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Note that he found the carb diaphragms 'siliconed'; it may mean you need new ones. Put in fresh spark plugs and do a plug chop, IOW, hold it at whatever speed it will reach for a couple miles, then hit the run/stop switch at the same time you pull the clutch in, and coast to the roadside. Safely, of course. Let it cool and pull the plugs out; let us know what they look like.
 
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And Trials is right about the foam filters; they often block the air jet inlets, causing a poor mixture under high flow conditions. Finding a set of OEM type filters would be better than stacks, but at least you'll know if the filters you have are the problem.
 
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Unifilters can, and do, block the air jet ports. Many have the step from where they fit around the end of the intake to the main airflow tube too large, and cause the main airflow to divert inside the ports. The stacks Honda provided not only increase air velocity at the intakes and reduce turbulence, they make for a smooth air flow that does reach the air jet intake ports. The increased air velocity will also lift the CV pistons more effectively, therefore providing a richer mixture at high throttle settings than without it. CV - variable venturi - carbs are used instead of fixed venturi carbs because of the wide range of rpm, therefore wider range of air volume, than carbureted auto engines. Without variable venturis, the air reaches sonic speeds, which is bad for the atomization function.
 
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First, when the air velocity exceeds sonic speeds in the venturi, the shock wave after the venturi causes a greater drop in temperature. It chills the vapor to the point that it turns into something more like rain, which will cling to the intake tubes and doesn't burn well. My FIL was an engineer at Carter Carburetor, and explained this to me. It is also one reason many intake manifolds are heated. No one cares if this happens in lawn mowers; they aren't that efficient to begin with, and are governor controlled and rarely reach this condition. Performance car engines are a different matter.

Direct-lift carbs do go lean when the throttle is lifted quickly, because the air velocity over the jet briefly drops, so doesn't draw as much fuel as when it reaches the steady state, when the jet/needle combo takes full control. If you snap the throttle too quickly, it can be very lean, making the engine feel fuel-starved.

CV carbs are briefly rich under the same conditions, because of the opposite effect. When the piston lifts from the vacuum above the piston, which increases from the increasing air velocity, the opening of the venturi is larger, the velocity drops, and mixture control returns to the jet/needle. Velocity stacks, by decreasing turbulence and increasing air velocity ahead of the venturi, increases air density and pressure entering the venturi, so the total velocity through the venture is greater, and air velocity is what causes the piston to rise and open the jet/needle, until you approach WOT. The increased density of the air leans the mixture some, but also compensates for the reduction created by the air filters. The increased velocity at the entrance of the venturi will increase the total velocity through it, making the piston rise a bit higher, countering the leaning effect. It's all a careful balance, which is why taking the filters off, or using low-resistance filters, can make it so hard to re-tune a CV carb. You can't just throw in a bigger main jet and lift the needle some with most of them, you have to adjust the whole airflow vs. fueling curve. Unless you are only going for WOT, as in dragging the 1/4 mile.
 

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Yes, air at higher velocity is less dense, but the velocity stack compresses the air somewhat, increasing the density before it enters the venturi.
 

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The stack gets smaller at the carb end, so the air entering has to fit through a smaller opening. The engine vacuum mostly applies to the other side of the venturi, so even though the stack increases the velocity of the air, it also compresses it to fit into a smaller area. The big velocity increase is in the venturi, where it reduce the air pressure the most, and enables the fuel to be drawn through the jets. In both the direct lift and CV carbs, the air velocity doesn't change as much as with a fixed venturi, which is why the opening above the main jet has to change by withdrawing the needle from the needle jet.
 

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The issue with pods is many that are sold as 'fits', and they do actually fit, is that they partially block the air jet ports of the old Keihin design carbs, which are very near the perimeter of the intake end. This keeps the air from flowing at the designed speed into those air jets, and the emulsion tubes from working the way they should. Note that this will have little effect on the carb shown in that video, as its air jet ports stand very proud of the edge, while the ports on the old Keihin design are actually embedded into the sides, where the step in the boot of many pods, that form the locator stop can, and in many cases does, block them from the direct air flow. There are pods with a smaller step, but you won't know until you have them in hand if they will actually work. This is a known issue among vintage Hondas. With skill, you can reduce the size of the step, but you have to know to check.
Also, with these carbs, the stack is not part of the venturi, which starts some distance inside the intake.
Art Font Engineering Parallel Auto part
 

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The CB450 carb does not have air inlets that 'wind around' the intake, it has three small holes centered about 3mm from the outside edge, two near the top to feed the underside of the CV piston, one at the bottom with a brass jet pressed in to feed the main jet emulisifier; If the step in the mounting boot for the filter is more than 2mm high, the air mostly flows past them at higher airflow.
The needle/needle jet has nothing to do with that; it only adjusts the fuel ramp itself. You can dump it and go to adding more emulsion bleed holes to do the same. Either or works and depends on what the designer wants
Really? The volume of fuel drawn through the main jet is determined primarily by two things: the air velocity passing over it and the area of the jet. Since the air velocity doesn't change much in a CV carb, the jet size must increase, which is the function of the jet needle and needle jet, until the piston rises near the top of its motion. If you want a proper air/fuel ratio throughout the full range, you have to adjust the fuel volume; adding air bleed holes to the emulsion doesn't provide this control. The air velocity through a direct-lift carb may change more, but doesn't change enough to pull enough more fuel through a fixed jet, which is why they also have needles.

As to changing air filter restriction not affecting fuel/air ratio, consider that the ratio is between the fuel and air mass, not volume, so if the air pressure changes at the carb inlet, the ratio also changes. This is why engines with simple carbs run rich at high altitude, if you don't make changes in the jetting system; less air density, less mass flowing through the carb, and less mass means higher fuel/air ratio. A high restriction air filter also reduces air pressure at the carb inlet at higher throttle settings, and reducing the restriction increases that pressure, so more air mass enters the engine; more mass requires more fuel to maintain the fuel/air ratio, which is why CV carbs run lean when they have high-flow filters.
 
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The carbs on my VS800 were worse for changing mixture with low-restriction intakes than the old Hondas. Whenever someone complained about running problems after taking the stock filters out, we told him to just put them back.
 
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