Begin with knowing what you currently have. A 76 CB550 could be a F model or a K model. The carbs could be set up as 022A, 087A, or 069A. The set up determines what internal parts they have and how they were adjusted by the factory.
ALL the set ups sized the fuel metering devices inside to flow properly with the barometric pressure drop in the carb throats provided by the stock air filter arrangement. There is no Pod filter offering that duplicates this pressure drop. And, there isn't even any consistency between brands of pod filters. Therefore, tuning to changes are done on an individual case by case basis.
In the FAQ you will find a chart of the known adjustment and internal parts changes among the different carbs available to the CB500/550.
This is a starting point. The next step is to determine the effects of the changes you've made to the air induction and exhaust restriction has had on the combustion needs of the motor. This is done either with a Dynomometer and instrumentation. Or, with a test track of some sort and frequent "readings" of the combustion spark plug deposits. The deposits tell how the engine is running with the last changes made to the carb adjustments. It is a reiterative process, test run, read, adjust, and repeat until the plugs yeild the correct color with performance, and throttle response either meeting the driver's goals or he tires of the process and accepts the way it currently runs as good enough.
There are three aspects of carb fuel metering to be addressed in response to engine "breathing" changes. They are interactive to some extent. But, dominate at certain throttle position settings.
1) The idle, slow or pilot circuit. Active from idle to 1/8 throttle. This is a separate parallel circuit from the main and throttle valve fuel delivery circuit (discussed later). It is comprised of the idle, slow or pilot jet orifice size and the pilot or Idle air bleed adjustment screw. (alternately an Idle Mixture Screw in some carbs.)
Depending on the individual carb, the adjustment range may be limited, requiring changes to the idle, slow or pilot jet orifice size to achieve proper behavior. The extra tricky bit with the 550 carbs is that they have no enrichment pump to compensate for the rapid loss of vacuum when the slides are opened quickly. Therefore, these carbs are adjusted sufficiently rich to achieve adequate throttle response with moderate changes in throttle position.
2) The throttle valve controls mixture from about 1/8 to 3/4 throttle positions. It is comprised of the slide needle, and the slide needle jet orifice. The taper, needle width, and depth position in the jet orifice determine the fuel delivery at the various throttle positions.
3) The Main jet determines the maximum required fuel draw required at 3/4 to Wide Open Throttle (WOT). The slide's throtte valve is withdrawn from the throtle valve orifice to present less restriction to fuel flow than the main orifice.
Now it gets more complicated:
Float bowl fuel level also effects mixtures throughout the throttle range as the fuel has to be drawn up into the carb throat bore. The longer the lift, the more pressure is required to make the lift. Given the same pressure, less fuel is delivered for a longer lift.
Emulsion tubes:
Both the idle, slow, or pilot circuit and the Throttle valve/Main circuits have emulsion tubes. These tubes premix air into the fuel creating a froth that is more easily drawn into the carb throats and more thoroughly atomized after throat insertion. These emulsion tubes have cross drilled holes for air instertion. The number and size of these holes determine how much air is frothed into the fuel delivery mix and this also effects the ultimate fuel mixture delivered to the engine. The factory used different emulsion tubes in setting up the carbs for different machines.
There are also air jets that supply air for the emulsion tubes. These are usually drilled metering orifices in the carb bodies and is part of the set up configuration stamped on the carb body.
One final note about why many of the above metering orfices have to be readjusted.
The partial vacuum in the carb throats is a barometric pressure change in the carb throats relative to the surrounding atmosphere. The depth of the pressure change determine the rate of flow though metering orfices, as the surrounding atmosphere pushes the fuel into the carb throats as it tries to equalize the pressure difference. Nature abhors a vacuum and will rush to fill it, using everything in it's path to do so. In this case, fuel is part of what is in the path.
The intake stroke of the engine creates this partial vacuum. Resistance to the inward flow deepens the partial vacuum. The intake runner, carburetor venturi, slide, and air filter media all present resistance to the air flow and thus the deepening of the vacuum in the carb throat. In this way a filter that presents less restriction to airflow does not increase demand for fuel from the engine. However, it makes the existing fuel metering orifices deliver less fuel. Increasing the orifice size is a compensatory measure.
Honda engineers determined all the settings in the carb to work properly with the engine's breathing charateristics, exhaust, and intake restriction. Change these and you must find new settings for the changes you've made. Changing air filters is not like changing the color of the bike. If you don't enjoy the engineering set and adjust process, get the stock air box arangement, set to book values, and enjoy the ride. Otherwise, be prepared to tinker for a good while before focusing on ride enjoyment.
Cheers,
Unless I hear objections, I may make a variation of this post available for a FAQ submission. The subject certainly comes up often enough...
