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 Impact type float bowl vents on a N/A application 
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Joined: Fri Feb 08, 2013 5:52 pm
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Why? If I'm not mistaken they cause richening of the mixture with CFM increase; an elementary carburetor without any air correction does the same thing so why deliberately do it? :ummm?:


Fri Nov 14, 2014 2:28 pm
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If you are talking about vents with multiple holes it's to maintain a balanced pressure with the surrounding air versus seeing turbulent air. Turbulence usually causes reduced or fluctuating pressure, neither of which you want. A smooth rise due to scoop pressure increase as MPH is not an issue, turbulence is.


Sat Nov 15, 2014 2:23 pm
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Thank you, it clears another question I had. I actually meant those carburetor designs where vent tubes protrude into the center of the bore and have a cut at the end parallel to the air filter mating surface. Something like this:
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A smooth rise due to scoop pressure increase as MPH is not an issue, turbulence is.

Same principle as I understand?


Sat Nov 15, 2014 3:07 pm
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In the case above turbulence would probably not be an issue, or if it is it will impact airflow in the carb as well.


Sat Nov 15, 2014 3:35 pm
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Then this design is kind of an additional means to make fueling curve richer at high CFM? This particular carb is a compensation jet type, but exactly the same impact vent was used on emulsion Pierburg carbs for air cooled VW's. Wouldn't it be more prudent (and cheaper in mass production) to just set up the emulsion package to achieve enrichment at high loads high RPM? Carter carbs have this setup too, not sure what metering system they use though.


Sat Nov 15, 2014 3:51 pm
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A simple pipe, like in the picture, is cheaper to make than a precision emulsion tube requiring several machining and drilling operations.


Sat Nov 22, 2014 2:48 pm
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Oh yes I see, thank you!


Sat Nov 22, 2014 5:28 pm
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Imagine what the effect is if the main nozzle (in the booster) is turned facing up like that. Some AFB secondary nozzles are cut straight 90 degrees square on the end and angle of the booster leg is upward, so the end of the nozzle is seeing impact at about a 15 degree angle.

I once upon a time found a nozzle in a QJet upside down, so the discharge hole was facing up instead of down. The truck had been back to the dealer numerous times with complaints of everything you might imagine, including burned valves, and they didn't find it. After the warranty expired the owner wanted the power he was missing so I got a swing at it. He wanted a cam and intake, headers, etc, but all he needed was a tuneup and a carb body that had both nozzles installed properly. The dealer and GM said "too bad" when we showed them what the problem had been all along.


Sun Nov 23, 2014 11:44 am
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Location: Phila. Penn.
Webermaniac, I don't follow what you mean "enrichment"? A simple jet restriction on the fuel side usually produces an richer and richer mixture as a the rpms climb. With the opening turned up, the pressure drop created would be less, resulting in a flatter curve. Full load enrichment would have to be achieved by another means. So in Tuner's example, the burned valves were from too lean a mixture under partially and full load conditions. Correct me if I'm wrong.


Sun Nov 23, 2014 10:09 pm
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Initially I asked about a bowl vent tube seeing impact pressure and that's what it is in the photo. Mixture discharge nozzle came up later in the last couple of posts and I'm glad it did because I now can see how by using bowl vent tube and discharge nozzle with different upward and cut angles it's possible to alter fueling curve. Cool idea!
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It's exactly how they did it in this carb.


Mon Nov 24, 2014 5:36 am
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