DIY! How to Tune the Carburetor of Vintage Motorcycles

There are many tools that have the potential to be used as "special tools" in the world.

For example, disposable "cotton swab" can be useful during maintenance. I tried "throttle valve tuning" of carburetors with a self throttle cable, which is common for vintage models, especially for 2-stroke engines!

KAWASAKI had released the 500SS Mach III, an air-cooled 2-stroke triple-cylinder engine in 1969 prior to the high-speed period in the 1970s. The model type H1 of 500cc had been released from 1969 to 1976.

In the inline-3 engine, not only tuning on idling but also "cable adjustment" for tuning the vertical movement of the throttle valve is important. It is also the same for 2-cylinder.

1. The broken "metal hack-saw" is useful!

In the case of a multi-cylinder model 250cc or more, cut the broken or dulled blade of the metal hack-saw into the same length, and prepare it the same amount as cylinders. This is the completion of the special tool for the play adjustment of the cable wire.

In case that the throttle valve of carburetors is small like a 2-stroke 2-cylinder engine of 125cc or less, it is recommended to prepare a "thick wire or annealing wire" cut by 70 to 80 mm. If it is a model of 250cc or more, since the bore of the carburetor is large, it is better to use the cut-off blade of metal hack-saw that is easy to check during adjustment. The idling adjustment of a multi-engine starts from the uniformizing of the engine rev of each cylinder by adjusting the air screw, pilot screw, and the throttle stop screw.

According to a motorcycle manufacturer's service manual, it is indicated that the height of the throttle valve = "engine rev" makes match by putting on your palm to around the exhaust port and comparing the exhaust pressure. It is mostly possible to adjust idling by such verification methods, but there is a more accurate method, which I will explain later.

After the idling adjustment, try to operate the throttle to check the engine response. The most important of the throttle feel is the "cable tuning" for each carburetor. The wire cable that controls the throttle valve does not move at the same time, and if only one cable moves quickly or is not balanced, the engine cannot work comfortably.

It is also not uncommon that the throttle response delay with 1 tempo or 2 tempos. By properly tuning the throttle cable "when roll-up", the engine response will be surprisingly good. In such a case, this "metal hack-saw" adjustment will be useful.

In addition, The throttle stop screw of the old 2-stroke engine often has an adjustment section on the throttle valve cap. Since the operability of this throttle valve is greatly changed in the condition of a rod (a rod-shaped like an inverted nail) that is a substitute for the stopper, be sure to check the "bend" of the rod when disassembled.

2. Hook the blade of metal hack-saw into the throttle valve.

Adjust the play of the wire cable by hooking the "blade of metal hack-saw" to the throttle valve of each carburetor and slowly operating the throttle. As the cable play matches, 3 blades of metal hack-saw will be shaken at the same time. Moving quickly is considered playing less.

3. There is the idle screw on the top cap.

In most of the models that have appeared after the 1980s, the throttle stop screw is attached to the side of the main body, but in the case of the old carburetor, the adjustment bolt is attached to the top cap.

The engine speed will be high if turning the bolt in the loosening direction to the left, the valve will be lowered and the rev will be decreased when tightened to the right.

4. Throttle valve "adjustment rod" penetrating up and down.

A rod that is like an inverted nail has been inserted in the vertical direction of the throttle valve. The idling rotation is adjusted by the throttle valve pressed against the tip of this rod (a nail head). By tightening the adjustment screw from above, the whole rod is gone down, and the throttle valve is pushed in, and the idling rev will be down.

5. Do not use a bent rod!

The rod that adjusts the closed position of the throttle valve is always wanted to keep straight for good condition. If the rod is dirty or bent, it leads directly to the operability degradation of the throttle valve.

Please be careful because the rod is often bent when the desorption of the throttle cable. It is a typical example that the rev decreasing gets worse.

6. Malfunction after warming up? The cause is this!

If there is an insufficient play of the starter cable (the starter function that replaces the choke), the starter valve will open and the gasoline will be richer at all times. Even engine condition is good during warming up, you should doubt the starter if the engine will be bad condition after warming up. The play of cable needs around 3mm.

7. Knacks to install the starter valve

If the sealed rubber sheet of the starter valve tip is deformed to round, replace it with a new valve. When installing the valve, it is better to press the spring end face to the valve side hole instead of contracting the spring by a fingertip and set the valve by twisting while pushing the cable end into the side hole.

 
▼ Convenient for carburetor setting ▼
Click here for KEYSTER Fuel Tuning Kit