Lean idle. Rich cruise.
The outer passage goes to the idle port and is restricted by the needle valves (idle mix screws).
The inner passage goes to the transfer slot and is restricted by the slot width, depth, and amount exposed to manifold vacuum by the throttle plate position.
Here's a simplified illustration of the relationships without a TSR.
The Transfer Slots Explained
By Tuner,
... The magazine articles tell us the slots obviously deliver more fuel as more of the slot is exposed to the vacuum at the edge of the butterfly and slot is exposed to the manifold vacuum below it. The real deal (which I’ve never seen described in the funnies) is that the portion of slot above the butterfly is air bleed and that below the butterfly is fuel feed. As the throttle blade changes position up and down the slot, the ratio of air bleed to fuel feed changes. As the butterfly opens air bleed is being cut off and fuel feed is being increased.
Usually (almost all Holley’s & clones), the slot and the curb idle adjustment are fed by the same idle jet and air bleed. If the slot is too long there is too much bled air from the slot above the butterfly at idle and small throttle openings. Because so much bled air is getting in the circuit from the slot, less is needed from the IAB for the same size IJ to function correctly at the dead idle. The result of less bleed area at the IAB to get the same size IJ to function at a dead idle is too much fuel when the total t-slot is exposed to the manifold and there’s a stronger vacuum because engine revs are higher. The mixture gets richer as the blade goes up the slot. Ideally, the mixture is richest at the dead idle and then gradually leans as the throttle is opened.
Its big because that is just meant to be a passageway, not a restriction.
The outer passage goes to the idle port and is restricted by the needle valves (idle mix screws).
The inner passage goes to the transfer slot and is restricted by the slot width, depth, and amount exposed to manifold vacuum by the throttle plate position.
Here's a simplified illustration of the relationships without a TSR.
The Transfer Slots Explained
By Tuner,
... The magazine articles tell us the slots obviously deliver more fuel as more of the slot is exposed to the vacuum at the edge of the butterfly and slot is exposed to the manifold vacuum below it. The real deal (which I’ve never seen described in the funnies) is that the portion of slot above the butterfly is air bleed and that below the butterfly is fuel feed. As the throttle blade changes position up and down the slot, the ratio of air bleed to fuel feed changes. As the butterfly opens air bleed is being cut off and fuel feed is being increased.
Usually (almost all Holley’s & clones), the slot and the curb idle adjustment are fed by the same idle jet and air bleed. If the slot is too long there is too much bled air from the slot above the butterfly at idle and small throttle openings. Because so much bled air is getting in the circuit from the slot, less is needed from the IAB for the same size IJ to function correctly at the dead idle. The result of less bleed area at the IAB to get the same size IJ to function at a dead idle is too much fuel when the total t-slot is exposed to the manifold and there’s a stronger vacuum because engine revs are higher. The mixture gets richer as the blade goes up the slot. Ideally, the mixture is richest at the dead idle and then gradually leans as the throttle is opened.