Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Idle circuit screw response

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raf2330:
So I bought Cliff's kit, and rebuilt my 17055748 for my 78 chevy dually. Truck runs really good. Good throttle response, secondary's kick right in, its working nice when driving. Idles nice also. Problem is when I turn the air screws all the way in there is no response. This is with the idle set at 700 rpm & the timing at 8 BTC as the stinking emissions standards here in CA call for. If I turn the idle down to about 550-600, barely running, the screws start to have some effect. If  I put more timing into it to bring idle up some I won't pass smog, but without screw adjustment I can't tune to pass at higher rpm. I've checked for vacuum leaks, nothing. Had some warping, fixed that. I had the same problem with it at idle before I rebuilt it. So I think its set past the idle circuit and idling on the primary's, am I right? Idles really good. I'm certainly no carb guru, any thoughts would be appreciated. What am I missing?

Frank400:
I'm no expert like Cliff, but if you would add some idle air bypass, you could " back up" the throttle screw, and then maybe your idle mixture screws would have more effect ?

   In other words, to get the faster idle speed you want, give it some air thru the idle air bypass instead of the throttle screw.

  Have you read Cliff's book ?

   Frank.

raf2330:
I have the book. Will look at that. I'm just afraid to start drilling on things and really mess it up, when it comes to emissions in this state. But I'll look at that. Thanks for the reply.

Cliff Ruggles:
Most likely it is getting too much fuel from the idle system from the exposed portion of the transfer slots.  Lowering the throttle plate and starting to see some control indicates it may respond well to more bypass air.  To find out remove a small vacuum hose, re-set the idle speed, and see if the mixture screws gain response.  If the results are positive, add some idle bypass air to accomplish the same thing....Cliff

raf2330:
Removing vacuum hose did gain some response at screws. Did some reading on this, and if I understand how these systems work, this later model APT carb has the bypass air circuit and the orifices are already fairly large, as in recipe 2-3 size. The idle tube is already .040. Maybe because of the truck motor, 454? Guess it really doesn't matter why, but I want to be clear what you are meaning by adding bypass air. If these passages are already this large, are you talking about drilling plates?

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