Idle Mixture Screws Do Not Adjust Anything on 1974 454.

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Joe Baring

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Hello Everyone first post here! I bought a rough, but not rotten 1984 C20 Squarebody! I bought it thinking that it was a 350 small block, but it turned out to be a 305 ): I wanted a bit more get up and go than what it had so I bought a 1974 454 Big Block for it. Once installed in the truck I bought a brand new Ebay Quadrajet measuring 750CFM. (Bare with me! I am still learning about Quadrajets and carbs in genral.) The internet thought 750 CFM was fine for this engine so on it went. Immediately after starting it would not idle without some choke and even with choke it would run stumbly. As soon as you open the choke it would die. After some looking and researching I adjusted the timing and idle mixture screws. Timing went fine and I set it to 12 Degrees with Vacuum advance disconnected. One thing I noticed is that it ran better the more timing I gave it past 12. I tried to adjust the mixture screws, but nothing was happening when I would turn them. Vacuum, engine speed and quality would stay the same. I consistently have 15-17 on the vacuum gauge no matter what I do with the screws. After some playing and some substantial run time I was able to get it to idle without choke. It spits and sputters like its really lean. I assume lean because it gets substantially better and idles much higher when the choke is partially closed. I have not found any vacuum leaks, but I would assume that the leak would have to be very drastic to cause this type of problem. In higher RPMS it runs good! Sidenote that big block is a force to be reckoned with! Nearly broke the tires loose in a hard shift to 2nd...and its stock! I am all out of ideas the carb is brand new, but who knows what that means anymore. The timing is dead set at 12 degrees. The mixture screws do absolutely nothing unless almost taken out. It only runs not awful with choke and if I get it to run without it, it sounds awful. Where should I check first? Reading some of these forum post I can see a lot of you are brilliant when it comes the notoriously misunderstood Quadrajets so your help would be greatly appreciated! I have a lot more work to do on this truck, but I will post about those plans another time. Thanks!
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fast 99

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Know you said vacuum leaks were checked but try disconnecting anything that uses vacuum except advance assume you checked that, brake booster all of it. Check fuel pressure and volume. Was intake and carb base checked for leaks?

Although don't think timing is related to idle issues 12 degrees sounds a little high, what is the total?
 

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What is your idle speed,is your vacuum advance working and connected to manifold vacuum,not ported vacuum? Also spray around carbureator base,and intake ports with carbureator cleaner,listen for idle change,do not use brake clean.
 

Joe Baring

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Here's what I did for a vacuum check. I sprayed carb cleaner around the base of the carb and carb shafts. No change in RPM. What's funny is there is not much change even if I spray it right into the intake. I have not checked by disconnecting and blocking off all vacuum lines I will be doing that shortly. For some reason I did not think of that. (I'm still learning bare with me) The PCV valve should not use vacuum Correct? It is connected to the base of the carb. As for timing my vacuum advance is broken and does cause a major vacuum leak. Currently it is disconnected and blocked off. Because of this I do not know my total timing. If I go anywhere below 12 degrees I have a difficult time getting it to idle, to the point where I wonder if the balencer has slipped on its rubber inside. I will be checking that later. What timing should be/do you run on a big block?
 

fast 99

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Need to fix or check base items before proceeding.

Carb cleaner into carb should have some effect, raise RPM if lean, lower if rich.

PCV works on vacuum. Have seen issues if incorrect one is installed, or flow is too high. If in doubt replace it they are cheap.

Need to fix that advance.

When the throttle plates are open too far because of other problems, carb idle mix screws will have little or no effect. It's sounding more and more like that is what is going on.

Do need to check balancer if it is suspect.

12 degree base timing is a little too much but without vacuum advance total will be low. Has the centrifugal advance been checked for movement.
 

Joe Baring

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Yes. The timing will advance without the advance being hooked up. I am assuming that means the centrifugal advance is working like it should be. I do not know how far it is increasing because my timing light has no adjustment. I can only see timing by the marks by the harmonic tab. What should be my initial timing?
 

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I would verify the distributor is installed correctly by bringing #1 up to TDC and seeing where the rotor is at. Then verify correct firing order.

It was noted if the throttle plates are open too far, it takes it out of the idle circuit and the screws make little difference, this is true and it will never run right. It also would not be the first big block Chevy with a bad cam, something else to look out for
 

fast 99

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Wouldn't dwell on the timing. You need to do some diagnostics to verify or eliminate. 1STLS1 brought something up related to the cam. What is the vacuum at idle and is it steady?

Timing would be safe at 6 degrees, but it is really a mute subject right now. Without vacuum advance and not knowing the total can't properly set initial. You said it won't run there and it should. Something else is going on.
 

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What you said about the timing makes sense and I appreciate your explanation. My vacuum is about 15-16 at idle. It is extremely steady and does not move at all. Vacuum hardly moves while turning idle screws. the only time it increases is when engine speed changes. Unfortunately I will not be able to do more diagnostics on this engine till Saturday. (work can make hobbies difficult sometimes) I will try everything listed in this post and by then I should have some much better information to work with. I am starting to wonder about my throttle blade being open to far.
 

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Here's what I did for a vacuum check. I sprayed carb cleaner around the base of the carb and carb shafts. No change in RPM. What's funny is there is not much change even if I spray it right into the intake. I have not checked by disconnecting and blocking off all vacuum lines I will be doing that shortly. For some reason I did not think of that. (I'm still learning bare with me) The PCV valve should not use vacuum Correct? It is connected to the base of the carb. As for timing my vacuum advance is broken and does cause a major vacuum leak. Currently it is disconnected and blocked off. Because of this I do not know my total timing. If I go anywhere below 12 degrees I have a difficult time getting it to idle, to the point where I wonder if the balencer has slipped on its rubber inside. I will be checking that later. What timing should be/do you run on a big block?
You need to fix the vacuum advance first. It's timing first then carbureation. The vacuum advance will add a bunch of timing at idle. Thus your statement it runs better I'd I give it more advance,yep that's what the vacuum advance is supposed to do. With the timing more advanced ( vacuum advance) you'll be able to close the throttle plates more and still idle,then you'll be in the idle circuit and able to adjust idle mixture. As for timing best is to give it what it wants,but you need your vacuum advance working first and carb dialed in. This will work even if your balancer has slipped for setting total,which is what you want. Vacuum advance disconnected and plugged,put a vacuum gauge on manifold vacuum twist the distributor until you have the highest steady vacuum reading, then twist the distributor clockwise until vacuum drops a pound. See what your timing at idle. Find a stretch of road with 2 land marks about a football field apart. Accelerate dead stop from 1st land mark check your speed at the 2nd landmark. Advance the timing at idle 2 degrees,check again. Repeat until your speed starts dropping or you get spark knock. Retard the timing to lowest timing that gave you best speed or 2 degrees before you got detonation. Do not worry about the number at idle,total is what's most important and you just set it.
 
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Joe Baring

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That makes a ton of sense! Thanks for explaining that. That may just be my issue. I will be able to work on it Saturday and Ill let you know how it goes. Thanks again for both of your help!
 

Matt69olds

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Fix the obvious things first, starting with the vacuum advance.

While the distributor cap is off, make SURE the distributor weights under the rotor are free to move, and return to the rest position. If the weights are sticky or frozen, it’s going to run like crap.

It’s easy to make your own total timing marks with some simple math. Read this:

 

Joe Baring

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Well I'v got updates. The replaced vacuum advance definitely helped the issue! It now will idle with choke off! But it still does not idle smoothly at all. It shakes and misses and just generally runs badly. I have tried everything. I have moved the timing every which way and that did not improve it. The mixture screws still do nothing at all. I can run them all the way in and out and they have little to no effect on the engine and vacuum. (still at 15) I made sure the idle was low enough to be in the circuit to the point where the adjustment screw was not making contact withe the throttle. (Just barley staying alive and screws still have no effect) I checked for vacuum leaks. (PCV, Climate control, transmission, throttle linkage, mounting surface) I could not find one and if there is one hiding its so faint that I do not think it would create this problem. (I could be very wrong feel free to correct me). Engine runs great at higher RPMs, and each cylinder is firing. I did the remove each plug wire and see if it makes a difference trick and engine runs too good in higher RPM's to have a miss. Plugs and wires are new. The only thing I can think of is I have a faulty carb. It was 200 on Ebay and said it was rated for 750CFM. Any ideas or things I missed in checking? I beleive this porblem means the idle curcit is failing to work. Thanks for all the help as always!
 

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Check fuel pressure,what fuel pump are you using,?
 

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OK, lots to read here and I didn't read it all so.... I might have missed if someone already made the suggestion. So have you used carb cleaner, or seafoam or WD-40 and sprayed around the carb base gasket while it's running? I'm wondering if you don't have the wrong baseplate gasket. I've seen it happen several times.

As far as Qjet, they are great carbs. They get a bad wrap due to the couple of known issues they have. Once you handle those issues permanently, then you have a great carb. Well plugs need fixed, usually coverred in epoxy to sell them permanent, and then use bronze bushings to bush the throttle shafts where they tend to wear the base plate and have an ever lasting vacuum leak.
 

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