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Old 07-18-2019, 11:11 AM
many birds many birds is offline
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Default Q-Jet Adjustable Air bleed question

What does the adjustable air bleed in the air horn on older Q-jets affect? I’m talking about the one under the triangular cover that is adjusted with a small flat screwdriver.

How do I go about correctly adjusting it?

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Old 07-18-2019, 11:43 AM
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Is that a air bleed or does it adjust the idle postion height of the metering Rods like in the later but non computer Carbs?

If it truly is a air bleed then I can't see how you will get it adjusted as needed but for having a Ox sensor or sniffer in each tailpipe.

I have played around with the big air bleed screw on the face of the original AFB Carbs and on those I think the base line starting point was 3 turns open .

If you have not touched its stock setting an you now have a bigger Cam in the motor and or a bigger cid motor then I would do this.

Flip the Carb over and set the throttle blades to just enter the transistion slot in the base plate of each throttle Bore, then bolt it back on the motor and see at what rpm it idles , if it's too slow then raise the idle speed setting by no more then 1/2 a turn,
If at that point the idle is too slow then open up that idle air bleed 1\2 turn at a time and make note of how much the rpm comes up with each 1\2 turn and report that info back here.

This all assumes that the motor has enough timing to atleast be in the ball park for what's needed and that you have no vacuum leaks screwing things up.

Maybe chase down Cliff and he can give you some tips on where to start with this.

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Last edited by steve25; 07-18-2019 at 11:53 AM.
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Old 07-18-2019, 12:33 PM
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If you're talking about the tiny screw near the bowl vent it affects a couple things depending on the year.

Earlier ones affected the upper idle air bleeds and later ones affected the main air bleeds.

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Old 07-18-2019, 01:20 PM
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What is the carburetor part number?....

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Old 07-18-2019, 01:57 PM
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Quote:
Originally Posted by Cliff R View Post
What is the carburetor part number?....
It’s a 7043266. Modified per recipe #2 in your book.

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Old 07-18-2019, 02:37 PM
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The adjustable airbleed screw on those leads to the main airbleeds. They are pre-set at the factory to precisely calibrate the carb using the stock parts and settings......Cliff

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Old 07-18-2019, 02:51 PM
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Quote:
Originally Posted by Cliff R View Post
The adjustable airbleed screw on those leads to the main airbleeds. They are pre-set at the factory to precisely calibrate the carb using the stock parts and settings......Cliff
So, that’s out the window, now the carb and the engine are modified. And I’ve loosened and tightened it a dozen times and have no idea what previous owners may have done with it. What should I do? Is there any way to tune it?

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Old 07-18-2019, 03:55 PM
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You may use the "tip-in" test at 2200 rpm´s to fine-tune the main metering jetting with this adjustable part throttle screw.
Start with the screw closed.

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Old 07-18-2019, 04:49 PM
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I would gently seat it and set the carb up exactly for the engine combo and not worry about it.......Cliff

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Old 07-19-2019, 05:49 AM
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Cliff, so on these year Carbs with that type of adjustment point that is a fine adjustment to the main air bleeds on the factory calibration?

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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs!
And he was not talking about 1/8 or 1/4 mile ETs!

1) 1940s 100% silver 4 cup tea server set.

Two dry rotted 14 x 10 Micky Thompson slicks.

1) un-mailed in gift coupon from a 1972 box of corn flakes.
Two pairs of brown leather flip flops, never seen more then 2 mph.

Education is what your left with once you forget things!
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Old 07-19-2019, 07:01 AM
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The 73-74 Pontiac (non Super Duty) carbs were pretty lean anyhow, but there are three different configurations used all with a slightly different calibration. I don't pay any attention to all that and simply seat the adjustable airbleed screw and recalibrate them for what they are going to be used on. Even with a stock calibration on a stock engine (pretty rare occurrence) they will be too lean on this new fuel........Cliff

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Old 07-19-2019, 07:26 AM
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Thanks!

Got ya on that!

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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs!
And he was not talking about 1/8 or 1/4 mile ETs!

1) 1940s 100% silver 4 cup tea server set.

Two dry rotted 14 x 10 Micky Thompson slicks.

1) un-mailed in gift coupon from a 1972 box of corn flakes.
Two pairs of brown leather flip flops, never seen more then 2 mph.

Education is what your left with once you forget things!
  #13  
Old 07-19-2019, 09:34 AM
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I tried the “tip in” at 2200 last night with the screw seated. The rpms go up a just a little. I’m guessing this means the jetting is close or maybe just a tad on the lean side.

Then, just for giggles, I loosened the air bleed screw 3/4 turn and took the car for a gentle spin at up to 60 mph. Seemed like it had less throttle response—just seemed softer even when climbing hills. No impact on idle at all. I did not try any full throttle testing. I wonder if it had any effect on gas mileage.

Did the carbs that had APT also have this air bleed adjustment?

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Old 07-19-2019, 09:55 AM
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It does have a very minor affect on fuel mileage and the overall air fuel ratio. I can watch the AFR change on my wide band when turning the screw. It makes small changes but it's there.

Just an extra tuning tool. Like Cliff said, not really needed if the rest of the carb has been recalibrated for the combination.

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