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Old 10-05-2019, 05:20 PM
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Default Quadrajet stumble off idle

I have a 7040264 Quadrajet on my stock YS 69 GTO engine. It idles good and does fine cruising. However, there is a stumble just off idle when I take off. The calibration is stock on the carb. Does anyone have any suggestions as to what I might need to modify to make it stop stumbling and having a flat spot when I take off?

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Old 10-05-2019, 06:43 PM
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Stock setup?

What have you checked, so far?

Any crap in the float bowl?
Maybe take the check ball out squirt some carb spray there.
Shoot some carb spray into the idle tubes.
Blow all the passages out, after generous spritzing with carb spray.

Have you shortened the acc pump shaft?
Acc pump diaphragm OK?

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Old 10-05-2019, 07:01 PM
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What are the degrees if timing fir initial, and vacuum advance, and which port is the vacuum advance on, ported (wrong), or, full manifold vacuum (correct when the degrees are set right).?

Reason I ask, if the timing is not right, the whole ball of whacks goes out the window and doesn't work right.

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Old 10-05-2019, 07:37 PM
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Accel pump material maybe failing from the gas

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Old 10-06-2019, 02:36 AM
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Thanks for the replies. The engine is a completely stock YS 10:1 69 400. My total timing is about 36 degrees. It did have a lot worse of a stumble so I replaced the umbrella on the accelerator pump today. The old pump umbrella was maybe 5 years old. I have never run ethanol fuel through it. The old umbrella didn’t look horrible. The stumble is nowhere near as bad but it is still present just off idle when I take off. The inside of the carb did look very clean. I blew all of the passages out with carb cleaner and compressed air. One thing I did not mess with was the little screw under the metal plate on the airhorn. I’ve worked with the air mixture screws but that did not help. The accelerator pump rod is in the inner slot on the arm. It did appear to be wore some. It idles great and runs great cruising. The flat spot taking off is just annoying.

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Old 10-06-2019, 03:09 AM
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Two things you can try:
Grind an 1/8 of an inch off the top of the accelerator pump shaft or if you have extra carb parts take the lower accelerator pump spring from the spare carb and put it inside of the upper spring of the one you're using. Willing to bet one of these methods will work.

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Old 10-06-2019, 03:56 AM
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The little screw under the metal plate on the airhorn is used for off-idle calibration. If open it adds air to the main circuit on 1970-74 Pontiac Q-jets.
Most if not all hesitation driving is due to lean carburetor calibration.
You may first try to set the off-idle screw closed lightly seated to make off-idle somewhat richer.
This may help some, but in these units the idle tubes are really too small for todays fuel, so i recommend pulling the tubes and enlarge the openings from .029" to at least .033" and at the same time open the idle down channels from .046" to .052" to cure the off-idle issues.
Also, with this re-calibration you will find the factory ignition settings working properly.

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Old 10-06-2019, 10:31 AM
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Quote:
Originally Posted by John Milner View Post
I have a 7040264 Quadrajet on my stock YS 69 GTO engine. It idles good and does fine cruising. However, there is a stumble just off idle when I take off. The calibration is stock on the carb. Does anyone have any suggestions as to what I might need to modify to make it stop stumbling and having a flat spot when I take off?
John, I had this issue and my cure was to re-seat the check ball located under the accelerator pump…

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Old 10-06-2019, 08:28 PM
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Quote:
Originally Posted by 72LuxuryLeMansLa. View Post
John, I had this issue and my cure was to re-seat the check ball located under the accelerator pump…
That also can be an issue...

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Old 10-06-2019, 10:34 PM
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"36 degrees" isn't saying much. What is the initial timing, and how many degrees does the vacuum advance have?

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Old 10-06-2019, 11:00 PM
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Quote:
Originally Posted by 72LuxuryLeMansLa. View Post
John, I had this issue and my cure was to re-seat the check ball located under the accelerator pump…
That is one thing that I didn’t check when I had the top off of it this weekend. I’ll re-seat the check ball next time I work on it and see what that does. I am right at 15 initial on my timing as well. It seems to start fine and run fine with my timing there. The stumble is there right off idle.

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Old 10-06-2019, 11:22 PM
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Quote:
Originally Posted by FrankieT/A View Post
Two things you can try:
Grind an 1/8 of an inch off the top of the accelerator pump shaft or if you have extra carb parts take the lower accelerator pump spring from the spare carb and put it inside of the upper spring of the one you're using. Willing to bet one of these methods will work.
These will give you more pump shot...usually a stumble off idle is a lean condition.

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Old 10-15-2019, 11:48 AM
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Quote:
Originally Posted by John Milner View Post
That is one thing that I didn’t check when I had the top off of it this weekend. I’ll re-seat the check ball next time I work on it and see what that does. I am right at 15 initial on my timing as well. It seems to start fine and run fine with my timing there. The stumble is there right off idle.
Any progress John?

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Old 10-15-2019, 12:14 PM
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Quote:
Originally Posted by 72LuxuryLeMansLa. View Post
Any progress John?
I completely disassembled the carb this weekend, blew out all passages with carb cleaner and compressed air, re-seated check ball and put everything back together. I re-checked the float and it was set at 1/4 inch. I drove it around this weekend and the stumble is still there. I am going to try to grind some off of the accelerator pump next and see if that helps. It will probably be next weekend before I get the opportunity to do that.

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Old 10-15-2019, 12:21 PM
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How's it jetted & rod?

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Old 10-15-2019, 12:22 PM
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Guess you didn't get a chance to try Kenth's recommendation for the idle circuit?

With todays ethanol fuels, they have a richer stoich than the good gas does, and as a result the carbs need to be recalibrated a bit richer than the factory had them 50 years ago. Cures a bunch of funky drivability issues that I see pretty frequently these days.

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Old 10-15-2019, 12:29 PM
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Try ported vacuum if you havent already. My vacuum advance is limited to 10 degrees and with manifold vacuum it would give that same off idle stumble as yours does, but switching to ported it totally cured it.

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Old 10-15-2019, 12:43 PM
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Quote:
Originally Posted by shaker455 View Post
How's it jetted & rod?
It has 70 jets and 41 rods in it. As far as I know, I believe that was how it came stock. I have not pulled the idle tubes yet either to see about opening them up some. I forgot to try the little air screw under the metal plate on the airhorn. I'll play around with that next time I work on it.

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Old 10-15-2019, 06:21 PM
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John Milner, Great movie American Graffiti!

If your completely stock then about 73-74 jet is a better place with your 41 rods & today's fuel.

Simply try that and see where your at.

Also, good idea to know all your restriction sizes & airbleeds.

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Last edited by shaker455; 10-15-2019 at 06:30 PM.
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Old 10-15-2019, 06:35 PM
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Accelerator Pump Shot...

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