Be aware that fuel /mixture requirements change with the load placed on the engine. While you'll make adjustments in your garage, testing the machine for correct jetting (besides idle) needs a dyno or track and stopwatch.
Perhaps this will help with better understanding.
Your carbs have three fuel metering devices as well as several air metering devices. The fuel metering devices supply fuel all the time while the engine is running. But each dominate the equation for their intended RPM range.
You have a main jet. This is primarily selected to provide the maximum required fuel at WOT (Wide Open Throttle) when the slide has fully opened the carburetor throat.
When the slide is partially closed, not only does it restrict the air going through the carb throat, but the attached tapered needle (Jet needle) then inserts into the needle jet. The needle jet also has what is called an emulsion tube. The emulsion tube has several holes in it where air is premixed into the fuel before being passed to the carb throat. The midrange mixture ratio is thereby primarily determined by the taper profile of the slide needle, the needle jet orifice, the emulsion tube hole sizes, and their location in the emulsion tube where it is determined how much air is premixed with the fuel for the various slide needle positions. Finally, we come to the slow jet which is selected to provide the correct fuel mixture for idle speeds. This jet also has a small emulsion tube on it. The air that goes to it can be adjusted with the pilot screws found on the side of the carb body. While the slow jet system does provide fuel over the whole RPM range, it’s contribution compared with the midrange or main fuel metering systems is very small. Conversely, there is leakage from the main and midrange systems that can have an effect on the slow jet metering.
So, how do you select the correct jetting? Well, in the ideal world, you would put the machine on a dynamometer that will load the engine at whatever speed you wish to examine and provide you with a measurement of power output. Then you would use instrumentation to measure intake and exhaust gas temps, and the exhaust gas composition to determine how thoroughly the fuel is being converted to power. Too many Hydrocarbons would tell you to lean the mixture. And, perhaps, detonation sensors attached to the cylinder head would warn you if it was way too lean. You’d lean down until you got max. power.
For your CB, you’d start with Main jet selection at WOT. Then repeat the tests at midrange rpms to find the correct needle taper profile and emulsion tube hole sizes and locations. Finally, measure idle exhaust gasses for low hydrocarbons with the smallest size slow jet for optimum and the accompanying pilot screw setting. This would have to be balanced with off idle response. The carbs are set toward the rich side to acheive this.
What? You can’t find one of these dynamometers at the local hardware store? Well, 30-40 years ago these kind of adjustments were made with the seat-of-the-pants-and-stopwatch-dyno. Coupled with the ability to read your spark plugs to determine what the heck was going on in the combustion chambers of the engine after that last change you made. This approach can be cheaper than renting Dyno and instrumentation time. But, it might be more costly in terms of personal time and effort. However, depending on your talent, patience, and perhaps stubornness, the results can be just as satisfying.
Hope this helps,
