Cliff's Quadrajet Parts and Rebuild Kits

Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: raf2330 on June 12, 2017, 05:02:47 PM

Title: Idle circuit screw response
Post by: raf2330 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?
Title: Re: Idle circuit screw response
Post by: Frank400 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.
Title: Re: Idle circuit screw response
Post by: raf2330 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.
Title: Re: Idle circuit screw response
Post by: Cliff Ruggles 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
Title: Re: Idle circuit screw response
Post by: raf2330 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?
Title: Re: Idle circuit screw response
Post by: blazer74 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.
Title: Re: Idle circuit screw response
Post by: raf2330 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.
Title: Re: Idle circuit screw response
Post by: raf2330 on June 24, 2017, 10:20:35 AM
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.?
Title: Re: Idle circuit screw response
Post by: 77cruiser on June 24, 2017, 03:40:23 PM
How far are your throttle plates open now? I'd guess if you went up to ..090 you see some affect.
Title: Re: Idle circuit screw response
Post by: raf2330 on June 24, 2017, 04:48:45 PM
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.
Title: Re: Idle circuit screw response
Post by: 77cruiser on June 24, 2017, 05:51:44 PM
Is emission testing required? If not you can up the timing & then you'll be able to close the throttle some.
Is the vacuum advance hooked to ported or manifold vacuum? Guessing it's supposed to be ported for emissions.
Title: Re: Idle circuit screw response
Post by: raf2330 on June 24, 2017, 07:19:22 PM
Yes on emissions. Have tried messing with timing as you stated. Have it to manifold normally. Put it on ported for test. Either way I've had this problem since going to the manifold and very mild cam (cam just to match Edelbrock 2.0 so it would still pull), just enough to mess me up on proper idle the last couple of years (doesn't get driven much at all) and be a pain for emissions, although we have made it pass, barely. I want to get it into the idle circuit at idle.
Title: Re: Idle circuit screw response
Post by: blazer74 on July 02, 2017, 10:21:36 PM
Better to go up gradually on the bypass air.
Time consuming yes.
Harder to go back smaller, you would need to bush and re drill
Or add screw in set screws from Cliff.
My guess would be .086 to start but I'm a novice.
Title: Re: Idle circuit screw response
Post by: raf2330 on July 04, 2017, 10:11:20 AM
Thanks again guys for the reply's. I have gone up gradually. It is responding some to the air screws now. Both shut will bring RPM's down, but not kill it. This is where I'm at.

Bypass air: .090 (stock was .050)
Idle Mixt. hole: .078 (stock was .050)
Upper air bleed in air horn: .110 (stock)
Lower air bleed: .060 (stock)
Idle tube: .050 (stock)
Idle down channel: .040 (stock)
Changed hot air choke to electric, plugged would be vac. leak.

So right now the screws are out 1 turn and my AFR meter/gauge says I'm a little rich, 13.5, so I feel like I need to open up the bypass air a little more. Definitely have more control, but feel like I need more air. Am I on the right track?
Title: Re: Idle circuit screw response
Post by: 77cruiser on July 04, 2017, 02:43:55 PM
Sounds like the right track. When you have the idle rpm where you want it, how much transfer slot is showing?
Title: Re: Idle circuit screw response
Post by: blazer74 on July 05, 2017, 04:44:07 PM
.050 idle tube doesn't sound correct. That would be a lot of idle fuel and the down channels are only  @ .040

To properly measure the idle tubes you need a 2" long # drill bit (Taper Length they are called.
Other wise you would need to remove the tube to measure the restriction which is at the lower end.

You refer to upper idle bleeds in the air horn that would actually  be the main air bleeds @ .110
The upper and lower air bleeds are in the float bowl housing. uppers at the top above the down channels and the lowers in the actual bore itself.

Title: Re: Idle circuit screw response
Post by: raf2330 on July 06, 2017, 08:13:20 PM
Switched the numbers, oops. .040 idle tube. Had them out during rebuild and checked with Cliff as he  sent me a pair with .036 in kit. He told me to put the .040 back in. Thanks blazer74.

77cruiser, I don't know about the slots. I'll check when I pull it off this weekend.
Title: Re: Idle circuit screw response
Post by: raf2330 on July 09, 2017, 08:53:09 AM
Been messing with truck this morning and think I have it to where it will pass the smog right now, but would like some input as I have not used WB's, A/F gauge before this. I did not pull the carb to check transfer slots, so not sure where I'm at but it idles good and I still have response from the screws. They are out 1 1/4 right now and I raised my PP a turn. So at idle now my A/F is 14-14.5 and at 2500 RPM, A/F is 15+-. This truck is HD/454 and gets tested at idle and 2500 RPM in CA. I think those #'s are OK to pass, am I right, close? I have ordered more bits to be able to open bypass air in smaller increments if needed. But seems to be working.

So smog aside, just driving the truck it feels good. When I leave the line the A/F is good 14.7, but getting up to speed and cruising in leans out to 16.8-17. Is that OK? When I go into secondary it feels good and A/F goes to 12.8-13, but I would think I would want it a little rich there for towing.?

One of these days I will get out of this state and be able to have some fun with this truck.

Any thoughts? Thanks for your help guys.
Title: Re: Idle circuit screw response
Post by: raf2330 on July 15, 2017, 05:05:49 PM
So I didn't do anything else to the carb and just went ahead and took the truck to my smog test guy. The #'s on my A/F gauge matched his and truck passed no problem. So with Cliff's kit, taking care of warping, a lot of tinkering and this new AF gauge/meter I feel confident in keeping this thing running correctly for a long time. Really nice to NOT have it fall on it's face when I mash the pedal and it transitions into the secondaries smoothly. May still need a little more bypass air at idle for more screw control, but my big problems are gone and I may just leave it alone. Thanks for all the help guys.
Title: Re: Idle circuit screw response
Post by: Hillbillyenginering on October 12, 2017, 09:53:09 PM
You asked if your numbers are OK at 16.8/17.
17.2 is stigmatic. Which means it is at maximum efficiency of burning air to fuel. If the numbers rise it goes into lean burn. Cruise and accel numbers are good. As long as you don't see signs of lean burn or experience ping\knock your good. Doesn't get much better than you have.
Your hampered by strick emission standards.
If you pull a long hard grade. Get out and look at the exhaust manifolds and header pipe. If they aren't glowing red your spot on and not going lean.
I worked on natural gas ford 460's. The exhaust glowed red about three feet back for at least a half hour after its run.