the left-hand pair of carbs to the right-hand pair of carburetors. It may well take some experimentation to get all four matched up.

2. Remove the syncro tool and reassemble the bike in the reverse order it was taken apart.

You're finished! Synchronizing the carburetors should not take more than an hour or two. It should take even less after you've done it a couple of times and have learned how to use the adjustment screws to the best advantage. If you are doing your own maintenance, another article that might be of interest is one on how to change the brake pads on a Ninja ZX7R as well. If a brake job is in your future you might give it a look.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: No.1 carb doesn't seem to alter when adjusting screw. What am I doing wrong?

Answer: No.1 is not adjustable. Adjust #2 to match #1, adjust #3 to match #4 and then adjust the 1-2 combination to the 3-4 combination.

© 2011 Dan Harmon