How to Set Quadrajet Idle Mixture Screws on a Classic Chevy (and Get the Choke Right)

If you want to know how to set Quadrajet idle mixture screws on a classic Chevy, here is the short version. Get the engine fully warm with the choke wide open and the ignition timing already right, hook a vacuum gauge to a manifold vacuum port, and set both mixture screws on the front of the carb base the same number of turns out. Then turn each one in slowly until the vacuum (or rpm) just starts to drop, and back it out about a quarter to a half turn until the idle smooths out again. Reset the curb idle to the number on your underhood decal or in the service manual, and you are done with the idle. The choke is a separate job: make sure the blade closes fully when cold, the vacuum break pulls it open a crack the moment the engine fires, the fast idle cam steps down as it warms, and the unloader cracks the choke when you floor it during cranking.

That is the whole job in one paragraph. The rest of this article is about doing it right the first time, on whichever Quadrajet you actually have, and about what it means when the screws do not do what they are supposed to do. Because a lot of the time, a Q-jet that “won’t idle right” is not a mixture problem at all.

I have been fiddling with these carbs since I was old enough to borrow my dad’s screwdriver without asking. The first one I ever really learned on was on a ’72 Chevelle with a tired 350 that a neighbor had already “tuned” by turning everything on the carburetor at least twice. Both mixture screws were out different amounts, the choke rod had been bent into a shape I still cannot describe, and the fast idle screw was cranked down so far the car idled at about 1,200 rpm hot. It took an afternoon, a borrowed vacuum gauge, and a phone call to an old Chevy dealer mechanic to get it sorted. Once it was set properly, that carb started on the first crank on cold mornings and idled like a sewing machine. The Quadrajet was never the problem. The people adjusting it were.

First, figure out which Quadrajet you have

Before you touch a screw, know what you are working on. Rochester built Quadrajets for decades, and the adjustments are similar but not identical across the generations. According to Cliff Ruggles of Cliff’s High Performance, who has probably rebuilt more of these carbs than anyone alive, Rochester used a simple code system:

• 4M series: the original design, found on GM cars through 1974 and on GMC and Chevy trucks through 1977.
• M4M series: the “Modified Quadrajet” or “Mod-Quad,” introduced in 1975.
• E4M series: the electronic, computer-controlled Quadrajet that showed up around 1980.

The last letter tells you the choke type. A V (as in 4MV) means a divorced choke, with the thermostatic coil mounted on the intake manifold over the exhaust crossover and a rod running up to the carb. A C (4MC, M4MC) means the choke coil lives in a housing on the side of the carb itself, heated by hot air. An E at the end (M4ME, E4ME) means a carb-mounted electric choke. A carb with no letter after the 4M or M4M had a manual choke.

Most classic Chevy passenger cars from the late ’60s through 1974 with a factory Quadrajet carry a 4MV with the divorced choke. That is the carb most people reading this will have on a Chevelle, Camaro, Nova, Impala, or Corvette of that era. But do not assume. Look at the passenger side of the carb. If there is a round black cap with index marks on it, you have a carb-mounted choke. If there is a thin rod going down to a little metal box on the intake, you have a divorced choke.

It is also worth knowing that the other GM divisions did not all follow Chevy. High Performance Pontiac (on Hot Rod’s site) notes that Pontiac ran a divorced choke through 1972 and moved to a carburetor-mounted choke coil in 1973. Cliff points out that Buick kept manifold-mounted chokes through 1974. Oldsmobile used the 4MC with the choke housing on the carb on many of its engines, including the 1970 Cutlass. From 1975 on, almost every passenger car Quadrajet went to a choke housing on the carb, though Lars Grimsrud notes that heavy-duty truck carbs kept the divorced setup after 1975. So if your Chevy has a swapped-in carb from a Pontiac or a later truck, the choke section below may not match what your factory manual shows.

One more warning on the E4ME and other computer-controlled Quadrajets. Those carbs have a mixture control solenoid run by the engine computer, and the idle mixture is set with factory procedures that involve a dwell meter and a sealed idle air bleed. That is a different animal, and this article is about the non-computer carbs that are on most classic muscle cars. If you have an E4ME, get the factory service manual for that car before you touch anything on it.

How the Quadrajet idle circuit actually works

A Quadrajet is really two carburetors in one. The two small primary bores up front handle idle, cruise, and light throttle. The two huge secondary bores in back stay shut at idle and only come into play when you lean on it. The factory Delco service manual is clear on this: the secondary throttle valves remain closed during idle, and on some applications they were even coated with a graphite material so they would seal tight in the bores for minimum air flow at idle.

That matters because it means your idle mixture screws only control the primary side. There are two of them, one for each primary bore, and they thread into the front of the throttle body (the base plate), down low on the side facing the front of the car. Turning a screw in (clockwise) makes the opening smaller and leans the idle mixture. Turning it out (counterclockwise) richens it. That is the same on every Q-jet.

What the screws do not control is just as important. The idle system on a Q-jet is not a stand-alone circuit. Fuel for the idle system comes from the float bowl through the main jets and up the idle tubes, past restrictions and air bleeds, and then out under the mixture screws. Lars Grimsrud’s well-known tech paper “How to Tune a Q-Jet” makes the point that the primary metering rods and power piston affect how the mixture screws respond. If the cruise metering is way too lean, or the power piston is not seating, the mixture screws can act lazy or do nothing at all. Keep that in mind when we get to troubleshooting.

Limiter caps, sealed plugs, and why your screws may be hiding

If your carb is from the early ’70s or later, there is a decent chance you cannot just stick a screwdriver on the mixture screws. Starting in 1971, according to the Delco Quadrajet service manual, Rochester put plastic idle limiter caps on emission control carburetors to discourage field adjustment. The caps only allow a small amount of rotation. Later carbs went further, and the manual says the idle mixture screws were sealed to prevent readjustment from the factory setting.

Here is what that means in the garage:

• Plastic limiter caps (early ’70s): these are the plastic caps pressed over the screw heads. They can be carefully pried off with a small screwdriver. Go slow and do not bend the screw. Once the cap is off, you can adjust the screw normally.
• Hardened steel plugs (mid to late ’70s and later): on a lot of 1975 and newer carbs, the screws sit in recesses behind hardened steel plugs. A H.A.M.B. thread on a 1978 Quadrajet confirms that a drill bit will not touch these if they are the hardened type. The factory removal procedure, described in the Delco service information, requires taking the carb off the engine, cutting into the throttle body casting beside the locator points, and driving the plug out. The plug usually shatters rather than coming out whole.

If you have a sealed carb that runs well and passes whatever inspection your state still requires, think hard before you start cutting. If it really needs adjustment, set it up on the bench, follow the factory procedure for your carb, and expect to need a special idle mixture tool or a deep socket for the screws. Old-timers on the H.A.M.B. remember parts stores selling modification kits for exactly this job, because the later screw heads are not a simple slotted head.

And if your state still does emissions testing on classics, check the rules first. Some do, and messing with sealed screws can create a headache you did not need.

Tools you need

You do not need a lot of fancy equipment for this. Here is what I keep on the cart:

• A vacuum gauge. Any decent dial gauge will do. This is the most sensitive and honest tool for mixture setting. Cliff’s forum regulars have said for years that a vacuum gauge is far more sensitive than a tach.
• A tachometer. A handheld or clip-on tach is fine. You need it to set the actual idle speed. Do not trust the dash tach for this.
• A long flat-blade screwdriver that fits the mixture screws, and a shorter one for the idle speed screw.
• Vacuum caps and a short length of vacuum hose, plus a longer piece of about two feet for testing the choke pull-off.
• Drill bits to use as gauges for choke settings. Grimsrud’s choke paper uses an eighth-inch, a quarter-inch, and a five-sixteenths bit as simple gauges.
• Your factory service manual and the underhood emissions decal, if the car still has one.
• A fire extinguisher within reach. Not optional.

Do these things before you touch the mixture screws

This is where most people go wrong. They go straight to the mixture screws and start twisting. A carb can only be tuned on an engine that is otherwise healthy, and the Q-jet idle circuit is sensitive to things that have nothing to do with the carb.

Set the ignition timing first. Both Grimsrud and Jeff Smith at Street Muscle say the same thing: you cannot fix timing problems by tweaking the carb, and carbs often get blamed for problems caused by ignition. If you have not checked it recently, go through our guide on how to set ignition timing on a classic carbureted V8 with a timing light first. Then come back here.

Find and fix vacuum leaks. A vacuum leak is unmetered air. The engine idles lean, the idle speed wanders, and the mixture screws stop making sense. Check every vacuum hose, the PCV valve and hose, the brake booster hose, the carb base gasket, and the intake manifold gaskets. The troubleshooting notes in Mike’s Carburetor Parts’ Quadrajet manual suggest spraying carb cleaner around the base of the carb and the vacuum lines and watching for the rpm to change. If you do that, use short bursts, keep it away from the exhaust manifolds, and have that extinguisher handy.

Make sure the engine is truly warm. Not “the temp gauge moved.” Fully warm and heat soaked, as Cliff puts it. Drive it for 15 or 20 minutes if you can. The choke must be fully open, and the fast idle cam must be off its steps so the throttle is resting on the curb idle screw.

Check the hot idle compensator, if your carb has one. Some 4MV and 4MC carbs have a hot idle compensator in a chamber at the rear of the float bowl. It is a little thermostatic valve that opens when things get very hot to add extra air and offset fuel vapor richness. The Delco manual says it must be closed for a proper idle adjustment. To check it, hold a finger over its air inlet on top of the air horn. If the rpm does not drop, the valve is closed. If it is open, plug the hole or let the engine cool a little until it closes. And if you plug it with something, use something you will see, like a pencil, so you remember to pull it out before the air cleaner goes back on.

Know your idle spec. Curb idle speed on these cars is specific to the engine, transmission, and year. It varies, and some specs are given in Drive for automatics and in neutral for manuals. Look at the emissions decal under the hood (GM put them on 1968 and later cars) or in the factory service manual for your exact car. I am not going to give you a single number, because there isn’t one.

Where to hook up the vacuum gauge

Connect the vacuum gauge to a manifold vacuum source, not ported vacuum. Manifold vacuum comes from below the throttle blades and is there at idle. Ported vacuum comes from a port just above the throttle blades, and at idle it reads little or nothing because the throttle has not opened past the port yet. A BITOG forum thread on this exact job lays it out plainly: ported vacuum is not available at idle, which is part of why it is used for the distributor vacuum advance on some setups.

On a Q-jet, you usually have a few options. The ports on the base of the carb and the large PCV fitting are manifold vacuum on most applications. A tee in a known manifold vacuum line also works. If you are not sure, hook the gauge up to the port and look at it at idle. If it reads strong steady vacuum, it is manifold. If it reads zero or close to it until you crack the throttle, it is ported.

Do not get hung up on a specific vacuum number. A stock small-block might pull a healthy steady reading, and one with a big cam may pull a lot less and bounce a bit. You are not chasing a certain number. You are looking for the highest steady reading you can get with the leanest screw setting.

Initial settings: how many turns out?

If the screws have been removed or nobody knows where they are, you need a starting point. This is where you will see a lot of different numbers thrown around, so let me give you the real story.

The Delco service manual’s own advice is to count the number of turns it takes to lightly seat the old screw before you remove it, then put the new one back to that same count. That is good advice. Note the turns, and write them down.

If you have no idea where they were, here is what the experts say:

• Lars Grimsrud’s “How to Tune a Q-Jet” says most 4MV carbs will run well with a base setting of about 4 turns out from lightly seated, and most M4M carbs at about 6 turns out.
• Cliff Ruggles has written on his forum that most carbs end up about 2 1/2 to about 4 turns out with the 10-32 thread screws, and about 4 1/2 to 7 turns out with the fine-pitch metric screws used on later carbs, and that the normal range depends on the size of the holes under the screws and the taper on the screw tips.
• Plenty of forum folks start lower, around 1 1/2 to 2 turns, and work out from there.

Here is how I look at it. The starting point is just a place to get the engine running smoothly so you can do the real adjustment. The number of turns where it ends up is whatever your engine wants. Set both screws the same, to whatever the old count was, or to a conservative number in the range above. Then let the vacuum gauge tell you where they belong.

And always seat the screws lightly. The Delco manual specifically warns not to force them against the seat, because that damages the tip and the seat. The rebuild tips in the Mike’s Carburetor Parts manual also say a screw with scoring on the tip from over-tightening should be replaced. Just snug them until they stop, then count back out.

How to set Quadrajet idle mixture screws on a classic Chevy, step by step

This is the procedure I use, built on the method Cliff Ruggles has described for years and the same basic approach in Grimsrud’s paper and Jeff Smith’s Street Muscle article. Do it in this order.

1. Safety setup. Parking brake set, wheels chocked, automatic in Park and manual in neutral for the first passes. Fan shroud and belts out of the way of your hands and sleeves. Garage door open or the car outside, because carbon monoxide does not care how good your hearing is.
2. Warm it up fully. As above. Choke open, fast idle cam off its steps.
3. Hook up the vacuum gauge and tach. Gauge to manifold vacuum, tach to the coil per its directions.
4. Set the idle speed close to spec. Use the idle speed screw to get the idle somewhere near your spec. The exact number is not important yet.
5. Find the best idle. Turn one mixture screw out (richer) slowly, a quarter turn at a time, and watch the vacuum gauge. Then try in (leaner). You are looking for the point where vacuum is highest. Do the same with the other screw. If the idle speed climbs as you go, bring it back down with the idle speed screw, because, as a Corvette Forum regular pointed out, the vacuum reading goes up with rpm and you will get a false reading if you let the rpm drift up.
6. Find the lean drop. Now take one screw and slowly turn it in until you hear the engine slow or see a very slight drop on the vacuum gauge. That is the lean drop point. Cliff describes it as finding the edge of the lean side.
7. Back out until it smooths out. From that lean drop point, back the screw out about a quarter to a half turn, until the engine smooths back out. Cliff’s range is a quarter to three-quarters of a turn, depending on the engine. You are setting the screw just on the lean side of best, which is where most engines want to live for crisp throttle response.
8. Repeat on the other screw. Same procedure, same patience.
9. Go back and recheck both screws. Adjusting one affects the other a little, especially on single-plane intakes or open spacer setups. Cliff recommends balancing them in those situations.
10. Reset the curb idle speed. Bring the idle back to the spec on your decal or manual with the idle speed screw (or the idle stop solenoid, more on that below).
11. Check it in gear. For an automatic, put your foot firmly on the brake, have a helper make sure nobody is in front of the car, and shift into Drive. Jeff Smith’s advice is to check that the engine still idles comfortably in gear. If it lugs or nearly stalls in gear, the mixture or idle speed needs a little more tweaking.
12. Write down the final turns. Turn each screw in lightly, count, and back them out to the same place. Write it down on a piece of masking tape on the inside of the glove box or in your shop notes. Future you will thank you.

If you have no vacuum gauge, you can do the same procedure by ear and with a tach, looking for the highest smooth rpm instead of the highest vacuum. Grimsrud even says that if you have an ear and a feel for the car, you do not strictly need a gauge. I still like the gauge because it shows small changes faster than my ears do.

The factory “lean drop” method

You may run across the term “lean drop” in GM service manuals and on emissions decals from the early ’70s. On a lot of those cars, the factory procedure was to set the mixture for the best idle, then lean the screws in to drop the idle speed by a specified amount. A member on the Corvette Forum summed it up as setting the idle mixture just a bit leaner than ideal, which was mostly for emissions. Grimsrud says that if you need to pass an emissions test, run the screws in to get a 50 rpm drop after best idle quality is achieved.

The exact numbers vary by year, engine, and transmission. Some decals list one rpm before the lean drop and another after. If your decal is still there and legible, use those exact numbers. If it is gone, check your factory service manual. If you are just after the best street idle, the Cliff method above is what most experienced Q-jet guys use today.

Idle speed screw, idle stop solenoid, and the fast idle cam

There are a few different screws on a Quadrajet that people mix up. Let us sort them out.

The idle speed screw (sometimes called the curb idle screw or throttle stop screw) is on the driver’s side of the carb on most Q-jets, according to Grimsrud’s choke paper. It sets how far the primary throttle blades are held open at hot idle.

The idle stop solenoid is an electric solenoid mounted on a bracket on some Quadrajets. When the key is on, it pushes out a plunger that holds the throttle open to the curb idle position. The Delco manual explains that curb idle is set with the plunger screw on the solenoid while the solenoid is energized, and the low idle is set with the idle stop screw on the carb with the solenoid de-energized. When you shut the key off, the solenoid drops out, the throttle closes further, and that helps prevent the engine from dieseling. Your decal will tell you which speed goes with which screw.

The C.E.C. valve is a different solenoid used on some early ’70s Quadrajets. It looks similar but controls vacuum advance and acts as a throttle stop in high gear. The Delco manual is blunt about it: the valve has a label that says never use it to set idle. If you have one, set the idle with the idle stop screw, and leave the C.E.C. valve alone.

The fast idle screw is on the passenger side of the carb, on the primary throttle lever below the choke linkage. It rides on the fast idle cam, a stepped piece that rotates with the choke. When the choke is closed, the cam holds the throttle open a bit on its high step so the engine can idle when cold. As the choke opens, the cam rotates down through its lower steps until the screw is completely off the cam and the throttle rests on the curb idle screw.

For a bench starting point, the factory Rochester and Pontiac 4MV shop manuals both say to close the primary throttle valves, put the cam follower on the high step of the fast idle cam, and turn the fast idle screw in a certain number of turns after it makes contact. One manual says two turns, another says three. Grimsrud’s paper suggests two and a half. Then you set the actual fast idle rpm on the car with the engine cold, using the speed and cam step on your decal or in your manual. Different years specify different cam steps, so check yours.

When you set the fast idle on the car, make sure the engine is cold, the choke is working, and the cam follower is on the step called for. The fast idle rpm is a “cold start” speed, so trying to set it on a warm engine with the choke propped open will give you the wrong answer.

Setting up the choke

The choke is where a lot of Q-jets get butchered. People bend rods trying to fix symptoms, and then the carb never works right again. Grimsrud’s choke setup paper (hosted by the NCRS Southeast chapter) is one of the clearest write-ups on this, and the steps below follow his order. One important caveat from his paper: the choke setup assumes the timing is right and the carb is jetted and set up correctly. If the carb runs rich or lean, the choke will not work right even when it is adjusted to spec.

Also understand the measurements below are Grimsrud’s recommended street settings, which he presents as a simple, proven setup. Factory specs for choke rod, vacuum break, and unloader openings vary by carburetor number and year. If you want factory numbers for a numbers-matching car, look up the specific carb number (stamped on the driver’s side of the float bowl) in the Delco specs for that year.

Divorced choke (most 1967 to 1974 Chevy 4MV carbs)

On a divorced choke, the thermostatic coil sits in a little box on the intake manifold. As the crossover heats up, the coil expands and pushes a rod up, which opens the choke on the carb. Here is the setup:

1. Disconnect the divorced choke rod from the lever on the passenger side of the carb. Leave it attached to the coil on the manifold.
2. Close the choke. Open the throttle slightly and push on the carb lever where the rod normally attaches until the choke blade closes completely.
3. Bottom the rod. Push the choke rod all the way down into the coil until it hits its stop. On a dead cold engine it may already be there.
4. Check the rod position. At this point, Grimsrud says the top of the choke rod should be level with the bottom edge of the hole in the carb lever. Bend the rod to get that relationship.
5. Reconnect and check. Hook the rod back up and push it down to the seated position again. The choke blade should be fully closed. If it is not, Grimsrud says to bend the intermediate choke rod that comes up through the carb body to the choke blade lever, until it is.

The divorced coil is not adjustable like a carb-mounted cap. If the choke stays on too long or opens too fast after you have done the above, the coil itself may be tired, or the crossover may be blocked so the coil never gets hot. A lot of old small-blocks have had the heat crossover blocked with gaskets or carbon over the years, and that will make a divorced choke act lazy.

Integral hot-air or electric choke (most 1975 and later carbs)

On these carbs, the coil is in a housing on the side of the carb. On the 1975 to 1979 hot-air version, Grimsrud explains, clean air is pulled from a nipple on the carb, through a steel tube and a heat exchanger in the manifold crossover, and back into the choke housing. That airflow depends on a vacuum bleed in the housing. His warning is worth repeating: the hot-air choke will not work at all if the whole tube system is not hooked up and working. Starting in 1980 the electric choke replaced the hot-air system on most passenger cars, and Cliff notes that the cover screws were replaced with rivets around then.

1. Remove the cover. Take out the three screws holding the black choke cover, or drill out the rivets if it has them. Grimsrud notes the rivet holes can be tapped for screws.
2. Set the coil lever. Crack the throttle slightly and push up on the choke coil lever inside the housing until the choke is closed. There is a small recess inside the housing below the lever. Grimsrud uses an eighth-inch drill bit in that recess and lets the lever rest on it. The choke blade should be fully closed in this position. If not, bend the intermediate choke rod until it is.
3. Set the choke rod (fast idle cam) clearance. Remove the bit, crack the throttle, and rotate the fast idle cam so the cam follower sits on the second step, right up against the high step. With light pressure closing the choke, Grimsrud’s setting is a five-sixteenths opening measured between the rear upper edge of the choke blade and the rear airhorn wall. Adjust it by bending the small tang on the lower side of the fast idle cam.
4. Reinstall the cover. Make sure the coil tang in the cover engages the lever inside the housing. For an electric choke, Grimsrud and Cliff both say do not use the gasket, because the electric choke grounds through the housing.
5. Index the cover. Grimsrud’s starting point for hot-air chokes is the cover mark on the second dot clockwise of the center dot on the housing. For electric chokes, which have no line, he lines up the indentation in the cover with the screw and clip location toward the front of the housing. Your factory spec may differ, so check if originality matters to you.

The cover setting versus the pull-off

This was a big lightbulb for me when I first read Grimsrud’s paper, and it is the most useful choke tip in this whole article. On an integral choke, the cover rotation and the vacuum break do two different jobs:

• Rotating the cover changes how long the choke stays on before it fully opens. Clockwise shortens the duration. Counterclockwise lengthens it. It only takes a mark or two to change it. Contrary to what a lot of folks believe, Grimsrud says rotating the cover does not change how rich or lean the carb runs with the choke on.
• The vacuum break (pull-off) controls how rich or lean the engine runs right after a cold start. If it runs rich and sooty when first started, open the pull-off a little more. If it starts and wants to die, close it a little.

So if your car starts fine but the choke stays on halfway to work, rotate the cover. If it starts and immediately loads up or stalls, look at the vacuum break.

Vacuum break (choke pull-off) adjustment

The vacuum break is the round diaphragm on the carb with a vacuum hose going to it. When the engine fires, manifold vacuum pulls the diaphragm in, and that cracks the choke blade open a set amount so the engine gets enough air to keep running cold. If the diaphragm is torn, the choke stays fully closed, and the engine loads up and dies or blows black smoke. If it opens too far, the engine stalls cold because it is too lean.

Here is Grimsrud’s procedure, which works for both choke types with a small difference at the adjustment end:

1. Test the diaphragm first. Pull the short vacuum hose off the pull-off and put on a longer hose, about two feet. Suck on it. The plunger should pull in and stay in. If it does not, the pull-off is bad and must be replaced. No amount of bending will fix a leaky diaphragm.
2. Close the choke. On a divorced choke, crack the throttle and push down on the divorced choke rod to close the choke, and keep light finger pressure on it. On an integral choke, the cover set closed as described above does that job.
3. Pull the diaphragm in with vacuum. Suck on the hose and hold it, or use a hand vacuum pump.
4. Simulate airflow. Lightly push down on the forward lower edge of the choke blade, the way air rushing in would.
5. Measure. Grimsrud’s street setting is a quarter inch between the forward lower edge of the choke blade and the forward wall of the airhorn. A quarter-inch drill bit is an easy gauge.
6. Adjust. On a divorced choke carb, he bends the tang on the choke linkage where it meets the pull-off rod. On an integral choke carb, he turns the screw on the end of the pull-off lever. Reapply vacuum each time you change it and remeasure.

The factory manuals measure the vacuum break similarly, with the diaphragm seated and the choke closed, gauging between the edge of the choke valve and the air horn wall. Which edge of the blade the factory gauges from can differ by year, and some carbs have both a front and a rear vacuum break, so if you are restoring to factory specs, use the specs and diagrams for your carb number. Some 4MV carbs also used a fast idle cam “pull-off” feature, where the vacuum break plunger kicks the cam down from the high step to the second step as it pulls in, and the Delco manual notes that a slightly different vacuum break adjustment applies on those.

Choke unloader adjustment

The unloader is your get-out-of-jail card for a flooded engine. When you hold the throttle wide open, a tang on the throttle linkage forces the choke blade partly open so extra air can clear out the excess fuel. The Delco manual’s description is simple: to unload the engine, the accelerator must be pressed so the throttle valves are held wide open.

The factory check, as written in the Pontiac and Rochester 4MV shop manuals, goes like this:

1. Hold the choke valve closed. The Rochester manual uses a rubber band on the vacuum break lever to hold it.
2. Open the primary throttle to wide open.
3. Measure the gap between the edge of the choke valve and the inside wall of the air horn.
4. Bend the tang on the fast idle lever (or throttle lever, depending on the carb) to adjust.

The specified gap is listed in the manual for your carb number. One tech write-up on Corvette Q-jets notes that up through 1976 the measurement is taken at the bottom side of the choke butterfly, and from 1977 on it is taken at the top side, so be careful which edge you measure.

The practical lesson here: if your car floods on a cold start, do not pump the gas pedal like a slot machine. Every pump squirts more fuel from the accelerator pump. Push the pedal to the floor, hold it there, and crank. That opens the unloader and clears the engine. And on a properly set Q-jet, you usually only need to press the pedal to the floor once before a cold start to set the choke and the fast idle cam, then let it come back up and crank.

Troubleshooting: when the mixture screws do not work

This is the part most “how to” articles skip. If the mixture screws do not change anything, or the idle is still rough after you have set everything, here are the usual suspects.

The screws do nothing when you turn them out

If you can back the screws out and out and nothing changes, and the idle never goes rich, Cliff says the idle system is not getting enough fuel. The most common causes:

• Clogged idle tubes. The Mike’s Carburetor Parts troubleshooting notes call the idle tubes the item most often missed by rebuilders. They are tiny passages that clog with old fuel residue and ethanol varnish. Clean them with thin wire and blow them out with compressed air.
• Too much throttle opening and nozzle drip. If the idle screw is cranked open far enough, air through the primary venturi can start pulling fuel from the main nozzles at idle. Cliff calls this “nozzle drip,” and it means part of your idle fuel is not coming from the idle circuit at all, so the mixture screws lose authority. With the engine idling and the air cleaner off, look down the primaries with a light. If fuel is dribbling from the main nozzles at idle, that is the problem. On a cammed engine, Cliff says the fix is usually more idle bypass air and sometimes more idle fuel, not more throttle.
• Low manifold vacuum. Cliff also points to low idle vacuum from too little timing, a bigger than stock cam, or low compression as causes of insufficient idle fuel. Recheck your timing.
• Lean cruise metering. Grimsrud’s paper says if the primary jet and rod combination is too lean, the pressure drop in the idle system can keep the screws from flowing enough fuel. His quick test: run the screws out to where more turns have no effect, then cover part of the carb inlet with your hand. If the idle improves as you restrict the air, the metering may be too lean or the power piston may be set too deep.

The screws do nothing when you turn them in, or it always runs rich

If you turn the screws in and the engine never leans out, or if it smells rich and blows a little black smoke no matter what you do, fuel is getting in somewhere the screws do not control:

• Leaking well plugs. This is a big one on Quadrajets. Several passages in the bottom of the main body were sealed at the factory with soft plugs, and Hot Rod’s Quadrajet coverage explains that years of heat cycling make them leak, which leads to over-rich running and difficult hot starts. Cliff told High Performance Pontiac that the secondary wells on carbs through 1968 were sealed with brass cup plugs, and that all of them leak horribly. Aluminum plugs came in 1969 and were better, but still leak on some carbs. The proper fix is to seal them with threaded plugs and fuel-resistant epoxy, as kits from Cliff’s and others do. Hot Rod now considers resealing the well plugs a mandatory part of any Q-jet rebuild.
• Power piston not seating. Grimsrud explains that if engine vacuum at idle is low, like with a lumpy cam, the power piston may not get pulled all the way down because its spring is too stiff. Then the metering rods sit in the rich position, the car runs rich at idle, and the mixture screws have little effect. The idle can float down until the engine stalls. A softer power piston spring often fixes it.
• Float level too high or a leaking needle and seat. If the float level is too high or the needle does not seal, fuel can spill out of the nozzles. The Mike’s Carburetor Parts manual lists fuel pump pressure, a bad or heavy float, float level, and a binding float as things to check when a carb floods.
• Choke not fully opening. If the choke is even partly closed on a warm engine, the engine runs rich. Look down the carb with the engine warm. The blade should be straight up and down.

The idle surges or wanders

Surging and hunting at idle usually points to a vacuum leak, a restriction in the fuel supply, a dirty or sticking power piston, or a float level problem. The Mike’s Carburetor Parts list also includes kinked fuel lines and plugged fuel filters, including the filter in the carb inlet. Check the easy stuff first.

Hard starting when hot

If the car fires right up cold but cranks and cranks after it sits hot for a while, and it acts flooded, leaking well plugs are a prime suspect, according to both Hot Rod and Cliff. Fuel drips from the bowl into the intake while the engine is heat soaking. Before you blame the plugs, also check the float level and the needle and seat.

Choke problems

• Hard cold start, engine will not catch: check that the choke blade closes fully when cold and that you press the pedal once to set it before cranking.
• Starts, then immediately dies cold: the vacuum break may be opening the choke too far, or the fast idle speed may be too low.
• Starts, then loads up and blows black smoke: the vacuum break diaphragm may be torn and not pulling the choke open at all, or the gap is too small.
• Choke stays on too long: on an integral choke, rotate the cover clockwise a mark. On a hot-air choke, check the whole hot-air tube system and the vacuum bleed. On a divorced choke, check the coil and the heat crossover.
• Choke opens too fast and the engine stumbles cold: rotate an integral cover counterclockwise a mark. On a divorced choke, the coil itself may be weak.
• Electric choke does nothing: make sure there is no gasket under the cover and that it is getting 12 volts with the key on. Cliff suggests the windshield wiper motor’s switched feed is a good power source.

Safety notes

None of this is dangerous if you respect the basics, but people get hurt doing carb work every year.

• Fire. Gasoline and carb cleaner around a hot exhaust manifold is a real fire risk. Keep a fire extinguisher close, never spray cleaner near the exhaust, and clean up spills before you start the engine.
• Moving parts. Your hands will be close to the fan and belts the whole time you are adjusting the mixture screws. Roll up sleeves, take off rings and watches, and tie back long hair.
• In-gear checks. When you check the idle in Drive, have your foot hard on the brake, the parking brake set, and nobody in front of the car.
• Exhaust fumes. Do this outside or with the garage door wide open. Carbon monoxide builds up faster than you think, especially if you are making the engine run rich while setting it.
• Fuel spills. If you pull the carb to work on sealed plugs or well plugs, drain the bowl into a container, cap the fuel line, and keep rags away from any ignition source.

FAQ

Can I set Quadrajet idle mixture without a vacuum gauge?

Yes. You can use a tach, or even your ear, by finding the highest smooth idle and then turning each screw in until it just starts to slow, and backing out a quarter to a half turn. Grimsrud says an experienced ear can do it without a gauge. A gauge just makes the small changes easier to see.

Should both mixture screws be at the same number of turns?

Start them the same, but they do not have to end up exactly the same. Each primary bore can be a little different. They are usually close, though. If one ends up several turns different from the other, look for a vacuum leak on one side, a clogged passage, or a worn throttle shaft.

Do I need to set the mixture in Drive?

Some factory specs give the idle speed in Drive for automatics. I set the mixture in Park or neutral for safety and then check the idle in gear with my foot on the brake. If it idles poorly in gear, touch it up. Follow your decal for the actual idle speed, including whether it is in Drive or neutral.

My Quadrajet has sealed plugs over the mixture screws. Should I drill them?

Not without a reason. If the engine idles well, leave them alone. If you need to adjust them, the factory procedure involves removing the carb and cutting the casting beside the locator points, then driving out a hardened plug. It is not a job for a drill on the car.

Why does my choke stay closed on a warm engine?

On a divorced choke, check the coil, the rod, and the heat crossover in the intake. On a hot-air choke, the hot-air tube system has to be connected and working. On an electric choke, check the 12-volt feed and the ground. On all of them, check for a bent rod or sticking linkage.

Sources and further reading

I leaned heavily on a handful of real Quadrajet authorities to put this together. If you want to go deeper, these are worth your time:

• Cliff’s High Performance Quadrajets: Quadrajet Model Designation Descriptions and Quadrajet History and Design, plus years of mixture and nozzle drip advice from Cliff Ruggles on the Cliff’s forum.
• Lars Grimsrud, How to Set Up a Q-Jet Choke (hosted by the NCRS Southeast chapter) and his “How to Tune a Q-Jet” technical paper.
• Delco/Rochester Quadrajet 4M, 4MC, 4MV service manual, as reproduced by Mike’s Carburetor Parts.
• Hot Rod: How to Restore Lost Performance to Your Quadrajet, and High Performance Pontiac’s Used Quadrajet Shopping Guide and Home Remedy Quadrajet Carburetor.
• Jeff Smith, Tune the Quadrajet, Street Muscle.

Related reading on CNC

If you are tuning the carb, you are halfway to a full tune-up. Start with how to set ignition timing on a classic carbureted V8 with a timing light, since the carb cannot fix a timing problem. If you are hunting for vacuum leaks and the engine bay is a greasy mess, our guide on how to clean an engine bay on a classic or older car will make leaks easier to find. And if you are building or rebuilding the motor underneath that carb, read common DIY mistakes on V8 engines first.

Final thoughts from my garage

The Quadrajet got a bad reputation for a lot of years, and almost all of it came from people who did not understand it. They called it the “Quadrajunk,” swapped it for something with more sight glasses, and then wondered why their mileage went in the tank. Set up right, a Q-jet gives you great throttle response from those small primaries, decent mileage, and plenty of air at the top end from the secondaries.

The idle mixture and choke are the two adjustments that most affect how the car feels every single day. When they are right, it starts on cold mornings, steps down off fast idle on its own, idles smooth in gear at a light, and pulls clean off the line. When they are wrong, it is a pain to live with. Get the timing right, chase down the vacuum leaks, warm it up all the way, and take your time with the screws. Your Chevy will thank you, and so will your neighbors on Saturday morning.

That old ’72 Chevelle I mentioned at the top? Once we set it up properly, it never needed a carb rebuild the rest of the time that neighbor owned it. A Quadrajet set right is a happy carb.

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