I have this booklet from Honda, circa 1975. Honda Technical Series, Motorcycle carburetion. Hear is an excerpt which may describe what you've identified:
AIR-BLEEDS Air under atmospheric pressure is bled into carburetor fuel passages to improve fuel atomization, stabilize fuel height in the jets, and provide corrections in the air-fuel mixture ratio. Air jets and/or air bleed adjustment screws control the relative amount of atmospheric air drawn into the fuel systems. Main Jet Air-Bleed: Fig. 15 illustrates a typical main jet air-bleed. Low venturi pressure. which causes fuel to rise through the main jet, also causes atmospheric air to to flow through the air jet. Air and fuel meet and mix together in a perforated tube above the main fuel jet. The aerated fuel released into the venturi is more easily atomized than a dense unaerated stream of fuel. Aerated fuel also has less tendency to fall back down the jet tube between intake strokes, thus stabilizing fuel height in the jet tube. The same effect can be observed when drinking beverages through a straw. When you remove your mouth from the straw, a frothy beverage tends to remain in the straw, But an unaerated beverage will fall back down the straw into the cup. <<<<<fig 15 is a cross section drawing of a slide carb showing the main jet, perforated tube, main slide needle, slide, venturi. There is a tube from the intake runner to the perforated tube that contains a restriction and is labeled "Air Jet">>>>> Here is another section that may be of interest to you: INTERMEDIATE SYTEM WITH SLIDE TYPE THROTTLE VALVE When the throttle valve is opened, there is a transition from the "low speed system" to an "intermediate system" which meters fuel from the main fuel discharge jet (needle jet). In carburetors with slide type throttle valve, the intermediate sytem uses a tapered fuel metering rod (jet needle) that works in connection with the variable venturi and choking action of the throttle valve. The jet needle meters fuel flow, maintaining the correct air-fuel mixture ratio through most of the carburetor's operating range. At full throttle opening, the jet needle is fully raised, and fuel flow will be controlled solely by main jet diameter (high speed system). Throttle Valve Cutaway: The needle jet begins to discharge fuel at about 1/8 throttle opening and supplants the low speed system as the chief fuel supply. It is important to have a smooth transistion from the low speed system to the intermediate system as the throttle is opened. Otherwise, there would be a momentary fuel delivery failure causing a flat spot in acceleration. The height of the throttle valve cutaway is crucial in obtaining smooth system transition and good initial acceleration. While the position of the throttle slide detemines the venturi constriction in the carburetor bore, the shape of the bottom edge of the slide determines the extent to which induction port vacuum is maintained at the needle jet. The bottom edge of the slide, on the upstream side (air cleaner side) of the needle jet acts as a choke, but is "cut away" to a height that will produce the exact degree of choking needed to ensure proper fuel delivery from the needle jet at low throttle openings. Increasing the height of the cutaway reduces the choking effect, producing a leaner air-fuel mixture ratio, and vice versa. The throttle valve cutaway controls the air fuel mixture ratio primarily at 1/8 to 1/4 throttle openings and ceases to have any effect beyond a 1/2 throttle opening. Hope this helps, Lloyd SOHC4 #11 [EMAIL PROTECTED] wrote: >There is a jet in the bell-mouth of the Keihin carb of my CB750. It looks >like it directs air from the filtered atmosphere into the emulsion tube. > >Do any of you carb-knowledgable people know what it does, and why it's a >fixed size? (ie not replaceable?) > >Phill >AOL Screen Name rickmancr750 > > >
