Author Topic: still cannot control mixture correctly  (Read 482 times)

Offline PonchoVia

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still cannot control mixture correctly
« on: February 11, 2020, 12:56:35 PM »
17058276 Previously installed Cliff's kit, 73 jets, 44 rods, float level 17/64.

OLD SETUP:
(Engine Pontiac 400 0.040+, 96 heads, performer intake, comp 262 cam, HEI with centrifugal coming in at 1000, all in at 2900. I've been using manifold vacuum for advance. 2004r, 9 inch 3.25.)

OLD ISSUE:
Had an issue with idling like crap but driving perfectly. Called Cliff to order a choke pull off, turns out I   didn't need it. Recently installed AFR/wideband found my idle problem was WAY too lean, mixture screws did next to nothing to correct. Added Cliff's recipe to open idle down tubes and mixture holes to add fuel for the old cam which helped the idle quality, but never solved my lean issues.

NEW ENGINE:
461 forged rotating assy from Butler, Kauffman Dports, 310 cfm, Comp roller 232-238 @ 0.050, 113 LSA .590 lift with the 1.65's.

JUST got the engine put back together. STILL cannot control the mixture well at idle. I've got wide bands in both banks. Left shows fairly well around 12.1-5, right shows 15-16-sometimes 17.

Idle vacuum is 15 inches, idle quality is decent.

Swapped o2 sensors bank to bank, same readings.

Sprayed brake cleaner around idle mixture screws, right bank goes crazy, left bank not so bad, but they both get LEANER with the brake clean.

I   am concerned I   have a problem in the carb, and I   clearly do not want to hurt this engine for something silly, but I'm no guru, need some guidance.

Need some guidance for the steps to diagnose
1980 Trans Am WS6
17058276 Q-Jet, Edelbrock Performer,
400 +0.040, Comp xe264h, 8.5 compression
Ram Air exhaust
2004r by CPT, PTFB 9

Offline Cliff Ruggles

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Re: still cannot control mixture correctly
« Reply #1 on: February 12, 2020, 03:34:31 AM »
What size are the idle tubes?

DCR's?

Bypass air?

Idle airbleeds?

Offline PonchoVia

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Re: still cannot control mixture correctly
« Reply #2 on: February 12, 2020, 07:50:04 AM »
 I used .038" idle tubes, .055" DCR's and .110 idle bypass air, all airbleeds are stock.

This was info you passed on to me previously.
1980 Trans Am WS6
17058276 Q-Jet, Edelbrock Performer,
400 +0.040, Comp xe264h, 8.5 compression
Ram Air exhaust
2004r by CPT, PTFB 9

Offline PonchoVia

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Re: still cannot control mixture correctly
« Reply #3 on: February 12, 2020, 11:33:19 AM »
Just found rear vacuum tee loose. I'll fix that and recheck it.
1980 Trans Am WS6
17058276 Q-Jet, Edelbrock Performer,
400 +0.040, Comp xe264h, 8.5 compression
Ram Air exhaust
2004r by CPT, PTFB 9

Offline Cliff Ruggles

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Re: still cannot control mixture correctly
« Reply #4 on: February 14, 2020, 03:13:23 AM »
The rear vacuum T connects to both sides of the intake so it shouldn't cause one bank to show leaner than the other.

Check the power piston hanger arms to make sure one metering rod isn't deeper in the jets than the other.  I've seen hanger arms bent down enough to block off fuel flow to the idle tubes and cause lean running issues at idle speed.  Normally it's not much of a "player" at idle but keep in mind that all the fuel to the idle system has to go past the jets first so make sure everything there is set up correctly.

Also make sure the throttle plates are well sealed when closed and the shaft isn't twisted.  I see quite a few twisted primary throttle shafts from folks taking them apart without holding the parts on the choke side with pliers.  If it takes a LOT of effort to remove the screw on that side of the shaft it can get bent.  This sets one throttle plate open further than the other and messes with idle fuel delivery as different amounts of air is getting down each side at idle and the amount of transfer slot exposure isn't the same......Cliff

Offline 77cruiser

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Re: still cannot control mixture correctly
« Reply #5 on: February 14, 2020, 06:39:46 AM »
Any exhaust leaks on the lean side?
Jim

Offline PonchoVia

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Re: still cannot control mixture correctly
« Reply #6 on: February 26, 2020, 02:58:24 PM »
No exhaust leaks.

Ill check the hanger when I   get the car back from getting another issue fixed.

When I   first got this carb it was a virgin old unit with the calcium deposits in the bottom.

I   cleaned it with ospho which worked great, no I'm wondering if there is gunk in an unsee-able passage.

I'll check all this and reply when I   get it back.
1980 Trans Am WS6
17058276 Q-Jet, Edelbrock Performer,
400 +0.040, Comp xe264h, 8.5 compression
Ram Air exhaust
2004r by CPT, PTFB 9

Offline tayto

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Re: still cannot control mixture correctly
« Reply #7 on: February 26, 2020, 09:59:04 PM »
did you pull idle tubes and clean put passages. i did a marine qjet this past summer and not only were the idle tubes plugged but underneath them were plugged as well.

Offline Cliff Ruggles

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Re: still cannot control mixture correctly
« Reply #8 on: February 27, 2020, 04:09:36 AM »
I've ran into that issue with Marine units as well, plugged solid in and under the tubes.  I've even ran into Marine units with plugged up main nozzles.

They live in a chitty environment so extra care and attention to detail required........Cliff

Offline Cliff Ruggles

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Re: still cannot control mixture correctly
« Reply #9 on: February 27, 2020, 05:37:55 AM »
I'd add here that I'm suspecting a fundamental issue someplace since you really aren't getting good results with either combination.

I use the same carb on my own engine and it is set up nearly identical and it is FLAWLESS now for coming up on 30 years. 

I actually use it as a "dyno-mule" for engines built here and have loaned it out a number of times for folks dynoing Pontiac engines with stock intakes up at KRE.

To date nothing has outran it, dyno or at the track, and every engine I've sat it on idles perfectly and full control of the mixture screws from rich to lean.

So time to go back to basics, including making sure you don't have a secondary throttle plate xxxxed open slightly, twisted primary shaft, idle airbleed too large, or blocked up passage someplace.

Making 15" vacuum would hardly require any modifications to the idle system with that carb other than a slightly larger idle tube and opening up the DCR's to .055" or so.  The big engine will like .110" idle bypass air if you aren't using the hot-air choke system as blocking off that air (converting to E-choke) reduces bypass air some.........Cliff