Here's some info on how the carbs work, so you can see what happens when you try to use pod filters (or velocity stacks).
Refer to the picture(s) below, modified from my book.
The fuel mix depends on there being 3 distinct pressure zones in the carb, plus one intermixed zone that is made from 2 pressures in the airbox-to-carb throat area. In the accompanying pictures here, these pressure zones, relative to an imaginary 0 pressure, are:
ATMOSPHERE – about 14.7 PSI at sea level (BLUE in the pictures)
MIXED – about 9 PSI at half throttle, 12 PSI below ¼ throttle (BLUE+GREEN in the pictures)
AIRBOX – about 9 PSI at full throttle, 10 PSI at mid-throttle (GREEN in the pictures)
VACUUM – from less than 1 up to 3 PSI on the engine’s side of the slide (YELLOW in the pictures)
(NOTE: with apologies to those of you who are Blue-Green colorblind, sorry…)
Bernoulli’s principles are at work here: the faster air is moving, the lower is its pressure, and vice-versa. That’s why dead-still atmosphere air is the highest pressure involved here.
First thing to remember: the highest, ATMOSPHERE, pressure is fed to the float bowls via small vent hoses that draw their air pressure from a still, quiet spot on the bike, like behind the airbox, near the battery (some reach up under the seat). This is the force that pushes fuel up through the jets when the throat of the carb is at lower pressures.
Second thing to remember: the air filter must slightly restrict the air entering the airbox, to generate a slightly lower pressure for the venturi in the carb. This MUST happen so the higher pressure in the float bowl can push fuel up the jets and into the venturi. (This is why pod filters are trouble, more on that later).
Third thing to remember: the vacuum from the engine is PULSING at low engine speeds and smoother at higher speeds. This means the idle circuit mixes differently from the higher speed mainjet-needle system. They do NOT act the same.
Idle-to-1/4 throttle: the air-bleed screw adjusts the amount of air that is fed to the emulsifier chamber (those tiny holes) of the pilot jet, and corres[ondingly, how much air-fuel mix gets pushed up the jet. This air comes in through the bell area at the back of the carb, near the outside of the passageway where the air is most still (and therefore is at higher pressure than the moving air). When you close the air screw, it pushes LESS fuel up the pilot jet, and vice-versa. This pressure is lower than the float bowl pressure, so when a vacuum hits the top of the pilot jet hole, the high-pressure bowl pushes fuel up, the mid-pressure emulsifier air bubbles the fuel as it rises, and it FOAMS up into the throat of the carb, wetting everything with a mist. This mist gets sucked into the engine during the NEXT intake cycle, not the present one that supplied the vacuum. This is why there is a slight ‘lag’ when you blip the throttle: it is running one engine revolution behind your twistgrip.
Low Midrange - ¼ throttle to midrange, below the “wasp waist” zone in the carb body: by ¼ throttle, the air layer along the edges of the passageway are running slower than the air in the midst of the flow. The slow-moving layer is higher pressure than the moving air, so it matches the pressures on both ends of the pilot jet and that stops working. In effect, the air pressure at the air screw is now the same as at the pilot jet’s tip, so it quits pumping fuel up. Now, the taller needle jet under the slide is sticking up where the fast-moving, lower-pressure airbox air is streaking by, and the high pressure in the float bowl is pushing fuel up through the main emulsifier (also called the “mainjet holder”). The air into this emulsifier is fed on the opposite side of the carb’s bell area, across from the air screw, through a little brass metering hole. This is an airbox-pressure zone, so it is lower than the float bowl, but higher than the moving air in the needle jet’s tip: it has enough force to bubble the fuel in the emulsifier to aerate it as it gets pushed up the emulsifier. The needle in the slide controls how much of this fuel gets sucked out by the passing air, and the engine is running fast enough now that it is pretty much a steady vacuum.
Upper Midrange: Now the slide has raised above the wasp-waist zone of the carb throat, and the vacuum is very steady from the engine, so the whole throat of the airbox-side of the carb is at airbox pressure, which is starting to drop slightly as the filter restricts it: this is necessary in order for the fuel to aerate in the emulsifier, lest it be “clumpy” and rich, and not burn well. Most roadrace bikes with velocity stacks will not run at less than this throttle opening, which is why old movies show the bikes constantly blipping the throttle to keep it alive. (Watch a video of the RC66 Honda 250cc Six for examples). The ‘wasp waist’ in the carb throat causes some turbulence now, to help make the throttle feel linear: in effect, it disrupts the mixture a little bit from 45% to 55% throttle opening so the transition from low-speed mixing to high-speed mixing is not sudden, like a switch – or, like a roadrace bike with carbs. The needle in the slide is metering the amount of fuel to mix with the air at this point.
Full throttle: in the CB750 SOHC4 Honda carbs, there is almost no difference between ¾ and full throttle except for the amount of fuel the petcock can supply (to keep the float bowls full) at full throttle vs. ¾ throttle. The size of the mainjet determines the mix ratio with the passing low-pressure air, and the air must be at lower pressure in the carb throat than in the float bowl or the fuel will NOT rise to the venturi area. When a too-open intake airbox is used, the mixture gets lean above 5/8 throttle because there is no air pressure difference available to push the fuel up to the needle jet.
That is the basic operation of the 2-jet carb as is found in all of the SOHC4 Hondas.
The trouble with ‘pod’ filters or velocity stacks: removing the intake restriction that is intended as part of the mixing system also removes the device that generates the “middle” air pressure in between vacuum levels and atmosphere levels. This removes the ability of these carbs to push fuel up the emulsifiers and to also aerate the fuel, so the mixture becomes uneven and highly non-linear with throttle settings. There is no way for the emulsifiers to aerate the fuel, so the owner chases rich and lean spots, alternating between “flat” (lean) spots and too-rich mixtures that foul the sparkplugs quickly for lack of air. On racing bikes with velocity stack, the throttles are held more-than-halfway open most of the time, so the wasp-waist section of the carb throat generates enough turbulence to help aerate the too-rich fuel mix risng in the needle jet. The idle circuit will usually still work enough to let the bike run a ratty idle, but it requires frequent ‘blipping’ to blow out the fuel buildup that occurs, and the low-range throttle settings are always erratic. You can hear this when the bikes enter slow lanes (or the pits) and when they launch back onto the track from the pits: they burble and spit until they clean out and run again. On the street: this type of riding is impossible in traffic, and the result is fouled sparkplugs. In normal use the plugs should last at least 3000 miles in these bikes before cleaning the plugs: if yours don’t go that far, then something is ‘up’.
Another “anomaly” that confuses sometimes is: the pilot air screw has only a very narrow adjustment range. In the CB750K0-K6 “roundtop” carbs this is from ½ turn to 1-1/2 turns: less than ½ turn has already cut off most of the lifting air to the pilot jet, so the engine suction can only eke out a little bit of fuel at each suction, making it lean. Beyond 1-1/2 turns, the pilot jet’s size cannot pass any more fuel, so it is maxed out (presuming proper jet size). On a 4-cylinder engine, though, while you are adjusting ONE carb, there are 3 OTHER carbs pulling the air through that carb you are adjusting, but at THOSE carb’s speed. If you have a 4-1 pipe and an exhaust mixture analyzer, turning that one carb’s air screw inward too much finally leans out the mixture so far that it will no longer fire in the cylinder, and the unburned fuel dumps into the exhaust, raising the hydrocarbons. This “looks” just like what happens when you turn the air screw too far out, making the mixture too rich and unable to burn, causing too much hydrocarbons. Don’t let this confuse you: turning the air screw inward leans it out, just like in a car engine with a carb and a constant vacuum and an idle mixture screw (but that’s a whole ‘nuther mixing system, won’t go there just now…).
A side note 6/12/2022: Honda discovered in the lean-burning CB650 (with smog-controlled PD carbs as also found on the CB750F2/3 and K7/8) that the turbulence of air passing over the vent tubes for the bowls was causing the CB650 to suffer a "flat spot" in throttle response at hiway speeds when the throttle would get jerked open (like for passing another vehicle). So, they modified the vent tube air supply just like we did for racing, and I show in My CB750 Book, by plugging the bowl vent tubes into the bottom side of the airbox to pick up the most-still air that can be found on a moving bike. It's found in the Service Bulletin here:
http://manuals.sohc4.net/cb650/as "HSB 650 #1", and it works just the same way. The turbulent, moving air that is supposed to be supplying higher pressure to the bowls becomes lower pressure from the bike moving thru the air, so the fuel gets less 'push' up the emulsifier tubes. Pod-filtered bikes suffer even worse, as there is no airbox to quiet the air.
