Carburettor synchronization

I'm a happy owner of Yamaha XJR1300 which I still consider to be the most beautiful bike. Yeah sure it is not a beast one would take to challenge R1 or CBR1000 on the track days but it certainly is a machine that will take you hundreds of miles in the seat letting you to enjoy the ride without any back or wrist pain and still agile enough to put your knee down if you like.

Some time ago I found subtle vibrations noticeable at around 4000rpm clearly a sign of dis-balanced power delivered by the four cylinder engine. Well, it was a time to synchronize the carburettors. A few year back I went to a certified Yamaha dealer in London to do the synchronization but they said it was not necessary but after a few rounds of convincing talks they did it with a good result. The only thing was I had to wait a week or so for the appointment and then a day for the job to be done. Finally they charged me about £80 which was a bit expensive adventure for what it was. This time I decided to do it myself since I already had quite a bit of experience with engines and carburettors tuning. The only thing I didn't have was the carburettor synchronizer so I decided to make one. Here is the story.

Synchronize carburettors? Why?

For multi cylinder combustion engine, in order to run smoothly, all cylinders need to deliver the same amount of power regardless of the rpms or load. If one produces more or less power than the rest the engine will vibrate and won't run well. This will be noticeable mainly in the idle where the engine will keep slowing down and speeding up, just about staying alive. Some workshops solve this by increasing the idle rpms which is just like giving more painkillers to the sick person instead of treating him. There are many things that influence the power delivered by each cylinder such as good spark plug, ignition timing, valves timing and clearance, correct exhaust back pressure, good piston rings and, of course, the carburettors. If one of these is not the same (good or bad) for each cylinder they won't work together and the engine won't like you and make you suffer by vibrating your body in the seat. For now I'm gonna focus only on the carburettors.

Single vs. multi carburettors engines

There are many two, four and more cylinders engines with only one carburettor or a fuel injection with a single throttle valve. This works well and there is nothing to synchronize (considering we are not going down the route of the fuel injectors synchronization which is a bit different story).

Single throttle body (single point fuel injection)

Four carburettors body

So why to trouble with multiple carbs? Well there are basically two reasons. Usually the main one is to provide a larger volume airflow for each cylinder with as laminar airflow as possible to improve the engine performance and efficiency. If the intake manifold has bends the mixture will be hitting the walls, creating turbulences and the fuel gets washed down as it travels to the cylinder. Also the cylinders closer to the central carburettor get richer mixture since the path through the intake manifold is shorter and without any bends. As you can see in the image above some manufactures make all intake pipes the same length which in result creates more complex path for the mixture thus increases the turbulences which is not good. If we use one carburettor or throttle body for each cylinder we make sure the fuel gets diffused evenly without splashing on the walls reaching the cylinder along the shortest possible path.  We can also easily increase diameter of the intake pipe thus improving the engine performance.

The second reason is to compact the intake system into as small area as possible while preserving a reasonable performance which is usually the case of motorcycle engines. There is not enough space on the motorbike to run an intake manifold that would with reasonable efficiency distribute the fuel mixture to all cylinders. Maybe two but almost never for more.

How to measure the sync.

To synchronize the carburettors or multiple throttle valves in the case of the fuel injection system we need to measure power delivered by each cylinder independently and then have a way of adjusting this power per cylinder.

We can find the power of each cylinder by measuring the vacuum in its intake manifold. On the first sight this may sound a bit puzzling but if you think about it you realize it's actually a common sense. To be precise we won't find the power per say in for example kW or Hp but rather a pressure in kPa (or psi or mHg) that relates to the power produced by the cylinder. This pressures is lower then the atmospheric one and is created by the piston sucking air into the cylinder during the induction cycle. On the other hand the throttle valve is blocking the intake manifold thus making it harder for the piston to suck more air which creates vacuum in the manifold. Other contributing factors are:

  • The exhaust back pressure.
    The residual pressure of the exhaust gases that stay in the combustion chamber after the exhaust cycle decreases the vacuum the piston can produce during the induction cycle, consequently it also decreases the vacuum (or increases pressure) in the intake manifold.
  • Energy produced by the mixture burning.
    If the mixture is burning well and quickly the residual pressure of the exhaust gasses will be lower than if it's burning slowly which can curry on even during the exhaust cycle. Again different burning precess will be manifested by a difference in the intake vacuum.
  • Ignition and valve timing, quality of the piston rings, how well the valves seals etc. also influences the vacuum in the intake.

Basically everything that has some impact on the residual pressure of the exhaust in the combustion chamber will manifest itself as a change of the vacuum in the intake manifold.

In our case, to sync. the carbs, we don't need to really know even the exact pressure in the intakes, what we need is just to compare one cylinder with another and make sure there is the same pressure in all intakes.

The tools

There is lot to chose from. Analog vacuum gauges, mercury manometers called carb sticks or digital manometers. To save time and money I bought a cheap set of four analog gauges on the eBay for about £13. They proved to be more expensive than I hoped in the time I spent trying to set them up and use them. If you don't want to end up with extra grey hair go for other tool. The problem is that these gauges don't have any integration chamber and transfer the pressure changes straight to the needle movement. As the engine is running the needs is oscillating all over the dial making almost impossible to get any reasonable reading. There is a small dumping screw on each pipe for each gauge that reduces the diameter of the pipe virtually making the manometer itself an integrator. The problem is that it doesn't work well and the needle is either absolutely still or dreadfully lagging or shaking all over the dial. Of course it depends on the type of the gauges so I'm convinced if you get more expensive ones you will be ok. In any case don't buy the ones in the picture below.

These analog vacuum gauges are very hard to use

If you decide to buy your own manometer I would recommend the digital Tecmate Carbmate for $125.99 

This electronic manometer has only two inputs which is absolutely fine and can be used to sync even, three, four or more cylinders engines as I will describe in a minute.

Other good choice is the Morgan Carbtune for £57. This is quite popular and reasonably cheap and accurate tool. It works straight for four cylinders and yes you can use it for two cylinder engine as well, just connect two inputs only.

I would probably avoid the nowadays obsolete mercury manometers or other with liquid inside that can get sucked into your engine if you are not carefully enough. That happens quite easily when you quickly shut the throttle which creates high vacuum in the intakes and if they are a bit out off sync one of them will drink up all mercury from you manometer. This also happens if you forget to connect one manometer input. In this case the mercury, or other liquid, disappears as soon as you touch the starter button. Luckily it never happened to me 🙂

Since I like to make things I decided to build my own, simple, differential carburettor synchronizer. If you have a bit of spare time and some basic workshop I would suggest to go this way, you will end up with as easy to use and accurate tool as the commercial ones.

The differential manometer works by comparing pressure above the surface of some liquid in two tanks connected at the bottom. This is not by any means a new idea and you can find many commercial or home made devices of this type all over the internet. In my case I used four old 60ml syringes where two pairs are connected together by the opposite ends as you can see in the image below.

Testing the manometer by artificially reducing pressure in the left test tube

These two syringe-tubes are then joined together at the bottom and mounted on a solid piece of wood for support and filled with a liquid. It's important to use volume not larger then is about 3/4 of one of the tube. The reason behind that is that if during the tuning process one of the cylinders moves all liquid to one of the tubes it won't reach the input pipe at the top and get into the engine. For higher contrast I used a blue non toxic liquid from an old cooler box. Apart from the high contrast colour this liquid also has a high viscosity which makes it nicely stable while running the engine. There are more pictures from the building process of this tool at the end of this article.

Lets do it!

The synchronization process is very easy. Basically the steps are:

1. Warm up your bike and prepare your tools.

Going for a 10 min ride should be enough. Apart from the manometer you will most likely need some auxiliary petrol tank. I'm using a modified plastic bottle with a silicone house.

Auxiliary fuel tank

Another think that comes in handy is a fan to keep your engine cool while the bike is not moving.

Other tools that you will need depends on the type of your bike. For the XJR1300 you just need about 30cm long cross slot screw driver and that's it.

2. Connect the manometer to the intake manifolds.

Again this varies bike from bike. On the Yamaha XJR1300 there are already small brass tubes    mounted in the inlet rubber boots ready for this purpose.

3. Find the throttle flap linkage.

This is also different on each bike but it's quite easy to find. On two cylinder engines like the Kawasaki 250R the throttle valves are connected by single screw that changes relative position of the valves. This is the only screw you will use to do the synchronization.

Kawasaki 250R throttle body

For more cylinder engines the carburettors (or throttle valves of fuel injection systems) are usually synchronized in pairs first and then the two pairs of cylinders together.

[the above picture is a courtesy of Ben check out his blog]

Pick any pair you like and while running the engine in the idle make sure the vacuum in the intakes is the same. With the differential manometer set the ratio of the flaps as such the liquid is not moving form one tank to another. It doesn't matter what level is in each tank they just must stay the same. Rev the engine to 2-3k rpm a few times and make sure the carbs are still in balance.

If you have a two cylinders machine you finished. If you have four cylinders synchronize the second pair and then the two pairs together exactly the same way as you did before. If your manometer has only two inputs you will need to stop the engine and reconnect the manometer to the second pair. To sync the cylinder pair connect one input of the manometer to one cylinder in the first pair and another one in the second pair. It doesn't matter which cylinders in the pairs you choose.

And that's it, job done. As you are balancing the carburettors you will instantly notice the engine is running more easily and has more harmonic sound. It will also start much better and will keep just humming on rock stable rpms in the idle. You should also notice no vibrations while riding at any revs.

And now it's time to go for a test flight 🙂 Happy and safe riding....


Appendix - making the differential manometer

Here are some pictures from building the device. They are self explanatory so no need to comment to much.

The syringes are glued together by an ordinary gasket seal and fastened by brackets made of copper-clad laminate (PCB) Of course any reasonably strong material is ok for the purpose.

An now just pour some nice colour liquid in and the manometer is alive 🙂

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