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Old 10-23-2006, 10:57 PM
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Q-jet carb

On my 81 trans am here is carb # 17081285 and 1261dkj cant seem to find any info on it any help would be great.

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Old 10-24-2006, 06:50 AM
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Info from the carbguy.com


Rochester, Quadrajet
A/T Canada
Make & year Chevrolet, 1981
Displacement/cylinders 305/8
Carb number 17081285
Primary jet & rod combo NA / NA
Secondary jet & rod combo 0,135" / NA
Idle feed NA
Fuel inlet valve 0,135"
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Old 10-24-2006, 07:38 AM
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Thanks BBB,anyone have a idea of the flow cfm.
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Old 10-24-2006, 11:13 AM
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That carb has the potential to flow 750cfm. It's probably lower since the engine size is small. GM sets the top butterfly max opening to control the cfm of the carb according to the engine size.
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Old 10-24-2006, 11:17 AM
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Ditto... I don't think the q-jet was made any smaller than 750cfm.
I do believe some were 800 CFM.... correct me if im wrong.
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Old 10-24-2006, 10:18 PM
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That is correct... All Qjets are either 750 or 800 cfm, but like Jmark said the air doors only let it flow what the engine needs. That's why Qjets were used on everything from a caddy 500 down to a V6.

The way to tell for sure what cfm you have is to look down the primary throats and compare them to these photos. Down in the throats are a double booster, then a second "booster" cast into the venturi. If they are all three evenly spaced, its a 750. If the throat venturi is bigger like the other photo, its an 800.

Rumor has it that there was an 850 made for a while on those rare stage 2 buick 455 dealer options, but details are sketchy at best. Since it was a dealer option, the chances that rochester would completely redesign its most complicated carb for a handful of engines is rare... not to mention, 800 cfm was more than enough for them.
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Old 10-24-2006, 10:23 PM
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Quote:
Originally Posted by Jmark
That carb has the potential to flow 750cfm. It's probably lower since the engine size is small. GM sets the top butterfly max opening to control the cfm of the carb according to the engine size.
That is partially correct. What actually happens is that the secondary air door is only drawn open as far as the airflow dictates. Its on a light spring, so if you put the same carb on a 305 and a 454, the 454 will draw that door open much farther.

But all Qjets have the same air door stop position. Its cast into the carb itself. They all will open fully with your finger, but are only drawn open according to the airflow of the engine. That's what makes them so versatile.
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Old 10-24-2006, 10:25 PM
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thanks all for the replies,will have a look and compare.
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Old 10-25-2006, 11:01 AM
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Quote:
Originally Posted by curtis73
That is partially correct. What actually happens is that the secondary air door is only drawn open as far as the airflow dictates. Its on a light spring, so if you put the same carb on a 305 and a 454, the 454 will draw that door open much farther.

But all Qjets have the same air door stop position. Its cast into the carb itself. They all will open fully with your finger, but are only drawn open according to the airflow of the engine. That's what makes them so versatile.

Hi Curtis...nice to have another Qjet fan on here!

You are correct on the qjet only opening as far as the engine dictates, but there is a stop too.

On the top secondary air door, they are limited by a tab stop. Some will open a full 90 degrees vertical, others are limited to as low as 55 degrees, atleast according to some of the qjet manuals I've got. When I did my last one, it was limited by the tab to about 75 degrees, checking with a protractor. I did as suggested and slowly filed off the tab on the linkage and got the air door to open to 85 degrees as suggested as the max opening. Here is a pic of the limiting tab on my carb. I had to remove about 1/16" off it to get the correct opening angle.

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Old 10-25-2006, 11:38 AM
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Oh.. I learn something new everyday! Sorry for the earlier misinformation. I guess I've just never seen one. I usually deal with Caddy 500s and other bigger engines, so I guess they all open the full 90.

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Old 10-25-2006, 04:46 PM
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No problem. If you are working on big V8 qjets, I'd imagine they all do open up full. Some of the small cube engine qjets are really limited, but the potential is there.

Mark
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