Installing a big carburetor is one of the typical tuning menus for mopeds in the carburetor era.
It is exciting to increase engine power by enlarging the diameter of the OEM carburetor, which is so small that you cannot even pass your pinky through the venturi, but if you focus only on the carburetor itself, you will be caught flat-footed. Depending on the choice of a throttle cable, the engine may not start if you are not careful! If the throttle cable is not chosen correctly, the engine will not start.
- Inner cable problem that is hard to notice with piston valve carburetors.
- POINT
- When the engine does not start and the engine speed rises when the steering wheel is fully locked, suspect a lack of inners.
- POINT
Inner cable problem that is hard to notice with piston valve carburetors.
Once the engine is bored up and the big carburetor is installed, the carburetor setting is a prerequisite. The jets and needles installed in general-purpose carburetors are selected to be compatible with various engines and mufflers, but in order to get the perfect match, the jets must be matched based on the burnt condition of the plugs and other factors.
Failure to use the OEM throttle cable even though a big carburetor was installed. Even if the throttle stop screw is loosened all the way, the throttle valve does not close fully due to the insufficient length of the inner cable. This means that the engine cannot be started due to insufficient intake negative pressure.
Since the valve can be set in the carburetor even when it is this open, many users are unaware of the cause of the starting failure and are troubled by it.
Although it is a matter of consultation with engine performance, many motorcycles can realize an increase in performance by enlarging the carburetor bore size. Many motorcycles are surprised at how much better they run by simply oversizing the OEM carburetor, which is set up with maneuverability, safety, and fuel efficiency in mind! You will often be surprised at how much better the motorcycle operates.
Especially with small-displacement moped-class engines, even the slightest change can be felt easily. If the stock carburetor with a bore diameter of about φ16 mm is changed to φ20 mm in conjunction with engine tuning such as bore up, a dramatic change can be experienced.
This is true not only for carburetors but also for fuel injection, and big throttle bodies are sometimes available as aftermarket parts for popular custom models.
An essential part of installing a big carburetor that is both easy and effective is to check the throttle cable length.
Especially for piston valve carburetors, where the throttle valve is opened and closed directly by the cable, the relationship between the venturi diameter and inner cable length is severe.
In the case of a piston valve carburetor, the cable travel distance from fully closed to fully open is simply the same as the bore diameter. If the bore diameter is φ16mm, the piston stroke is 16mm, and the inner cable that pulls up the piston from the top also moves 16mm.
If this is changed to a carburetor with a bore diameter of φ20 mm, the piston stroke and inner cable travel will also increase to 20 mm. It is important to note here that the required cable length will be longer as the amount of inner cable travel increases. If the intake manifold and carburetor can be bolted on and the outer cable length and routing remain the same, it would seem that the throttle cable can be diverted, but this is not the case.
The basic installation of the throttle cable is to pass the inner cable through the top cap and return the spring of the carburetor, hook the tycoon at the end of the cable to the throttle valve, and then insert them as a set into the carburetor body.
If the cutaway at the bottom end is oriented correctly and does not interfere with the guide when the piston is inserted, the top cap will fit tightly into the body. Now you can set it on the intake manifold. Operate the grip to make sure the throttle valve opens and closes without getting stuck, and at the same time, visually look inside the venturi. You will be lucky if you notice any discomfort at this time. This is because it is possible that the throttle valve is not fully closed if the cable diversion that you were optimistic that would be okay has failed.
POINT
Point 1 – When a piston valve type carburetor is converted to a big carburetor, the throttle valve may not close completely due to insufficient throttle cable inner length
Point 2 – After replacing the carburetor and reattaching the cables, look inside the venturi with the throttle fully closed to check the position of the valves.
When the engine does not start and the engine speed rises when the steering wheel is fully locked, suspect a lack of inners.
When the inner cable is removed from the throttle sleeve and aligned with the throttle holder, the position of the tycoon is significantly displaced. You can see that the inner cable length must be extended because of the larger bore in the big carburetor.
Some aftermarket parts include a throttle sleeve that allows the tycoon to be set in multiple positions (right). This can only be used when there is enough inner cable length, so it does not make sense when upgrading the carburetor bore.
Correct valve position when the throttle is fully closed. By closing the venturi, the negative pressure on the engine side increases during startup and gasoline is sucked out of the pilot outlet. If the inner cable has just enough play in relation to the outer cable, simply locking the handlebars fully to the left or right and applying tension to the outer cable can pull the inner cable open, causing the valve to open and the engine speed to rise.
In diverting the throttle cable, it is important that the valve also opens fully when the throttle is fully opened, but it is more important that the valve close fully when the throttle is fully closed.
The throttle valve will not be fully closed if the inner cable length has to be lengthened by the big carburetor, but failing to do so will result in the throttle valve not being fully closed.
If a φ16mm bore inner cable that fits perfectly is assembled into a φ20mm carburetor, the valve will stop at 4mm open even when the throttle is fully closed.
This is about the same valve opening as twisting the throttle to a reasonable degree even while running, and the engine will not be able to start because it will be open too far when starting.
A carburetor with a butterfly-type choke would probably be able to start by forcibly closing the venturi, but when the choke is turned back on, the throttle valve is open too far to idle.
Do not be alarmed if the throttle cable is diverted and the throttle valve appears to be fully closed at first glance. If the cable is just barely balanced, the inner cable may stick out when the steering wheel is fully locked to the left or right while idling, causing the engine speed to increase on its own. The symptom may be corrected by adjusting the throttle housing or cable adjuster to make more room on the outer side, but the theory is to change the throttle cable when the bore diameter is enlarged.
Conversely, although there may not be many examples, what will happen to the cables if the carb is reduced in diameter? If the inner cable has extra length, the play between when the throttle is twisted and when the valve begins to open will increase.
In this case, the play can be reduced by adjusting the adjuster so that the outer cable extends, but if there is too much play in the inner cable, the tycoon at the end may come off the throttle valve, so it is important to use the appropriate length of cable as when installing a big carburetor. Therefore, it is important to use the appropriate cable length, just as when installing a big carburetor.
POINT
Point 1 – If the inner cable is short when converting to a big carburetor, the throttle valve will open, making starting difficult.
Point 2 – If the bore diameter of the piston valve carburetor is changed, check the required throttle cable length before replacing the carburetor.