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Old 06-11-2012, 02:08 PM
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External slash vent on quadrajet?

The quadrajet that I'm practicing with, I noticed it is different than my Buick quadrajet. I think it's called external slash vent. I tried find more information on it but no luck. Maybe someone can explain what it use for. My guess is it's controlled by accelerate pump to non pressurized fuel bowl while stomp on the gas to allow fuel drain faster to prevent fuel being glupping from bowl being pressurized.

What's pros and cons about this?

This seem a good feature but why not all quadrajet has it?

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Old 06-12-2012, 06:31 AM
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Most quadrajets have a vent tube sticking straight up from the top, and on Chevy's its usually in the front in the middle. Its just a vent for the float bowl, and it may be cut straight or slashed. On some non-Chevy quadrajets the vent tube is off center, so the air cleaner has to be designed to fit the carb.

Bruce
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Old 06-12-2012, 01:24 PM
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Quote:
Originally Posted by 75gmck25
Most quadrajets have a vent tube sticking straight up from the top, and on Chevy's its usually in the front in the middle. Its just a vent for the float bowl, and it may be cut straight or slashed. On some non-Chevy quadrajets the vent tube is off center, so the air cleaner has to be designed to fit the carb.

Bruce


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Old 06-12-2012, 03:10 PM
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This link on Cliff Ruggles site has some information on the slashed vent quadrajet used by Buick.

http://www.cliffshighperformance.com...history_3.html

Bruce
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Old 06-12-2012, 03:27 PM
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vent on quadrajet

Hi freddy,, my quadrajet has the style of vent your carb has .. It opens at WOT , and has a wire looped around the arm on top of the pump rod. it lifts the rubber cap off the vent..... I don't see the wire in your pic.... jim
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Old 06-12-2012, 03:33 PM
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Quote:
Originally Posted by 327 / 350 hp
Hi freddy,, my quadrajet has the style of vent your carb has .. It opens at WOT , and has a wire looped around the arm on top of the pump rod. it lifts the rubber cap off the vent..... I don't see the wire in your pic.... jim
It does have wire that connect to accelerator pump rod. I took them off to practice rebuilding. I just wanted to know what's this vent's purpose reason.

Last edited by Freddyfingaz83; 06-12-2012 at 03:39 PM.
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Old 06-12-2012, 04:18 PM
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Quote:
Originally Posted by 75gmck25
This link on Cliff Ruggles site has some information on the slashed vent quadrajet used by Buick.

http://www.cliffshighperformance.com...history_3.html

Bruce

Thanks for this link. I don't see any detail explain but it said external was changed into internal to be more durable. My guess That's because external vent doesn't have any dirt protection where it has an opening space to allow dirty come in that damaging engine. Just guessing. I'm trying find more solid information just for my curiously.
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Old 06-12-2012, 04:28 PM
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Quote:
Originally Posted by 327 / 350 hp
Hi freddy,, my quadrajet has the style of vent your carb has .. It opens at WOT , and has a wire looped around the arm on top of the pump rod. it lifts the rubber cap off the vent..... I don't see the wire in your pic.... jim

Here's other pic with fully assembly before I tear it apart.

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Old 06-12-2012, 05:31 PM
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Your carb has the relatively rare adjustable off idle air bleed circuit, the screw is under the triangular spring metal piece pushed over the front vent tube. A bit of info on it is here, under ADJUSTABLE MAIN WELL BLEED.

The front vent you are asking about is known as a Thermostatic Idle Vent, you can search for more on it. It was used through '74 according to my notes.

There was a later vent system used that was on the back of the carb, called the "hot idle compensator" vent. Q-jets all have the casting for it (like the front thermo vent on that style casting), but it's rarely actually used. This can cause guys to think their carb might be missing parts when they're actually not.
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Old 06-12-2012, 06:03 PM
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Quote:
Originally Posted by cobalt327
Your carb has the relatively rare adjustable off idle air bleed circuit, the screw is under the triangular spring metal piece pushed over the front vent tube. A bit of info on it is here, under ADJUSTABLE MAIN WELL BLEED.

The front vent you are asking about is known as a Thermostatic Idle Vent, you can search for more on it. It was used through '74 according to my notes.

There was a later vent system used that was on the back of the carb, called the "hot idle compensator" vent. Q-jets all have the casting for it (like the front thermo vent on that style casting), but it's rarely actually used. This can cause guys to think their carb might be missing parts when they're actually not.

Thank you for this information. Thumb up!
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Old 06-12-2012, 06:18 PM
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Depending on what year and engine it came off from, you might want to do a little research on your carburetor some of those older Buick Q-Jets were 750 CFM carbs. If it is don't mess it up. I believe most Q-Jets were 650's
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Old 06-12-2012, 06:33 PM
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Quote:
Originally Posted by boothboy
Depending on what year and engine it came off from, you might want to do a little research on your carburetor some of those older Buick Q-Jets were 750 CFM carbs. If it is don't mess it up. I believe most Q-Jets were 650's

I have no plans with it but to practicing rebuild to get better understanding how quadrajet work. It's actually is Carter qjet that I got from junk 69 Pontiac grand pix
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Old 06-12-2012, 06:40 PM
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Quote:
Originally Posted by boothboy
Depending on what year and engine it came off from, you might want to do a little research on your carburetor some of those older Buick Q-Jets were 750 CFM carbs. If it is don't mess it up. I believe most Q-Jets were 650's
Close- Buick first started using the 800 casting, then the other makes followed suit for their performance engines. The 'regular' casting is rated at 750 cfm.

Pontiac used a one-year-only Q-jet in '71 that had the primary boosters cut down to a single ring. They flowed a little more at WOT but lacked throttle response, so was quickly discontinued.

Strange as it sounds, the best carbs today to use came on the GM trucks up through '86. They were non computer controlled, and almost all were the single main air bleed-type carbs w/the large 800 cfm casting. The 800 cfm casting was even used stock on the 4.3L V6 engines in those trucks!

More here.
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Old 06-12-2012, 06:48 PM
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Quote:
Originally Posted by cobalt327
Close- Buick first started using the 800 casting, then the other makes followed suit for their performance engines. The 'regular' casting is rated at 750 cfm.

Pontiac used a one-year-only Q-jet in '71 that had the primary boosters cut down to a single ring. They flowed a little more at WOT but lacked throttle response, so was quickly discontinued.

Strange as it sounds, the best carbs today to use came on the GM trucks up through '86. They were non computer controlled, and almost all were the single main air bleed-type carbs w/the large 800 cfm casting. The 800 cfm casting was even used stock on the 4.3L V6 engines in those trucks!

More here.
Well there you go.
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Old 06-12-2012, 07:28 PM
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Quote:
Originally Posted by cobalt327
Close- Buick first started using the 800 casting, then the other makes followed suit for their performance engines. The 'regular' casting is rated at 750 cfm.

Pontiac used a one-year-only Q-jet in '71 that had the primary boosters cut down to a single ring. They flowed a little more at WOT but lacked throttle response, so was quickly discontinued.

Strange as it sounds, the best carbs today to use came on the GM trucks up through '86. They were non computer controlled, and almost all were the single main air bleed-type carbs w/the large 800 cfm casting. The 800 cfm casting was even used stock on the 4.3L V6 engines in those trucks!

More here.

Interesting. I gotta hit junkyard for that qjet for my modified 454.
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