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 Impact type float bowl vents on a N/A application 
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Joined: Sun Jun 23, 2013 1:41 pm
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Location: Phila. Penn.
I see. Yes I lost track of the original question. Thanks!


Mon Nov 24, 2014 8:47 am
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To go further with this discussion on orientation of vents, bleeds and nozzles in relation to velocity and impact pressure, study the nozzle in webermaniac’s carb picture (zoom in, increase your screen magnification 400%), it has two orifices in the nozzle tip, one above the other.

The several orifices, the bowl vent, the vent above the actual discharge nozzle, and the main fuel nozzle, are subject to variations of pressure due to their location in higher or lower velocity air flow. The bowl vent is up in the air horn where the velocity is low, so pressure is high, and the bleed/vent nozzle is in the venturi area where velocity is high, so pressure is low.

The lower nozzle, the discharge nozzle, is in the (double) booster vena contracta where the velocity is highest and so pressure is lowest. The upper nozzle is above the venturi, where pressure is higher, and is oriented to receive the effect of the high velocity air impact.

The effect of higher pressure in the upper nozzle is to cause air flow into the nozzle passage and discharge with the fuel through the lower main fuel discharge nozzle.

This nozzle arrangement has the effect of adding a third booster venturi because the air velocity through the tip of the main nozzle causes an additional depression in the nozzle well and passage from the main jet.

The combined effects of pressure variations, which change with flow velocity at each orifice, is a means of controlling A/F over a desired range, usually widest possible.

Look closely at a QJet primary nozzle (or T-Quad, or nearly any double-booster venturi carb, and some with single booster venturi), there is a small hole in the tip of the nozzle which is oriented nearly directly above the discharge orifice in the bottom of the brass nozzle (should be anyhow, LOL). The air flow through the small orifice in the tip of the nozzle must exit through the discharge orifice and this flow causes additional depression in the nozzle, therefore, another booster effect in addition to the triple-venturi in the zinc casting.

Carter Carb. Co. received the patent for the little orifice in the nozzle tip in the early 1930’s (as I recall). I’ve seen variations of the same 1 bbl. Carter carb for 6 cyl. Chevys and the early carbs don’t have the little hole, but since the 30’s nearly all double booster venturi carbs have that type of nozzle.


Mon Nov 24, 2014 11:40 am
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Joined: Fri Feb 08, 2013 5:52 pm
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Here's the original full size image under thumbnail:
Image


Mon Nov 24, 2014 2:14 pm
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Great picture!! What engine is that carb designed for?

In my post above I described the upper nozzle as a bleed/vent into the main nozzle passage to the lower, larger nozzle. However, does that carb have two separate discharge circuits like a comp jet arrangement, so the two nozzles do not communicate and they are not connected inside the nozzle leg?


Mon Nov 24, 2014 2:33 pm
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Joined: Fri Feb 08, 2013 5:52 pm
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tuner wrote:
Great picture!! What engine is that carb designed for?
Thank you. This carb was a factory item for a 2.12 liter inline 4 engine used in a military jeep in 1952-1972.

tuner wrote:
In my post above I described the upper nozzle as a bleed/vent into the main nozzle passage to the lower, larger nozzle. However, does that carb have two separate discharge circuits like a comp jet arrangement, so the two nozzles do not communicate and they are not connected inside the nozzle leg?
Yes, I think they are separate.

I took another one of those carbs apart. This is how it looks like inside. All images open to full size when clicked.

Nozzle, discharge end:
Image

Nozzle, side view:
Image

Nozzle, inlet end:
Image

Double stage booster with reeds, side view:
Image

Double stage booster, top view, one reed removed out of curiosity :-) :
Image

Booster and nozzle assembled:
Image

Nozzle installed, inlet end view:
Image

Nozzle with jet - mating surface view:
Image

Nozzle with jet - slotted side view:
Image

Jet in body and adjusting needle:
Image

Everything assembled:
Image


Mon Nov 24, 2014 5:26 pm
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