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So we have yellow arrow - main well. Black arrow - air well which is supplied by mab and thin blue line float level (approximate, likely a tad lower).
As cfm increases through venturi and booster pressure is reduced below atmospheric in the main well (with a kill bleed this small air leak has to be overcome first). Air well is near or at atmospheric pressure thanks to mab. Bowl fuel is also atmospheric pressure. Some fuel in the higher pressure air well moves to the lower pressure main well exposing top emulsion bleed to air from the mab. Air bubbles enter the main well lifting fuel higher and allowing some of that lighter fuel to be carried through to the booster and to repeat a well used phrase “like wind whipping the spray off the top of a wave”. There at the booster it is further atomized as its sheared from the booster and broken down by colliding with airflow. The smaller those droplets are from main well the easier and better they will atomize leading to a more homogenous mix.
When we are talking about the start of the mains the main well does not need to overcome mab to get fuel moving, there’s an e-bleed between the two, often submerged. Air from the top emulsion bleed helps to establish flow to booster. A larger e-bleed means more air and bigger bubbles to activate sooner and lift more fuel (but enlarging gets to a point where it causes problems). Conversely smaller e-bleed is uncovered later and adds less air and bubbles for less lift for slightly later and leaner start up.
Increase the mab size and it makes it easier or increases the drive of the top e-bleed to get air to the main well, reducing its density and getting that spray of fuel moving to the booster.
‘Some others’ might have a better wordy version than me but they are not confusing air bleeds with emulsion bleeds, they are understanding how the mab and emulsion work as applied to Holley modular carburetors.
Statistics: Posted by Right hand drive — Mon Mar 29, 2021 12:13 pm
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RHD, I will give it one more try:
Quote [ not by me ] "The AB reduces the signal from the discharge nozzle, so there is less effective pressure difference to cause fuel flow through the main jet.....A large AB leans the mixture, particularly at higher engine rpm"
And from the original Holley book by M. Urich, p. 23:
"The strong signal from the discharge nozzle is bled off or reduced by the MAB so there is less EFFECTIVE pressure difference to cause fuel flow. The mixture will become leaner as the size of the bleed is increased. Decreasing the bleed size increases pressure drop across the MJ to pull more fuel through the main system, giving a richer mixture.
What you are confusing is what I would call head pressure [ effective pressure ] with the working pressure at the nozzle. The fuel is 'lifted' in the well to the nozzle by atmospheric pressure pushing the fuel through the MJ, after satisfying the pressure bled off by the AB. Further flow [ the richening you talk about ] happens ONLY when air speed through the vent increases [ pressure decreases ], achieved by an increase in throttle opening.....which might be so slight as not be noticed or felt.
Statistics: Posted by GTO Geoff — Mon Mar 29, 2021 7:50 am
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