How do I synchronize the carburetors?

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Carburetor sync is matching the butterfly valves in the two carburetors so they both open the same amount. Your bike will not run at its optimum level if one cylinder is running harder than the other.

The easiest way to understand how the adjustment works is to look at your carb linkage. There's a screw adjustment in between the two carbs that adjusts one butterfly in relation to the other.

Carb adj screw.jpg

Carb sync is adjusted by measuring vacuum through each carburetor and setting the butterflies so vacuum matches. There are fancy gadgets to do this, but there are also a couple quick and easy ways to do it at home. Keep reading.

Hints & Tips

  • Carburetor synchronization is done with the bike warmed up.
  • “You shouldn't need to, but...” Doing this procedure shouldn't take much time at all, once you know how to do it. The first couple times you try it, however, you may find yourself running out of gas in the carburetors before you have a chance to properly adjust everything. Should this happen, you can use a funnel or a turkey baster (remove the bulb from the end after it's in place) that will fit inside the fuel line, and add more fuel that way. You can use tape or wire to hang it in a good position. This is greatly preferable to reattaching your fuel tank.
Under normal conditions, you should get several minutes' worth of idle from the gas in your carbs.
  • You don't want to let the bike idle a LONG time (while doing the job) and warm up above normal operating temps. It's a quick job on the 250 (only one screw), but get a 4 cylinder and you have to be quick with the adjustments.

Carb sync tool options

There are numerous tools which will help synchronize carburetors. These are all systems which measure the difference in pressure between two (or more) carburetors. All are functional and accurate. There is no advantage to buying the commercial models except that then you don't have to make it yourself.

Note that the only other tool required to synchronize the carburetors on the Ninja 250 is a #2 Phillips screwdriver, ideally with a longish shaft (6" or more). You'll also need the tools necessary to remove the fuel tank.

Of the commercial tools available, the most commonly used devices are:

The "carb sticks" have the advantage that they're designed for four-cylinder motors, which is quite handy when working with other bikes with 3 or 4 cylinders. However, for the Ninja 250 (or any other twin), they're overkill. The Twinmax's main advantage is its easily modified sensitivity, but it's also dependent upon battery power and could be damaged if dropped or used in severe environments (the carb sticks suffer from these flaws as well). The Twinmax can be used with a 4 cylinder bike, but it isn't as convenient and may be impossible to use if the manufacturer specifies different pressures for each cylinder. The vacuum gauges are useful, but if mistreated will go out of adjustment, possibly without you knowing it. They are also highly dependent upon being set up correctly when manufactured.

However, a system for a two-cylinder engine is quite easy and extremely cheap to build at home. One type is Ian J's simple water manometer. Others are described below. These systems are accurate, sensitive, and require no calibration, as well as being durable and inexpensive to build.


Go for a 30 minute or so ride to make sure your engine is nice and warmed up. Carb sync is usually done after a valve adjustment, but you don't have to do them together.

Remove your fuel tank. On the top side of each intake - in front of the carbs - is a vacuum nozzle.


Remove the black hose from each (plug the hoses to keep out dirt) and place one end of the tubing on each fitting.

The next bit is the actual adjustment. It is more or less the same, regardless of which of the tools (described in just a bit) you build.

Start up your engine for a moment. If your carbs are in sync you will have equilibrium: the water in the two bottles will be at the same level, or the oil in the oil-in-tube or the ATF in the manometer will not flow one way more than the other. If there's more vacuum from one carb or the other, the oil or water will flow that direction.

Turn the linkage adjustment screw (you'll need either a very long screwdriver or a stubby one) in between your carbs so the vacuum is equal, and you are all set. It only takes a slight amount of turning to affect the adjustment. Don't open the throttle (butterflies). This introduces other factors that affect vacuum. Assuming your vacuum is even between the carburetors at idle, you have adjusted it correctly.

Directions for adjustment: Turn one way, see the result. Turn the other way, see what changes. Adjust as necessary. When they're even, you're done.

The two-bottle method

Honestly, officer, that's what it is! It's not a drug apparatus, I swear!

Carb sync tool 1.jpg

Our thanks to Payne for the original writeup. This has become the preferred tool for this job in the club.

Payne: "The inspiration for this project came from a post a good while back, in which someone described this design using beer-making equipment. I always found the oil in the tube thing (below) to be a pain and not terribly accurate, and I certainly did not want mercury sticks because of the toxicity and possible engine damage if it gets sucked in. And I didn't want to spend a lot of money on a tool I'd rarely use”.


  • 2 empty Nantucket Nectar glass bottles (any similar GLASS bottle with a large neck should work, such as Snapple 16 oz. ones)
  • 2 rubber stoppers ($2.50 each at Lowe's or other hardware places)
  • 10 feet of 1/4" ID (3/8" OD) tubing ($2.00 at Lowe's; almost every hardware store has this)
  • Motor oil optional in place of water


  • Drill with 3/8" bit.
  • Pliers to pull hose through once it emerges.
  • Lubricant (such as cooking oil; extra-virgin olive oil optional) to help get the tubing through the stoppers.

Drill two holes in each stopper. The rubber is a little difficult to drill through; you may try smaller drill bits first, working up in size as needed. One short piece of tubing goes through each and will go down to the bottom of each bottle. A long piece goes though each just enough to go into the bottle. Put about 2" of water in each bottle, not so much that if all the water goes into one bottle it will make it to the upper tube. Some people have found clean motor oil easier to work with than H2O.

Carb sync tool 2.jpg Carb sync tool 3.jpg Carb sync tool 4.jpg

What happens is: if one side pulls harder, the pressure will decrease in the upper portion of that bottle, causing liquid from the other bottle to move through the transfer tube and raise the water level in that bottle. Since there can never be any liquid in the tubes connected to the carbs, there's no chance of the engine sucking in liquid. Plus, the movement is very easy to see and not jumpy, as can be the case with the oil in the tube trick. It's extremely easy to see in an instant which side is pulling harder and correct it.

Quoth Payne: “In my opinion, this is THE way to do this procedure. I think for the first time my carbs are truly synced. The way my bike runs backs that up; it idles and accelerates noticeably smoother”.


  • Baby bottles work well.
Baby bottles.jpg
  • So do various glass alcoholic beverage containers.
4um1s8w.jpg 53t1kz9.jpg
  • You can get an 1 1/8" Basin/Bath Stopper from Ace Hardware. Part# 40174. Fits Snapple 16 oz. bottles well.

Slack-tube manometer

One of the guys in the club who is a mechanic came up with this one. It's similar to the simple water manometer (above). Total cost was about $3.00. All you need is about six feet of clear vinyl tubing, a wooden yardstick (or similar), some nylon wire ties (zip ties), and automatic transmission fluid (ATF) for the fluid. Drill small holes near the edges of the yardstick to attach the tubing as shown.


Find the center of the tubing and make a loop at the bottom of the yard stick, securing the tubing up to the top. The open ends of the tubing will attach to the left and right carb. As usual, make sure the bike is warmed up before you start the adjustment process.


Once you start the engine, any difference in vacuum will cause the fluid to be pulled toward the cylinder with more vacuum. Simply turn the carb sync screw until the fluid is level on both sides of the tube as shown. If the carbs are really unbalanced, it is pretty easy to have some of the fluid sucked into the intake. That's another reason for using trans fluid. It won't really do anything to the engine; the oil will just burn off.


This tool works great and is cheap to build. You can use most any type of fluid you want, but an oil will work best. ATF was used here because the bright red color makes it easier to see. Our mechanic friend felt it was easier and cheaper to build than the bottle method, but both will get the job done.

Oil-in-tube method

This is considered old school by many, but it still works.

Get a piece of clear plastic tubing about 3' long. Place some thick liquid in it, like motor oil (something that won't hurt your engine if it gets sucked in). Remove the black hose from each vacuum nozzle (plug the hoses for the duration of the job) and place one end of the clear tube on each fitting.

Let the clear tube hang down so the oil gathers at the bottom of the loop.

Tubing setup.jpg

Start up your engine for a moment, and if your carbs are in sync the oil will not flow one way more than the other. If there's more vacuum from one carb or the other, the oil will flow that direction.

If you can't find rubber stoppers for the two-bottle method

Here's another solution:

Go to a place that sells home brewing supplies. They should have white rubber "gum" stoppers. These are quite soft. Therefore, you have to do them differently, so you don't just end up with some fairly mangled-looking rubber chunkage.

You need a little bit of 3/16" rigid brake line. This fits nicely inside the 1/4" clear tubing.

Two #6 1/2 stoppers fit in Welch's grape drink bottles. Put them in the freezer overnight. The next day, drill them with a 3/16" bit. Cut four 2" lengths of the 3/16" brake line. Put a small squirt of oil in each hole. Tap lightly with a hammer to drive the 4 tubes into the four holes of the two stoppers. Cut the vinyl tubing to length and stick it on the rigid tubing snugly.


Two #6 1/2 White Rubber "Gum" Stoppers

Gum rubber stoppers 1.jpg

Drilling holes into frozen stoppers with a 3/16" bit

Gum rubber stoppers 2.jpg

Rigid 3/16" brake line snugly inserted

Gum rubber stoppers 3.jpg

Add liquid and go.

Gum rubber stoppers 4.jpg

Bench sync method

By Ghostt

You can sync with the carbs in hand. Back the idle adjustment off to close that carb then match the second carb to it with the sync screw. You can then turn the idle adjustment up while using a thin feeler gauge (maybe .002") between the throttle plate and the throttle bore, adjust to just fit, and then check the second one and adjust to match. If you have good feel for this it will run fine once installed. 

It would do folks well to take a minute to review and understand what a sync tool does, how it works. And take another minute to look at the carb linkage and sync adjustments available to fix irregularities that might occur. It can help take some of the mystery out of this as well as stop unnecessary carb sync screwups. 

Meaning, that the carbs were right once, the butterflies were matched and it performed well, but then changed. Undecided And the problem somehow occurred in the butterfly direct shafts, or the sync screw turned or wore that pad that it rests on, or the carburetor pairing/ bridging brackets twisted in relation to each other, and enough so that the butterflies no longer operated in sync, such that performance noticeably suffered. And a fix will be accomplished by changing the butterfly position, the only thing the sync adjustment does. Hmmm. Really? Undecided 

Meanwhile, the tool measures vacuum at each individual runner. That's all it does, how it works. And vacuum is effected by a huge list of things, butterfly position being only one of them, and the only one addressed with that tool while turning those sync screws. The rest of that list includes, valve adjustment, jetting, float level, compression differences between cylinders caused by wear as well as factory CC differences in the head, intake runner, as well as flow by port variations, etc. Variations in cams, wear or factory tolerance. Then there's the potential big one, pilot jets and pilot screw mixture settings. All of those effect the vacuum and will be read by the sync tool, accurately identifyng the difference that exists, and with the only adjustment being used to "correct" the problem, move the butterfly relationship, one to the next. 

It's that clear understanding that has me of the opinion and long time practice to not touch the sync screws on a set of carbs that were right once and were never dismantled from the rack. And jet cleaning as well as float needle changing doesn't require that they be dismantled. And further, if I DID dismantle a set of carbs, I just matched the butterflies on the bench at assembly, using a feeler gauge, and then never touched them again, never gave them another thought. 

That method, properly executed, will have THAT portion of the complete equation, balanced flow to each cylinder in a multi-cylinder/ carb application due to butterfly position, satisfied entirely adequately. And in fact, it has worked for me every time I've done it on every bike (as well as individual runner/ butterfly V-8 intakes, all eight) that I've ever owned or any that I've fixed for others. And the times I fixed for others was usually after an attempt by others to sync the carbs, chasing a problem, or sometimes not even chasing a problem, but one they created wth a sync tool, chasing a problem from that list, that was never a butterfly position change problem. Those linkages are so direct and simple that they don't know how to screw themselves up. Not enough to care about. 

I'm sure this will start a lot of controversy so I won't argue it, just offering it for those who understand the whole picture that I outlined and might make good use of it. It is what I've done, on every engine I ever worked on, including blue printed racing engines as well as regular old, high mileage street stuff. I've had at least four, four cylinder bikes with over 100,000 miles that ran quite well, the whole time, and never had a carb sync performed. When they did finally get a total disassembly, this is how I set them up, on the bench. Over the years I've straightened out at least a half dozen messed up ones, probably more, and on the bench, after fixing the original problem which was fouled pilot jets. 

I only posted this because it seems like its coming up fairly often, especially with problems from a sync gone bad. One I would suggest was never a sync problem to begin with. A big or sudden change in performance is NEVER a sync problem. NEVER. Remember that. Cool If there is a problem, and you connect the gauges? YES, there will definitely be an imbalance indicated. But because that hole has a problem that is from the rest of the list. A bad plug, a plugged pilot jet, a screwed up float, a bad valve or setting, etc. Every one of them effecting the vacuum in the intake but NOT from a sudden move of a butterfly position, a sync adjustment.