A model
carburetor usually has two mixture controls. High speed and
low speed or idle. These mixture controls alter the fuel/air
mixture at the various carburetor openings. Usually the needle
valves will meter the amount of fuel allowed to mix with the
air. A needle valve is a tapered needle that is moved into
a hole to provide a very precise adjustment.
Less expensive
carburetors have an air bleed for the idle adjustment. This
is just that, it bleeds in extra air at idle to obtain the
correct mixture. The only issue with air bleed is you usually
won't get a good crisp transition to high speed or a good
mixture just above idle or mid range. The air bleed adjustment
screw richens the mixture as it is screwed in, closing the
bleed hole.
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