Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Idle circuit screw response

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blazer74:
Take another look at Cliffs book. Idle bypass in a function of the base plate/throttle body.
Each primary bore in the base plate has a bypass air channel drilled adjacent to the throttle plates and then turns upward to the float bowl. They are on the fwd side of the bores.

The bypass air could be around .086 for example, depends on your setup. DO NOT  drill your plates. You can enlarge these bypass channels to the next numbered drill bit size and check for improvement.

These channels are exactly what they are called and allow air to bypass the calibration circuits to add air to the mixture below the throttle plates when at idle leaning the mixture and increasing rpm to where you can back out your curb idle screw to allow your throttle plates to close further.

Basically gives you higher idle with less throttle plate opening keeping the main circuit out of the mix giving back control of your idle mix screws.

This has nothing to due with your idle channels or tubes at this point.

Also the early carbs also can have the idle bypass not just later carbs with the APT at the top.

raf2330:
Blazer74, thanks for the response. I won't drill plates. I'll have time this weekend to pull the carb and check out the bypass channels. What you've suggested makes more sense to me now after reading a lot more this week. Will post findings. Thanks again.

raf2330:
OK guys, so I pulled the carb (17055748, which is not original to this truck, someone replaced before I got the truck 20 yrs ago. # suggest Buick carb.) and the idle bypass air in the base plate is .050-.052 as best I can measure. I have a 17059513 from a 79 pickup that has bypass air of .110, this plate still has the factory caps in it over the air screws, so except for someone putting a kit in it, I don't think  it's been modified. I say all that to ask if I should open mine up to the .110 or go up in small increments? Maybe I should just rebuild the 9513 and put on truck.?

77cruiser:
How far are your throttle plates open now? I'd guess if you went up to ..090 you see some affect.

raf2330:
The slots are almost completely exposed. Plates are just at the top of them. I will open up the bypass  some tomorrow and test. Thanks for the help.

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