Quadrajet question

Slooptin · May 4, 2022

I just want to make sure my logic is sound here and I don't waste my time doing something that won't solve my problem. I have a mostly stock 454 (stock internals, long tube headers, carb spacer) and I have an AFR gauge hooked up. I've been trying to dial in my carb and it's consistently inconsistent. I'll set my idle mixture screws and the carb will sometimes be rich, and sometimes be lean.
I've got it leaned out in the idle mixture to try and prevent dieseling at shutdown but it will still read like 13.4-13.8 afr with the screws 1.5-2 turns out. Often, it will "jump" a little where the vacuum drops 0.5-1 inHg and the afr jumps 0.5-1, but it isn't consistent like a misfire or anything and I haven't found a vacuum leak to date. I've also pulled the valve covers with the engine running and verified I don't have a dead lifter or cam lobe or anything. I've had my timing dialed in for a while on a brand new MSD distributor so I don't think it's a timing issue either.
It also loads up when I brake (afr drops to 11.5-12.5) and that will temporarily screw up the idle mixture (makes it run richer for about 10-30 sec before it levels back out to ~14.5).

I'm thinking I've either got junk stuck somewhere in my carb, or my float is set too high and it's letting little bits of extra gas into the manifold. I have a rebuild kit on the way and I'm going to redneck hot tank my carb before I rebuild it to make sure any foreign debris gets cleaned out of all of the passages. Can anyone provide any insight?

AuroraGirl · May 4, 2022

Slooptin said:
I just want to make sure my logic is sound here and I don't waste my time doing something that won't solve my problem. I have a mostly stock 454 (stock internals, long tube headers, carb spacer) and I have an AFR gauge hooked up. I've been trying to dial in my carb and it's consistently inconsistent. I'll set my idle mixture screws and the carb will sometimes be rich, and sometimes be lean.
I've got it leaned out in the idle mixture to try and prevent dieseling at shutdown but it will still read like 13.4-13.8 afr with the screws 1.5-2 turns out. Often, it will "jump" a little where the vacuum drops 0.5-1 inHg and the afr jumps 0.5-1, but it isn't consistent like a misfire or anything and I haven't found a vacuum leak to date. I've also pulled the valve covers with the engine running and verified I don't have a dead lifter or cam lobe or anything. I've had my timing dialed in for a while on a brand new MSD distributor so I don't think it's a timing issue either.
It also loads up when I brake (afr drops to 11.5-12.5) and that will temporarily screw up the idle mixture (makes it run richer for about 10-30 sec before it levels back out to ~14.5).

I'm thinking I've either got junk stuck somewhere in my carb, or my float is set too high and it's letting little bits of extra gas into the manifold. I have a rebuild kit on the way and I'm going to redneck hot tank my carb before I rebuild it to make sure any foreign debris gets cleaned out of all of the passages. Can anyone provide any insight?

get it professionally rebuilt
Also maybe throttle shafts are leaking?

Also... what kind of fuel pump?
Allllsooooo is it the OEM qjet?

fast 99 · May 4, 2022

My first thought is it running out of fuel at the carb. When doing adjustments with an infrared carbs will fluctuate like that if the fuel level is low, bad pump or boiling.
no ethanol fuel?
old fuel?
fuel pressure ?
fuel volume?

Slooptin · May 4, 2022

Its the stock fuel pump driven off the cam, using regular 87 octane pump gas. Not sure about volume or pressure.

I don't think it's the original carb. The number on the side is 17080512 which tells me its from a 1980 my truck and mine's an 85.

I don't think its running out of fuel though. when I pulled the air horn off just now, the float bowl was full which also tells me it's not boiling since I thought that was an issue which is why I put the spacer on

fast 99 · May 4, 2022

The description is a surge or variation. Can't help unless some basic diagnostic procedures are performed. You are just chasing your tail otherwise. How was the fuel level checked by just taking off the air cleaner?

Slooptin · May 4, 2022

fast 99 said:
The description is a surge or variation. Can't help unless some basic diagnostic procedures are performed. You are just chasing your tail otherwise. How was the fuel level checked by just taking off the air cleaner?

taking off the top of the carb

Matt69olds · May 5, 2022

Try disconnecting the vacuum advance (assuming your using it?) and see if that changes anything. If it does, you probably need a vacuum advance canister that has a higher vacuum rating. The idle could be jumping around because the engine vacuum is really close to the rating on the advance canister. The timing might be jumping around a little as the advance reacts to engine vacuum.

Make absolutely sure there are no vacuum leaks anywhere. As Aurora Girl mentioned, leaky throttle shafts are a possibility. Spray a little carb cleaner around the carb base (make sure the air cleaner is on so the engine doesn’t ingest the spray they the carb) and listen carefully for any changes in idle. If the throttle plate doesn’t have bushings, changes are after 40 plus years of use the throttle bores are worn.

Don’t get too hung up on the actual numbers. Give the engine what it wants, not what you think it needs.

Take the truck for a long drive, make sure to run it kinda hard to ensure all the carbon is burned off the plugs. Connect a vacuum gauge, and slowly turn each idle mixture screw in or out a little at a time until you have the highest vacuum reading. Reset the idle speed and fine tune the mixture again.

You might try spraying the idle air bless in the throttle bore.

75gmck25 · May 5, 2022

I’ve had a wide band AFR gauge for quite a while. It’s useful for measuring AFR at cruise or while accelerating, but not that great for tweaking the idle.

AFR changes during off idle throttle or at times when you quickly drop off on the throttle. You can tweak it to a perfect idle AFR, but it will seldom find that exact AFR idle in normal operation. Every time you stop at a light it will be slightly different, so just experiment with about 1/4 to 1/2 turns on the mixture screws to get smooth overall operation. Use the vacuum gauge or idle speed as your guide.
For example, mine is running about 12.5 at idle, but that extra fuel makes it idle better when the A/C kicks on.

Slooptin · May 5, 2022

Matt69olds said:
Try disconnecting the vacuum advance (assuming your using it?) and see if that changes anything. If it does, you probably need a vacuum advance canister that has a higher vacuum rating. The idle could be jumping around because the engine vacuum is really close to the rating on the advance canister. The timing might be jumping around a little as the advance reacts to engine vacuum.

Make absolutely sure there are no vacuum leaks anywhere. As Aurora Girl mentioned, leaky throttle shafts are a possibility. Spray a little carb cleaner around the carb base (make sure the air cleaner is on so the engine doesn’t ingest the spray they the carb) and listen carefully for any changes in idle. If the throttle plate doesn’t have bushings, changes are after 40 plus years of use the throttle bores are worn.

Don’t get too hung up on the actual numbers. Give the engine what it wants, not what you think it needs.

Take the truck for a long drive, make sure to run it kinda hard to ensure all the carbon is burned off the plugs. Connect a vacuum gauge, and slowly turn each idle mixture screw in or out a little at a time until you have the highest vacuum reading. Reset the idle speed and fine tune the mixture again.

You might try spraying the idle air bless in the throttle bore.

Vacuum advance canister is brand new. still the same issues when I disconnect vac advance, just at a lower idle rpm due to timing retardation. I think it may be throttle bushings though, I'll going to check when I get home. I already ordered a "new" quadrajet from SMI for peace of mind plus I want to start from scratch since I feel like I've been chasing carb/fueling issues since I bought this truck.

75gmck25 said:
I’ve had a wide band AFR gauge for quite a while. It’s useful for measuring AFR at cruise or while accelerating, but not that great for tweaking the idle.

AFR changes during off idle throttle or at times when you quickly drop off on the throttle. You can tweak it to a perfect idle AFR, but it will seldom find that exact AFR idle in normal operation. Every time you stop at a light it will be slightly different, so just experiment with about 1/4 to 1/2 turns on the mixture screws to get smooth overall operation. Use the vacuum gauge or idle speed as your guide.
For example, mine is running about 12.5 at idle, but that extra fuel makes it idle better when the A/C kicks on.

As far as the AFR, I figured out my apt screw was not able to fully seat with me turning it with the air horn on. With the top off, I was able to fully turn the apt screw and found out it was 6.5 full turns out from bottom, so that may have been some of the issue. The truck still will surge, but now my afr readings seem more consistent (not as much lean-rich jumping, or not as drastically as before).

I'll heed all that advice for tuning though. This is my first carbureted vehicle that isn't a motorcycle and I'm coming from a lot of diy ecu tuning so i think I just need to get in a different mindset when it comes to all of this

fast 99 · May 5, 2022

Use lean drop method to adjust the idle mix. Turn both screws out until the engine reaches the highest RPM. Then gradually turn them back in 1/4 turn or less at a time. May have to go back and forth a couple times. Final RPM should be around 50 less than highest.

Does this vehicle have an EGR or charcoal cannister? Both should be checked.

Have also seen weak PCV valves causing idle issues. Best PCV is Delco or GM. They don't rattle during operation.

Slooptin · May 5, 2022

I'll try that. No EGR, but it does have the charcoal cannister and the PCV valve does rattle so that may be my culprit

SirRobyn0 · May 5, 2022

@Slooptin I'm not sure how much value I can be to the conversation, because it looks like you have been given great advice so far, but maybe this will help some.

Literally anything that could cause a variation in the burn could be the culprit. Checking the throttle shaft and vac advance is easy. I'll also add that if the welsh plugs in the bottom of the carb are seeping a little, and maybe a bit intermittently that could cause a rich reading from time to time.

I also agree to some degree with @75gmck25 running it a little rich at idle can make that idle more stable, but to rich at idle will equal an overall loss in fuel economy.

idahovette · May 5, 2022

@SirRobyn0 , please define "fuel economy" for a square body........lol......thanks Rob

fast 99 · May 5, 2022

Unbolt the cannister check for liquid by shaking it.

SirRobyn0 · May 5, 2022

idahovette said:
@SirRobyn0 , please define "fuel economy" for a square body........lol......thanks Rob

HA, HA, Yes you are right, no square is an economy machine. For me economy means the best I can reasonably get out of my truck, nothing more. Even 1 MPG for most of us is 10% or so and is enough to add up over time.

Quadrajet question

Slooptin

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AuroraGirl

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fast 99

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Slooptin

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fast 99

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