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Posted by chriscase on Saturday, August 23, 2008 at 18:26:35 :
In Reply to: Idle screw regulates AIR - not gas posted by Paul Cook in Kempner, TX on Saturday, August 23, 2008 at 12:37:25 :
The throttle plate lets a small amount of air at idle, and the idle mix screw is the only source of fuel at idle.
If you have a disassembled carb, you can follow the systems. There is an air bleed in the air horn that allows air into the idle/transfer system, but there is also a passage that allows fuel in from the main jet passage. The idle/transfer circuit is the only source of fuel until the air flow is high enough to pull air past the venturi, sucking in gas too. The transfer port is the little slit directly above the idle screw hole.
The main flow/venturi system depends on the thickness of air to meter in the fuel, and so that system shouldn't need much adjustment for changes in elevation. The idle system lack the venturi, so would need a small adjustment for the altitude change. But hmmmm, maybe that is what the idle air bleed helps with? Thicker air, less air flow, more gas sucked in to match the thicker air flowing past the throttle plates?
Some Hollys and some furrin carbs do use air bleed screws. NOT B&Bs.
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