Author Topic: Idle circuit screw response  (Read 109 times)

Offline raf2330

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Idle circuit screw response
« on: June 12, 2017, 05:02:47 PM »
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?

Offline Frank400

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Re: Idle circuit screw response
« Reply #1 on: June 12, 2017, 05:19:53 PM »
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.

Offline raf2330

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Re: Idle circuit screw response
« Reply #2 on: June 12, 2017, 05:38:11 PM »
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.

Offline Cliff Ruggles

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Re: Idle circuit screw response
« Reply #3 on: June 13, 2017, 02:16:24 AM »
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

Offline raf2330

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Re: Idle circuit screw response
« Reply #4 on: June 18, 2017, 08:08:11 AM »
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?

Offline blazer74

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Re: Idle circuit screw response
« Reply #5 on: June 19, 2017, 09:37:15 PM »
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.

Offline raf2330

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Re: Idle circuit screw response
« Reply #6 on: June 20, 2017, 08:54:07 PM »
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.