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Old 07-27-2013, 03:22 PM
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quadrajet cfm

can anyone tell me if the quadrajet ever came in a size smaller than 750 cfm? If so how do you tell the cfm rating? i am looking for something around 550 cfm for a recently rebuilt 350 sbc.

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Old 07-27-2013, 03:49 PM
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All the Qjets only come in 3 cfm sizes 750 795 and 850. all rated @ 1.5"
But there are a few than had the secondary air door opening limited, that did restrict the air flow further.
(these Odd ball Qjets were only used on a few motors and you don;t need to look for one)

There are a few others that have a different WOT secondary throttle WOT position when wide open.
This does not really effect the air flow. it was done on some stock GM motors to get the fuel distribution right
for that motor. You don;t need to look for these either.

You do not need to resize your Qjet. When the air door is set up correctly it automatically sizes itself to your engines WOT air flow needs. The air door opening rate and opening start point delay point is fully adjustable.
The secondary metering rod and the sec metering rod hanger choise is highly dependant on how agressive
or tame you set up teh secondary air door opening rate.

Get them both right and it is a very smooth carb. a Smaller carb is not better.

Buy a book about the Qjet carb and learn how it works and how to tune it.

Last edited by F-BIRD'88; 07-27-2013 at 03:55 PM.
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Old 07-27-2013, 03:50 PM
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might want to study up on how a vacuum 2nd carb works..
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Old 07-27-2013, 04:55 PM
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poor acceleration

the reason I asked was that although the engine is recently rebuilt it stumbles on acceleration. I was told that it was probably due to a too large of a cfm on the carb.

I assume with a quadrajet this shouldn't be the problem?
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Old 07-28-2013, 03:56 PM
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The basic cfm size of a Q jet is not causing this.

Stumbling on inital acceleration is a function of basic ignition system condition, ignition timing set up
Carb set up. The Q jet is fully adjustable. Start by looking at the ignition system condition and setup.
if the plugs are now fouled, replace.
Is the engine to body ground wire installed? (engine to firewall) This is often left off of recent re-installs
and is a common source of pesky misfire, and easy to correct.
Make sure the brace is on your starter motor too.
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Old 07-28-2013, 05:23 PM
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Does the stumble occur under hard accel or normal driving?
If the secondary air door is opening too fast under hard accel, you get the infamous bog that gave us the name Quadra-bog. To control the opening rate you have to adjust the spring tension on the air door and/or the the rate of extension of the choke pulloff that is normally connected to the door linkage.
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Old 07-29-2013, 12:11 PM
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It stumbles under hard acceleration. The spring tension, should it be increased or decreased?

Yes I installed a new engine to ground wire. I don't remember a brace on the starter when I bought the vehicle.

you have to adjust the spring tension on the air door and/or the the rate of extension of the choke pulloff that is normally connected to the door linkage

I have a Haynes manual on rebuilding quadrajets but it doesn't contain tuning information. Can you suggest a book that does?

if it is a timing problem would that normally be too far advanced or retarded?

One last question. I noticed when I rebuilt the engine the vacum measured at the intake manifold increased to 25 inches while at idle (about 1000 rpm). Can this be part of the problem?
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Old 07-29-2013, 02:46 PM
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The air valve is opening too fast meaning the tension on the spring should be increased.


Turn the spring adjusting screw another 1/4 turn. Make sure to have the screw driver in the adj. screw when loosening the lock screw.

25" of vacuum at 1000RPM means you have a really mild camshaft.

Have you checked the air fuel ratio indications on the spark plugs?
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Old 07-29-2013, 03:11 PM
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Quote:
Originally Posted by rogus View Post
can anyone tell me if the quadrajet ever came in a size smaller than 750 cfm? If so how do you tell the cfm rating? i am looking for something around 550 cfm for a recently rebuilt 350 sbc.
Q-jets came in two basic castings, 750 cfm and 800 cfm. The difference is in the size of the primary venturi (shown below). There was a one-year-only Pontiac carb ('71 IIRC) that had a booster ring removed from the primary side that added a bit more flow- at the cost of off idle response. This wasn't used again in production Q-jet carbs.

You can restrict the flow of any Q-jet easily enough. This can be done by adjusting the air valve stop and/or the secondary throttle butterflies to not open fully. That said, you can tailor a Q-jet to work perfectly well on a mild 350 sbc w/o worrying about the cfm. The Q-jet was used on the Chevy V6 4.3L engines in trucks- and most of these carbs had the bigger 800 cfm castings, believe it or not.

If you have the carb number, I can give you a good baseline to work from.

More on the Q-jet here and here.

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Old 07-29-2013, 03:24 PM
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Quote:
Originally Posted by rogus View Post
It stumbles under hard acceleration. The spring tension, should it be increased or decreased?
Increased- IF it's being caused by the air valve opening too quick. But it may be the accelerator pump giving an insufficient shot or the power piston spring too weak, or the secondary choke pulloff is too fast, not enough timing or even too much timing, etc.

Start out by setting up the ignition advance first. THEN work on tuning the carb. Info on setting the advance curve here.

Quote:
if it is a timing problem would that normally be too far advanced or retarded?
Most often the initial advance is too low. But you can't just add a bunch of initial timing w/o seeing where that puts the total timing. This is covered in the link above.

Quote:
One last question. I noticed when I rebuilt the engine the vacum measured at the intake manifold increased to 25 inches while at idle (about 1000 rpm). Can this be part of the problem?
This could be caused by too much initial timing, idle speed too high (and too high of an idle speed can start the mechanical advance to come in). Unless you have a radical cam, the idle speed needs to be lower.
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Old 07-30-2013, 04:17 PM
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I appreciate all the help. Probably won't get to work on the car till friday. Will update then.
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