Hello, Everybody
So I have a 1979 Vette, L48, auto, all stock. California smog time. Cannot idle at 500 rpm for the life of me. Will not drop to the 500 rpm as required for smog at 4 degrees btdc. The timing is at 4 degrees but the rpms are at 700. My bad carburetor at least idled at 500. So checked the fast idle, works fine. cold start 1500. Drops to 700 when driving. Car runs beautiful. I backed off the idle screw. Not even touching. mixture screws are like 3 turns out. even all the way in she still idles at 700. Go figure! I checked the linkage at least 10 times. I also checked for vacuum leaks and found nothing. I know I am missing something here. Please help!
Thanks, Steve

 

With that retarded timing you are probably not idling on the idle circuit hence the lack of adjustment on the screws

 

Not sure about California requirements but the 500 rpm idle requirement is in gear not neutral right? That would be a very low idle if not...

 

Is this a replacement carb of some type..?

 

I see from your previous post that the carb you're running appears to be a "remanufactured" carb by Holley. As noted in my previous response, any Holley reman carb is going to have a massive amount of problems that you're not going to be able to fix. You are much better off having someone competent rebuild and test your original carb that came off the engine. If the well plugs are leaking (which is a very rare case), they can be fixed.

Lars

 

One of the things I've noticed on after market PVC valves.
They suck (no really) way too much...
As designed, they are a calibrated (meaning glorified) vacuum leak.
Pinch off the hose to it and watch all your problems go away.
I limited the available vacuum source so it would pull Neg hg, but
at a manageable level.
And all the adjustments became useful again As they can work in their
designed perimeters.............

 

This is the initial timing and rpm I have to achieve.

 

​​​​​​​I got it from Corvette Central. For or supposedly for a 78 and 79 L48 stock motor.

 

This is a supposed quadrajet replacment for my car. Parts mixed and matched. Runs nice though. Just the idle won't drop to 500 rpm. It's at 700 and way lean at idle. Or I think. Suspect Jet Performance Stage 1 type but sold as Corvette Central product.

 

That's the problem. It will never run right. It's a badly-built commercial carb. I've had dozens of those come through my shop, and I know exactly what you have. Get a good carb properly set up - you can fight that one you have until the end of time. No Internet advice or opinion is going to solve that problem.

 

loosen the butterfly screws and center the butterflies in the base plate bores.

 

He's got more problems than that. The issues are:

• High idle that can't be lowered
• Ineffective idle mixture screws, indicative that the carb is idling on the main metering circuit
• Lean idle (which is probably a "rich" idle if it's not idling on the idle circuit)

This is indicative of all of the classic problems associated with the Corvette Central rebuilt carbs - I've seen them several times, and they're pretty messed up. If he wants to retain that carb and pass emissions, it will need a complete teardown, and all of the problems will need to be corrected. This will involve replacement of several major components of the carb, and is usually not a cost-effective solution.

It's not "running nice" if idle cannot be controlled and the idle mixture screws are completely ineffective. That's running "all messed up," and it's not right... My suggestion: Have someone go through your original carb and set it up right. And get rid of the commercially rebuilt doorstop. Jet Performance does not build the carbs for CC.

Lars

 

Here is a simple list of the common commercially rebuilt carb problems that I always see. This is from my paper titled, "List of Problems I Have Seen with Commercially Rebuilt Q-Jets" and gives you an idea of what you're dealing with on the OPs carb:


List of Problems I have Seen with Commercially Rebuilt Q-Jets
by Lars Grimsrud
©2017 Lars Grimsrud.

Introduction
As the Q-Jet carbs keep getting older, fewer and fewer are original carbs that have not been tampered with or altered. Rebuilding a Q-Jet is becoming a significant problem for the novice, because the carbs cannot be simply bolted together the way they came apart: The carbs have been altered, and these alterations must be identified and corrected before the carb will run as it was designed to do.

The following is a simple, itemized list of the problems I typically see with these carbs. This list compliments the paper titled, “Problems I Have Seen with Commercially Rebuilt Q-Jets.” I have never seen a commercially rebuilt carb with less than at least a half-dozen of these problems, and a good portion of this list was compiled from inspection and repair of carbs coming from some of the major Corvette parts suppliers, such as CC:

• Altered (plugged or enlarged) idle air bypass channels
• Bent & altered secondary throttle connect linkage not allowing primary throttle to fully close
• Throttle blades misaligned in bores
• Bent throttle blades
• Bent APT needle (1975)
• Extreme low APT setting (low power piston height)
• Extreme rich jetted (often with “No-ID” “generic” jets so you can’t tell)
• Extreme lean jetted (often with “No-ID” “generic” jets so you can’t tell)
• Loose throttle plate, resulting in significant vacuum leak
• Bent primary throttle shaft & lever, resulting in binding preventing full closure of the throttle
• Stripped inlet threads “fixed” with self-tapping fitting
• Bent, destroyed or missing secondary lockout lever
• Sandblasted finish with all original plating removed and destroyed
• Extreme low or high float
• Incorrect float installed, or float jamming in the bowl
• Secondary airvalve loose or sticking/jammed
• Incorrect power piston spring installed
• Bent primary rods from incorrect attempt at installation into the jets
• Accel pump rod damaged, bent or broken
• Secondary throttles set to open over-center
• Broken/cracked throttle plate from bad installation of shaft bushings
• Plugged idle fuel tubes, forcing carb to idle on the main discharge circuit
• Bent secondary airvalve linkage & lever.
• Warped airhorn, resulting in fuel transfer passages not allowing idle fuel to transfer to the idle circuit
• Incorrect rods – both primary and secondary. “Generic “no-stamp” rods are often installed so you can’t tell
• Accel pump jammed in bore – inoperative
• Float clip installed backwards, jamming float
• Fuel filter spring missing, allowing debris to enter carb, jamming the needle/seat
• Power piston stop pin cut off - piston destroyed
• Secondary discharge tubes missing
• Secondary airvalve windup spring incorrectly set
• Bent/destroyed/altered/incorrect choke intermediate rod
• Secondary rod hanger bent or destroyed
• Secondary rod cam missing or broken
• Stripped Needle/seat threads
• Incorrect major components mated to a “correct” stamped float bowl
• Incorrect choke system for the model/series/brand carb
• Warped float bowl
• Incorrect or missing air bleeds in the idle system and main discharge system

 

also the wrong gasket between the throttle base plate and the main body will cause a vacuum leak and cause high idle speed. since a lot of rebuild kits include both you have to check to make sure you use the correct gasket

 

Your shop? I have not been on the forum for some time. You still in the rebuilding business? I searched the web prior to this awful purchase, everyone states you stopped building carbs. I hope this is not true.
First, thank all of you for your input. It means a lot to me. Carb issues for 3 years now! Augh!
The Holley rebuilt quadrajet, I purchased Cliffs book. Purchased a rebuild kit. Took the Holley apart. One air horn screw behind the primary would not come out. Those idiots torqued it down bad. Oversized tools to loosen the stripped screw. Rebuilt the carb myself. First ever. Big fail. I did see however the primary and jets were wrong as we're the secondaries. Used old parts put it together. No issues but.. airhorn gasket was saturated w gas. The torqued screw to the airhorn was and is stripped. Assume core is crap unless can be tapped to a larger size. Idiots! Besides the stripped body for that screw (possible fix) and the gas saturating the gasket, it idled proper, smooth, I did have to open it a few more times for the apt adjustment due to bog.
So Lars, if you are willing, I can send it to you for repair? I would send what I rebuilt and another core body which I believe isn't altered. If so I will send it to you. The parts are the correct proper parts.

​​​​​​​Thanks, Steve

 

Why don't you drop me an e-mail and we can discuss off-line. I don't rebuild those commercially rebuilt carbs due to the amount of damage and alterations, but I may be able to use some undamaged parts off it. If you have an original, non-altered carb, that would be your best bet.

Lars
V8FastCars@msn.com