The Key to Carburetor Adjustment for 2-stroke Twin is Tuning

Whether injection or carbureted, one of the requirements for running a two-cylinder or larger engine is to get the intake airflow just right, and while vacuum gauges are commonly used for four-stroke engines, two-stroke engines are more dependent on the operator's senses. This is where a synchro tester comes in handy. The mechanism is primitive, but it is reliable because you can see the intake air.

The negative pressure is strongest near idle. So delicate adjustment is necessary.

This vacuum tester is commonly used to tune 4-stroke, 4-cylinder carburetors. by adjusting the butterfly linkage adjustment screw while watching the changes in the 4 gauges, the variation in rotation at the start of throttle opening is reduced and the carburetor blows smoothly. This is a must-do when the carburetor is serviced.

Not only Suzuki RG250 Gamma, but also 2 stroke 2 cylinder carburetor opens and closes the piston valve by each independent throttle cable, so the synchronization is adjusted by each throttle stop screw. It is very difficult to adjust the throttle stop screw because it depends on individual feeling.

Some service manuals from the 1960's say to hold your hand over the exit of the exhaust system and if the pressure of the exhaust gas feels the same on your palm, you can judge that the exhaust gas is in tune, but it is difficult to feel the pressure difference if you are not used to it. But it is difficult to feel the pressure difference if you are not used to it. At first, you can feel the pressure difference by deliberately shifting your hand to a larger position, and then you can understand it gradually by decreasing the difference.

Even the most powerful engine starts out with the throttle fully closed; a 1000cc, 150-plus horsepower sportbike on the street should have a throttle opening of 1/If you open 4, you will get more than enough speed.

Whether it is a four-stroke or two-stroke engine, the negative inlet pressure generated by the engine is greater in the small throttle opening range. This is because the engine itself always wants to suck in the maximum amount of air mixture, which is blocked by the throttle valve. Therefore, in engines with two or more cylinders, the engine feeling is greatly affected by whether or not the throttle opening is in sync with the throttle valve opening when the throttle valve is opened slightly from fully closed, i.e. when starting from a stoplight.

In the case of four-stroke four-cylinder carburetors, there are exceptions such as the Honda CB750 Four K0, but for the most part, the butterfly is opened and closed with a single throttle cable by a linkage mechanism that links the four carburetors together. Therefore, a vacuum gauge that can measure the intake negative pressure is connected to the intake manifold to adjust the throttle valve opening.

On the other hand, most two-stroke models, whether two-cylinder or three-cylinder, are fitted with independent carburetors, with each piston valve opened and closed by an independent cable (although there are some examples of triple carburetors synchronized in the later Suzuki GT380 and 750 series).

The majority of 4-stroke 4-stroke carburetors use a single throttle stop screw to move all 4 throttle valves, so once you get them in sync, the intake airflow will be the same. However, in the case of 2-stroke multi-carburetors, where multiple carburetors is opened and closed by different throttle cables, it is difficult to synchronize them.

Using a parallel twin as an example, each of the two carburetors has its own throttle stop screw and can be adjusted independently. If the specified idle speed is 1300rpm, you may manage to idle with one carburetor even if one of them is fully closed in the extreme. If you open the throttle to a certain extent, the piston on the fully closed side will also open, so you may be able to get by once you start running.

However, as mentioned at the beginning of this article, it is important to adjust the carburetor near fully closed, where the engine's negative pressure is greater. By inhaling the same amount of mixture when the throttle valve is slightly open, each combustion chamber will do the same amount of work to achieve a pleasant running start. The basic maintenance procedure that has been followed since the 1960's is to adjust the throttle stop screw by placing the palm of your hand near the muffler exit and feeling the difference in exhaust gas pressure.

It is true that the human sensor is sensitive, and if the pressure of the left and right mufflers are completely different, we can tell the difference. However, when the difference in exhaust pressure becomes small, it becomes a vague area that "it seems to be right, but it doesn't seem to be right". However, some riders who use a vacuum gauge to adjust the 4-stroke tuning may want to know the difference not by hand but by numbers and figures.

Synchro tester that can measure the amount of air the engine draws in

The majority of synchro testers that you will find on the internet are of this design. The price goes up and down depending on whether or not you need an attachment at the end, but you can make your own attachment with materials from Home Depot. Even if the pipe is long and the intake resistance is large, it is not a problem because you are comparing two carburetors relative to each other.

The Yamaha AT90 was a two-stroke, 90cc two-cylinder motorcycle built in the 1960s. The only two-cylinder in the 90cc class, it was called the Little Twin and was used on the business model AT90 and the sport model HS1. the AT90's intake pipe formed a Y leading to a single air cleaner.

The diameter of the carburetor is small so I put a smaller adapter over it to prevent secondary air from being sucked in and start the engine. The red needle is at position 2 on the left carburetor, so measure it on the right carburetor as well. If it is uneven, use the throttle stop screw to adjust it.

Synchro testers are useful in such cases. The tester, which looks like a drum or a snail, is a measurement device that has been used in the tuning industry since the 1970s for adjusting carburetors of automobiles such as Weber and Solex.

The principle is very simple, you press the tester against the carburetor and as the air passes through the cylindrical body with slits etched in it, the needle indicates the amount. You may be worried that it is too simple to be reliable, but the concept of flow meter to measure the amount of intake air is a proven method that has been put to practical use as one of the methods to measure the amount of air in a fuel injection system, as the Muffsloe method.

When used in synchronizing a 2-stroke carburetor, you are not measuring the volume of air intake, but rather comparing the relative air volumes. Here we will show you how to use the synchro tester on two motorcycles, a 1980s Suzuki RG250 Gamma from the parallel two-cylinder era and a 1960s Yamaha AT90, but the basic procedure is the same for both.

When using the synchro tester, in order to measure the amount of air that each carburetor takes in, the pipe that connects the air cleaner to the carburetor is removed and the tester is pressed directly against the inlet of the carburetor. You may worry about the increase in intake resistance caused by this, but the intake slit of the synchro tester has a sufficient area, and even if resistance occurs, it will be the same resistance by hitting each of the two carburetors, so it is possible to compare relative air volumes.

However, if secondary air is sucked through the gap between the adapter at the tip of the tester and the funnel part of the carburetor, the reliability of the tester's indicated value will be lost, so it is important to keep the adapter and carburetor in close contact. Since this tester is made for motorcycle carburetors with a certain large diameter, it is not suitable for moped carburetors with a diameter of 20mm or less, but it can be used by procuring appropriate materials such as doorstop rubber at a home center.

When the tester is pressed against the engine and the engine is started, the needle moves according to the amount of intake air, so set it alternately on the left and right carburetors to get the current situation, and adjust the throttle or valve opening with the throttle stop screw. If the throttle stop screw is tightened, the valve will open, the engine speed will increase and the amount of intake air will increase. Conversely, if it is loosened, the valve will close, the RPM will decrease and the air volume will decrease. This happens as a matter of course whether you use a tester or not, but by visualizing it with a tester, you will be able to understand it more concretely.

If the two carburetors are out of sync, the amount of mixture flowing into the two cylinders will also be disproportionate.

When the throttle is opened, the explosive power of the two cylinders does not match each other and the engine becomes uneven. If the throttle opening becomes large to some extent and the amount of the mixture flowing into the engine becomes close, the difference in the explosive power becomes difficult to notice, but as I wrote at the beginning of this article, the difference in the synchronization near idling where the intake negative pressure is the largest will affect the engine feeling.

A large amount of starting torque is required when shifting into low gear from zero speed and connecting with a half clutch, and if the RPMs fluctuate here, it can be a concern when starting at traffic lights. When the carburetor is in sync with the throttle, whether it's a 2-stroke or a 4-stroke, there is no sluggishness or sticking at low throttle, and the motorcycle starts smoothly. This is why some users feel that it is not as exciting as it could be.

However, in terms of the relationship between the engine and the carburetor, it makes more sense for multiple carburetors to be able to breathe the same amount of air at the same time, and from this point of view, a synchro tester that can measure air volume is a useful device.

The RG250 Gamma's carburetor has a larger diameter than the AT90, making it easier to set up the syncro tester. If the distance between the carburetor and the air cleaner box is close, use an elbow-type adapter and swing the tester 90 degrees to make it easier to measure.

Adjust the throttle stop screw on the side of the carburetor body while watching the numbers on the synchro tester. It is important to adjust the idle speed and tuning first.

Cable adjuster to align throttle valve movement start

The cable adjuster on the top cap of the carburetor synchronizes the two throttle valves. Insert a wire or drill blade between the throttle valve and the venturi and adjust if they move in the same way when the throttle is opened or not. Depending on the model, the amount of cable play may be indicated by a numerical value, so check the service manual. If the play is too tight, the throttle valve may pull up even when the throttle is fully closed, so a moderate amount of play is necessary.

The top cap of the AT90 carburetor is a fixed nut type, and the cable adjuster (right) and throttle stop screw (left) are lined up on the top of the cap. This type of throttle stop screw lowers the RPM when tightened and increases it when loosened. The throttle stop screw is used to synchronize the throttle speed and the cable adjuster is used to adjust the play at the beginning of a movement.

When you realize that the throttle stop screw of a 2-stroke multi-carburetor not only determines the idle speed but also functions to determine the synchronization, you may notice that you cannot adjust the idle as easily as you can with a 4-stroke 4-cylinder where the throttle valve is synchronized by a link. This is because even if the idle speed is set in sync if either throttle stop screw is tampered with, the idle speed, as well as the speed, will be out of sync.

That said, the idle speed will go up and down a bit depending on the season and how warm the engine is, so if you do adjust the throttle stop screw, make sure the angle you turn it at is the same to minimize any misalignment. At the very least, be careful not to adjust either one of them to raise or lower the engine RPM.

Once you have determined the position of the throttle valve when the throttle is fully closed, you can adjust the cable adjuster on the throttle valve cover to synchronize the movement of the valve when the throttle is opened. This involves inserting a drill blade or hacksaw blade between the throttle valve and the venturi to see if they move in the same way when the throttle is opened.

Since the origin of the throttle valve is adjusted by the throttle stop screw, loosen the cable adjuster of the slower moving cable to pull the cable and check it again. Repeat this process to adjust the movement of the two cables, but be careful not to loosen the adjuster too much = pull the cable too much and pull the throttle valve itself up from the stop screw. It is also important to loosen the cable adjuster beforehand to prevent the throttle cable from pulling up on the throttle valve before performing the synchronization adjustment with the syncro tester and throttle stop screw.

Owners of two-stroke motorcycles who are fine with a larger throttle opening, but who are concerned about clutch operation because the engine falls apart when starting in low gear, may want to review carburetor tuning and throttle valve synchronization.