Main Air Bleeds (HSAB) - 1850-3 carb

[GTO Geoff]

No Harry & others, I didn't it backwards.

I am talking about the point where the system starts, not what happens afterwards. This is where the confusion is.
This is high school physics.
Atmospheric pressure [ AP ] acts on fuel in the fuel bowl. Once pressure at the nozzle [ from increasing air speed ] has reduced enough, AP pushes fuel & air out of the nozzle. The air bleed introduces air to [a] help lift the fuel & acts as an anti-siphon device. If you increase the size of the AB, more air speed is reqd to [b]start the fuel flow. The analogy is sucking liquid through a straw with a small hole above the liquid level. A certain amount of suction will be reqd to start flow. If you make the hole bigger, more suction is reqd to start the flow.

[Right hand drive]

[gntkllr]

0.031" PMAB's has been common in a number of 1850's that I've seen... though with #66 PMJ's (and 0.046" PVCR's)...

0.028" PMAB's have coincided with #65 PMJ's (and 0.046" PVCR's)...

[Right hand drive]

Would larger than expected .031” mab be used due to signal strength of a straight leg booster design to get it flowing when it needs to? Another way - do straight leg boosters need more mab start up assistance?

[GTO Geoff]

RHD,
What I said is CORRECT & so is the straw analogy. You are confusing emulsion bleed holes [ which maybe above or below fuel level in the well ] with the air bleed which is always ABOVE the fuel level.
But hey, don't take my word for it...

From Dave Emanuel's Carter book, P. 26: ' The function of the AB is not only to emulsify fuel. It also exerts control over fuel flow by bleeding off some of the suction force or fuel metering signal that is developed at the discharge nozzle......As bleed size is enlarged, the vac reqd to initiate flow is increased. Conversely, a reduction in bleed size reduces vac signal requirements & fuel flow is more easily initiated."

P .27: 'The airbleeds contained in prod carbs are precisely measured restrictions, sized such that fuel flow from the nozzle into the air stream is started at specific vacuum levels.....It should also be obvious that bleed size affects A/F ratios. A large bleed will admit more air than a small one, & consequently a leaner mixture will result.


Douge Roe's QJ book, p. 10: ' 'The strong signal from the discharge nozzle is bled off [ reduced ] by the main air bleed so there is less effective pressure to cause fuel flow. The mixture will become leaner as the size of the bleed is increased...

[Right hand drive]

Could someone please educate Geoff that in a Holley modular style carburetor a larger main air bleed richens the begining of mains start up more than a smaller main air bleed due to their effect on emulsion bleeds and therefore mainwell fuel. We all know the larger mab goes on to lean the top end more than the smaller.

Instead of quoting old Carter and Quadrajet books do real life testing on Holley carbs.

[Right hand drive]

Also Geoff, none of your quotes mentioned the effect of sequential jetting. Something that is in play with the Holley mab and emulsion bleeds.

Despite that emulsion acts on mainwell to lift fuel to boosters during low venturi flow and mab regulates how much air the emulsion has to do that at that stage of venturi flow. Bigger mab = more air available to emulsion to assist lifting fuel to boosters at low venturi flow. None of your examples discussed that use of air bleed, your examples are what happens with iab on Holley’s.

Apologies for interrupting your thread novadude.

[novadude]

[gntkllr]

[GTO Geoff]

RHD.

Still not getting it. AB size controls when the system STARTS.

Also, Doug Roe spent 4 yrs in the RPD [ maker of QJ carbs ] test lab, 12 yrs at GM on engine/carb development. He also worked for Stromberg carbs. You better contact him & 'educate' him about ABs, tell him how wrong he is.