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Old 09-22-2012, 01:06 PM
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Air Bleeds, why?

Why does a carb need air bleeds---both for the main circuit and idle circuits?

Is this just to help with the mixture or would the carb not function at all without them?

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Old 09-23-2012, 12:30 AM
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If you completely blocked off the air bleed the mixture would tend to get richer and richer as airflow increases, for a particular jet.

The air bleed is there to feed both some fuel and some air into the venturi.
With the correct sized air bleed you can get a much flatter fuel curve over a wider carb flow range.

The air bleeds will have little effect at low airflow, the jet size (and float level) determines everything.
As airflow increases, the air bleed has a larger and larger effect.
The result is, you can tilt the air/fuel ratio slope up or down by playing with the air bleed size.

Big air bleeds generally go with big jets, and small air bleeds with smaller jets to get a flat curve, but you can play around with both.
You cannot anticipate what a particular air bleed size is going to do without knowing if the main jet size is large, small, or typical.
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Old 09-23-2012, 08:02 AM
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First,define carb=== a pot holding fuel that is full of leaks,lol.
thats all it does is leak fuel with some control.
you know how a syphon works?
the jets would continuously allow fuel to flow into the engine once the syphon process started IE: low pressure air in the venturi starting the flow through the jets,and pump keeping float bowls full,gravity would take over,,,
The air bleed interrupts the fuel flow continously so fuel will only flow when low pressure draws fuel from bowl.
(technically higher pressure in the bowl forces fuel out through the jets)The air bleeds job is to balance the pressures to stop fuel flow.
Think of a garden hose used as a syphon to drain water from a pool. Once the process is started by applying a low pressure to the "lower" end of the hose,the water will flow continuosly from the pool as long as the end of the hose is lower than the water level and there is water
NOW,poke a hole in the top of the hose above the water level before in bends down below water level.If the hole is big enough the water will stop flowing,if the hole is small then the flow will slow down.If the flow stops because the air leak is large,then a lot more vacuum needs to be applied to the lower end of hose to get water to flow.
The rest is math, difference in pressure values of air inside the float bowl Vs pressure in the venturi. Our foot controls the venturi flow.
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Old 09-23-2012, 10:17 AM
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OK, i think i get it now. i was taking apart a carb to learn more and either nobody really mentioned air bleeds or i never really paid attention until now. But they are definitely there----both idle and main air bleeds.
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Old 09-25-2012, 09:45 PM
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Thanks Vinnie. Very cool analogy.

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