Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: jvmagic on February 01, 2018, 02:22:42 PM
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Hi,
I took my Q-jet carb in to be rebuilt (took carb off car) and after installation, the car idles high (2500) when cold and then dies when I hit the gas pedal to bring down the rmp. It will start again and I have to keep my foot on the pedal then it dies once I let off.
Carb was bench tested.
What's the ideal idle rpm for a cold start.
The other issue is why does the car die after I tap the gas pedal to bring the idle speed down (after 30 seconds or so of warming up)?
Video of car idling when cold https://youtu.be/bMsRMLBSkpY
Thanks in advance.
*1979 Trans Am 403 motor
*Q-Jet
*Everything is stock
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Sounds like a vacuum leak. Did you get all the hoses hooked up?
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as jim says vacuum leak , and throttle stop set to low ,with a vacuum leak as soon as you pop the throttle the chock will open and go extremely lean causing it to die.fwiw i like the phrase bench tested . no pun intended.
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There is not a good "bench test" for one of these units, and it really needs all adjustments made on your engine anyhow.
Even with that said it should run well enough if the basic settings were made or it was just left alone or idle mixture screws, fast idle and curb idle set back to the stock settings.
I do agree the symptoms indicate a pretty big vacuum leak, so correct that issue first, then things will start to go better for you.......Cliff
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Thank you all for your responses.
I forgot to mention a couple of things that now came to mind. The hot choke tube (I believe it's called) is broken. The link below is not of my car but it's to reference where the tube is broken. Would this cause the carb to behave the way it is? I already ordered a replacement.
Also, the shop gave me a carb base gasket that is much thicker than the oem. Is this an issue besides making a vacuum hoses stretch further than normal (hose going to canister)?
The only vacuum that was not connected from what I saw, was the hose going to the air filter housing. I connected this and it didn't make a difference.
Lastly, there is a strong gas smell.....enough to cause my eyes to get irritated (car in garage; garage door open of course).
http://s149.photobuc...xs1zpo.jpg.html
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We stock the correct carburetor to intake gasket for that application. I would not use a thicker one, it is always best to use the correct one.
A broken hot air supply tube will not effect idle speed or carb function, but it does allow unfiltered/dirty air into your engine, so don't operate the vehicle until the new one is in place.
If you verify no vacuum leaks, and the carb still smells of gas and not working correctly, and doesn't respond to making adjustments to the idle mixture and speed screw, then it's going to have to come back off and taken back apart to correct the issues with it......Cliff
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Found a vacuum leak and corrected the issue and now the car idles very nicely....very smooth!
Thanks for all the advice.
Onto the next issue for this 79' Trans Am.
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Good news!