Rochester 4 Jet Carburetor Adjustment? All Answers

Follow and the pictures helped a lot too. My engine and idle are working perfectly.”

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“Checking to make sure I know how to set the fuel/air mixture for my H30/31 Pick 1. Instructions were easy to follow

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Tuning a carburetor to provide an engine with the correct jetted air/fuel mixture has always been a task nearly impossible for most hot rod owners and tuners. In the past, most performance engine tuners would inspect the spark plug, exhaust port, and the first 6 inches of the header for the correct color and then make a guess as to what nozzle size change was required. One of the disadvantages of this method is that the header and spark plug can only show what the mixture was like at the exact RPM and load condition at which the plug check was done, so you had to mostly just tune by trial and error.

Now a new, more scientifically advanced way of checking the air/fuel mixture is to use an infrared exhaust analyzer and/or an extended range oxygen sensor in the exhaust system; Now the fuel mixture can be read at any desired speed and load condition. The content of the engine exhaust can be read to show what the air/fuel mixture is at any speed or load and how efficiently the engine is burning the fuel.

Proper tuning of any engine can mean the difference between an engine that runs well and one that always sounds and runs like it needs tuning. For most hot rodders, one of the biggest mysteries is how to jet the engine to get the proper air/fuel ratio your performance engine needs to deliver not only rideable horsepower when you want to go fast, but the engine with the right air-fuel mixture for driving in heavy traffic or cruising on the motorway.

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If the air/fuel mixture is too rich for the engine while cruising, the engine may have a tendency to overload and foul the spark plugs, while too lean an air/fuel mixture may cause misfires at idle at low loads or tend to overheat or overheat. Proper air/fuel mixture for all driving conditions will allow you to get all the horsepower out of the engine while allowing as many miles as possible on one tank of fuel without overheating or causing engine damage from too lean air. fuel mixture.

Recent advances in exhaust gas analysis technology and extended range oxygen sensor technology have made it possible to read and/or record the actual air/fuel mixture under almost all driving conditions. In the past, exhaust gas analyzers tended to be large and expensive, but the new devices on the market are not only compact and portable, but also affordable.

Performance and replacement carburetors sold today are generic tuned or jetted unless the carburetor is built for a specific engine package and fuel. A carburetor that is not built and tuned for a specific engine, exhaust system, and fuel should provide an air/fuel mixture that is rich enough for a wide range of engines (but this is not always the case). If the carburetor delivers an air/fuel mixture that is too lean, the engine will run sluggishly, overheat, or the lean mixture can cause engine damage. If the carburetor delivers an air-fuel mixture that is too rich, the engine can have a tendency to overload, foul the spark plugs, run sluggishly and suffer from a lack of power.

Proper carburetor selection can make the task of fine tuning the air/fuel mixture easier, my preferred replacement carburetors are: for a mild engine a Quadrajet, Edelbrock Thunder or Performer of 650cfm or smaller, for a high performance engine I prefer Barry Grant’s powerful Demon carburetor Inc. or the Holley 4150 hp, on a supercharged engine, forced draft carburetors available from Barry Grant have given us excellent results.

The fuel you use (pump or race), air density (i.e. altitude, barometric pressure, air temperature, humidity), compression ratio, camshaft, exhaust system, ignition timing curve, engine condition, fuel pressure, airflow through the air filter, etc .affect the carburetor adjustment required to obtain the correct fuel mixture for your engine.

The first task is to get the correct spark advance curve for the engine and fuel used, then the fuel pressure must be checked to ensure it has the correct system pressure at all engine load conditions. If the fuel pressure drops below the correct pressure, the carburetor air/fuel mixture will become lean, which can result in engine damage. After confirming that the spark advance curve is correct, many of the problems we see can be traced back to the fuel mixture not being suitable for the needs of the engine.

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Ignition timing and ignition curve Before checking the air-fuel mixture, the ignition timing and ignition curve must be correct. Regardless of which ignition system you use, if the spark timing does not match the engine’s requirements, the engine will not produce all of the potential horsepower that is built into it. With any distributor, power replacement or original equipment, the mechanical and vacuum advance curves must be verified and then adjusted to the engine and fuel used. (Note: MSD manifolds come with a very conservative mechanical advance curve and come with the bushings and springs to get the desired curve).

Barry Grant Inc. has a very good reference for recommended initial timing using camshaft duration at 0.050 valve lift which I find very useful, just go to the Barry Grant website and click on the demon selection guide. The advance curve we see most often on a 9:1 compression, mild cam Chevrolet V-8 engine is 12 degrees of initial timing plus an additional 24 degrees of mechanical advance at 3,600 rpm. If vacuum advance is used, it should they will provide a MAXIMUM of 10 degrees of EXTRA advance at engine vacuum over 12 inches! An engine equipped with a hot cam or air gap/race intake manifold may respond well to 18 degrees of initial timing combined with a shorter 18 degree mechanical advance curve at 3,200-3,400 rpm.

If the engine is not spark advanced enough it can lack power, have poor throttle response, consume too much fuel and cause the engine to overheat, while if the engine is spark advanced too much it can cause the engine to lose power lack, ping, consume too much fuel or let the engine overheat.

Proper ignition timing produces maximum cylinder pressure at about 12 degrees after the piston passes top dead center. Only then can you get all the energy out of the fuel and achieve maximum performance and engine efficiency. There are two methods we use to check the distributor’s advance curve. The best method involves using a manifold dynamometer to test and adjust both the mechanical and vacuum advance curves and the second choice is to use a recall advance timing light to check the advance curves while the engine is running at various engine speeds and negative pressure conditions running.

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Air/Fuel Mixture Reading A lean fuel mixture (insufficient fuel for the amount of air in the cylinder) can cause an engine to jerk or skip at idle and part throttle, stumble under acceleration, cause the engine to overheat and cause a lack of power, and cause possible engine failure due to the lean air/fuel mixture. A rich fuel mixture (too much fuel for the amount of air in the cylinder) can cause an engine to rev up when idling, fouling the spark plugs and also leading to lack of power or sluggish running. There are several methods to determine if the air/fuel mixture is correct, including the following:

1. Reading the spark plugs with a lighted magnifying glass. In this method, the base of the spark plug insulator (white portion of the plug) is inspected for slight discoloration of the insulator just above where the insulator comes through the steel case. If the mixture is too lean, it won’t leave any color, while a rich mixture will cause the fuel ring to be more prominent. Mixtures that are too rich give the spark plug a sooty appearance.

Stripping the manifold and looking at the color of the exhaust port in the cylinder head and the first 6 inches of the exhaust manifold is also used to determine the air/fuel mixture, but the color of the manifold and spark plug can only show how the air/ fuel mixture was at the load condition at which you performed the check.

In the days of leaded fuel and spot ignition this method worked well but today the use of unleaded fuel and high energy ignition systems has made this method much more difficult as there is very little color to be seen on the spark plug making it a job for an expert. Examining the spark plug insulator for signs of knocking, seen as aluminum spots, can be an effective way to determine if the ignition timing is too advanced for the octane rating of the fuel being used.

2. The second method is to use timed acceleration runs or maximum speeds for the propulsion system. This involves using trial and error jet changes to get the best results. It’s not that easy to get the right mixture for cruise (that’s the air/fuel mixture that the engine will operate on under light load conditions, such as z and faults, to get the best engine drivability.

When adjusting the Power and Cruise mixtures, it is always advisable to stay slightly rich to avoid engine damage. The idle mixture is adjusted with a tachometer to get the maximum RPM from each idle screw and then leaner to get a 20 RPM drop in speed. This is known as the lean drop method.

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3. The easiest and most accurate method we have found is to use an infrared emissions analyzer. This type of device allows us to determine the air-fuel mixture by reading the exhaust gases. By using an infrared emissions analyzer, the carburetor jetting curve (air/fuel mixture) can be checked at idle, cruising or power loads and then tailored to what your engine needs to perform optimally under all racing/driving conditions to run. A high NOx reading from the exhaust gas analyzer can be used as a method to determine if ignition timing is too early and creating excessive heat in a cylinder’s combustion chamber.

4. An optional method of checking air/fuel mixtures is to use a broadband oxygen sensor installed in the exhaust manifold. The sensor is read with a digital air/fuel gauge, the device that I have had the best results with is available motorsport refresh. In this method, the air/fuel mixture is determined by looking at the oxygen/unburned fuels in the engine exhaust; Readings can be very accurate, but false readings can be produced by an exhaust leak, engine misfires, or a high overlap camshaft at lower speeds (these false readings are caused by the oxygen sensor incorrectly measuring the extra oxygen in the exhaust due to the misfire). exhaust leak or high overlap cam)

Jetting with an Infrared Gas Analyzer or Broadband Oxygen Sensor The most accurate and easiest way to check an engine’s jetting (air/fuel mixture) is to observe the CO reading from an infrared gas analyzer and/or broadband oxygen sensor . First insert the sample probe into the tailpipe, and then the device will read the exhaust and provide the readings needed to determine the air/fuel mixture. The method of infrared exhaust gas analyzer and/or broadband lambda sensor allows checking the part load air-fuel mixture, which is otherwise almost impossible, the readings of both methods can be read in real time or recorded and later replayed. It is important to note that all changes other than jetting changes and other basic adjustments should be made by an experienced carburetor professional.

A starting point for air/fuel mixtures for most racing engines is: Idle: 1 to 3 percent CO or an air/fuel mixture of 14.1-13.4:1 Cruising: 1 to 3 percent CO or an air/fuel -Mixture of 14.2-14.0:1 fuel mixture Performance mixture and acceleration: 6.6 percent CO or a 12.0:1 air/fuel mixture for a normal engine; A high-performance engine with an improved combustion chamber design, such as a Pro Stock or a Winston Cup engine, will use a slightly leaner power mixture with 4 percent CO or an air/fuel ratio of 13.0:1 in some cases.

Adjusting the Air/Fuel Mixture Using an Infrared Exhaust Gas Analyzer An infrared exhaust gas analyzer reading indicates air/fuel ratio, engine misfire, engine combustion efficiency, and excessive combustion chamber heat (detonation) by looking at the CO exhaust. The reading of an infrared gas analyzer is the reading we use to determine the air to fuel ratio. (Note: CO is partially burned fuel.)

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The other readings that exhaust gas analyzers provide are: HC (Hydrocarbons): The amount of unburned fuel or an indicator of engine misfire, the best mixture gives the lowest HC.

CO2: The product of complete combustion, the best mixture gives the highest CO2 value

O2: A high O2 value indicates a lean mixture; an exhaust leak or the engine has a hot cam. Note: If O2 is above 2-3%, CO readings may not be accurate.

NOx (nitrogen oxides): A gas produced by excessive heat in the combustion chamber. In many cases, a high reading can be related to excessive ignition timing causing detonation that can lead to engine damage.

The best power and air/fuel mixture (CO) will burn all the oxygen in the cylinder and produce the least amount of engine misfire (HC). The ideal air/fuel mixture for any engine speed and load condition will also cause the engine efficiency (CO2) to be at its highest.

Tuning with a Digital Air/Fuel Gauge The digital air/fuel gauge method with an extended range oxygen sensor requires you to know what air/fuel mixture your engine requires for each driving condition. This data should be available from your engine builder or you can use an infrared exhaust gas analyzer which will help you determine what air/fuel mixture your engine needs to run at its best. The air/fuel measurement method uses a broadband oxygen sensor to determine the fuel mixture by analyzing the unburned fuels in the exhaust.

An extended range oxygen sensor can read air/fuel ratios of up to 9 to 1, or on the lean side air/fuel ratios of 19 to 1 or leaner (a standard oxygen sensor is only accurate at air/fuel ratios of about 14.7 to 1). This method has the advantage of extremely fast response times for the readings, but may be less accurate on an engine with a racing camshaft or supercharger application under light load/low speed test conditions due to the excessive oxygen in the exhaust generated by the cam overlap or the Supercharger blow-through effect at low engine speeds and low load conditions.

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The Innovate Motorsports Digital Air/Fuel Gauge allows you to sample air/fuel mixture data at a rate of 12 samples per second for up to 44 minutes, recording the ideal air/fuel mixture curve that gives you the infrared Exhaust gas analyzer can support the determination. While using an infrared exhaust gas analyzer has a slower response time, the advantage is that it not only measures the oxygen/unburned fuel content in the exhaust, but also allows you to determine the air/fuel mixture by measuring the CO -observe value; the misfire rate of the engine can be determined by observing the HC reading; Engine efficiency can be determined by observing the CO2 reading, and knock caused by excessively advanced ignition timing can be detected by observing the NOx reading.

In-Vehicle Testing After confirming that the basic engine condition and setting (fuel pressure, control curve, etc.) fuel mixture is idle to 3,000 rpm. If the cruise mixture is off, first swap out the nozzles to get the correct air/fuel -Mixture in the range of 2,500 to 3,000 rpm in cruise flight. Then check and adjust the idle mixture. If the air/fuel mixture is too lean at idle or part throttle and the idle mixture screws do not have enough adjustment, correction may include increasing the size of the idle jet.

If the mixture is still lean at 1,000-1,800 rpm, a carburettor such as e.g. B. a Quadrajet or Edelbrock of the Performer or Thunder series, the idle port throttle may need to be enlarged so that more fuel can be pumped at part load. This lean condition at part load will cause the engine to skip or stall, this is due to the lean air/fuel mixture, this problem is very common on many of the performance carburetors sold today. If the air/fuel mixture is too rich at idle and part throttle, the idle jet/restriction may be too large and may need to be replaced with a smaller one.

The next step is a road test using a handheld infrared emissions analyzer and/or a broadband oxygen sensor to check the main air/fuel mixture jets at cruising speed, followed by a load air/fuel mixture check. During a road test you can read the air/fuel ratio and then correct it to get it right at idle, cruising/easy throttle and full power.

Tuning The Carburetor A carburetor has an accelerator pump, idle, main jets and in most cases a drive system designed to provide the correct air/fuel mixture for the needs of the engine. An idle system has an idle jet/throttle that must be modified to deliver the desired fuel mixture for the needs of the engine at idle and off-idle. On a power valve carburetor, the size of the main jet determines the air/fuel mixture that is delivered to the engine at light loads/cruising speeds (1,500 rpm and above). The power valve restriction (below the power valve) is the determining factor for the air/fuel mixture that the carburetor delivers when the power valve is open; A 6.5-inch power valve is open, providing the richer air/fuel mixture needed at high power demands when the vacuum is below its 6.5 opening point.

A carburetor that uses metering rods in the primary jets, such as A Quadrajet or the Edelbrock Performer/Thunder series uses the metering rods to change the air/fuel ratio for both the engine’s performance and cruise mixture requirements. The larger the diameter of the metering rod, the leaner the air/fuel mixture becomes. The accelerator pump system adds fuel as the throttles are opened, adjusting the injection volume and duration of the accelerator pump tuning is done primarily through trial and error.

For a Demon/Holley style carburetor, the most common combination used is a .031″ squirt along with a pink pump cam. We frequently modify the accelerator pump duration spring on Demon/Holley style carburetors and Edelbrock Performer/AFB carburetors to make the pump more active and help eliminate lag issues during acceleration. The chart above shows the gases in the exhaust that an infrared exhaust analyzer reads and how the gases change as the air/fuel mixture changes.

If you are purchasing an engine package that has been tuned or developed on a dynamometer and is running on an engine dyno, it is a good idea to have the engine manufacturer tell you the initial ignition timing and ignition timing curve that they recommend for your engine and also find out which air/fuel mixture they recommend for the engine at both maximum power and cruising rpm loads, and then make sure they match the engine in the vehicle. If the engine builder is running the engine on the dyno, if possible have them use an air/fuel gauge such as the Innovate Motorsports unit and you can then use the recorded data to tune the fuel curve to run the engine with the same air /fuel mixture that the engine builder used on the test bench.

Many of the engine packages that we checked for spark timing and air/fuel ratio curves had the correct spark timing and air/fuel mixtures for high rpm/full throttle operation, but require a lot of tuning work at low rpm/part throttle /normal driving conditions. In most cases, when an engine is run on an engine dyno, they only test maximum power while using race-style headers with an open exhaust, and they supply the engine with outside air to a carburetor without an air filter.

The air/fuel mixture and ignition timing curves should be corrected for the real operating conditions of your car’s engine compartment, where the hot air from the radiator and exhaust system is produced along with the changes in exhaust back pressure created by the manifolds and mufflers you are using can result in the engine not operating at the same performance as seen on the dyno.

We get to the basics of how to tune a carburetor. We are on the final round of this 3 part series for a Rochester Quadrajet carburetor rebuild. This is definitely the easiest part, but probably one of the most overlooked for anyone who still owns a carburetor.

Disassembly – Part 1 to rebuild a Rochester Quadrajet carburetor

Assembly – Part 2 to rebuild a carburetor

Steps to tune a carburetor

Specifically, we’re talking about how to tune a Quadrajet carburetor, but really, these steps can be applied to most carburetors, including the Holley. There are really only 3 steps that need to be addressed. Of course, this assumes your rebuild is solid and you don’t have any intake manifold issues.

Step 1 – Choke

The first step in tuning a carburetor is dialing in the choke. The choke not only helps with cold starts, but also limits or enables the operation of the secondaries (4-cylinder). If you’ve ever not heard that 4bbl howl when you put your foot on the ground, it could be the choke keeping the secondaries closed.

Loosen the 3 small straight slotted screws on the plastic choke cover, then rotate the cover until the choke blade is barely open (almost closed). This assumes the Quadrajet is cold. Once the engine has reached temperature (160 degrees F), the choke blade should be straight and vertical and the secondaries should be free of binding. If necessary, loosen the 3 screws again and adjust rich/lean until the choke blade is vertical.

Step 2 – Idle Air/Fuel Mixture

Using a vacuum gauge, adjust the idle air/fuel mixture screws on each side until maximum vacuum is obtained. Locate a manifold vacuum connection on the Quadrajet, e.g. B. the PCV connector and connect your vacuum gauge. When the vehicle is warm, use a screwdriver to adjust the idle mixture screws. Begin by turning the screws in (clockwise) until they bottom. Then back out each idle mixture screw 3 or 3-1/2 turns (counterclockwise). This will ensure that both screws are initially set the same.

Begin by adjusting one of the idle mixture screws, but do not change them until you are finished with that screw. First, try turning the screw inward (clockwise) 1/4 turn. As the vacuum increases, turn in an additional 1/4 turn. If it decreases, go the other way. Find the maximum vacuum, then move to the other screw and repeat the steps. Once maximum vacuum is achieved for both screws, you are ready for the final step.

Final step in tuning a carburetor – fast idle

Finally, all we have to do is adjust the fast idle screw. This screw is spring loaded and is located on the linkage where it connects to your throttle cable or arm. You probably want to set the idle somewhere between 750 and 950 rpm. Turn the screw in (clockwise) to increase speed or out to decrease speed.

We bought our Quadrajet Rebuild Kit from Mikes Carburetor Parts.

This article was co-authored by Hovig Manouchekian. Hovig Manouchekian is a car repair and design specialist and manager of Funk Brothers Auto, a family business established in 1925. With over 30 years of experience in the automotive industry, Hovig specializes in the process of car repair and maintenance. He is also very knowledgeable in general automotive issues and needs including engine repair, battery replacement and windshield accessories and maintenance. Hovig’s knowledge and hard work have helped Funk Brothers Auto win Angie’s List Super Service Award five years in a row. This article has been viewed 1,050,080 times.

Article overview

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Before you adjust your carburetor, make sure you remove your car’s air filter so you can access the carburetor. Then start your car and let it warm up. When it’s up to operating temperature, you’ll find the 2 screws on your carburetor that adjust the air and fuel mixture. You can then use a screwdriver to turn both screws a quarter turn at a time until your motor purrs evenly. To set your idle speed, locate the idle mixture screw, which limits fuel flow at idle. Do no more than half a turn clockwise and adjust just until your engine idles smoothly. To learn how to tell if your engine is running too lean or too rich and how to correct it, read on!

The road to SEMA 2019 has been a long and arduous struggle, but here we are. Jesus in Lopez…

“Quadrabog”, “Quadrajunk” and many other tried and tested nicknames define GM’s most infamous four-barrel carburetor. Enthusiasts love it or hate it. However, the Quadrajet is by far the most tunable spreadbore out there if you pay close attention to what makes this big-throat atomizer tick.

The Quadrajet four-barrel carburetor was manufactured by General Motors’ Rochester Carburetor Division from 1965 to 1990. It was made in many forms, including electronic control from 1980 to 1990, for a variety of GM applications. Additionally, the Quadrajet carburetor was used by every single GM division during its long 25-year production life.

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In its most basic form, the Quadrajet was produced as the “4MV” (automatic choke/distributor choke oven), “4MC” (automatic choke, integrated choke heating coil, heater tube) and “4M” (manual choke). The first round of Quadrajet improvements came in 1968 with the APT (Adjustable Part Throttle) feature, which allowed more finite tuning and a smoother transition from idle to power circuitry.

computer command control

Before we continue, the Quadrajet Electronic Control “CCC” (Computer Command Control), known as “E4M”, cannot be used in a non-CCC vehicle. The CCC Quadrajet is a “feedback” carburetor introduced in 1980 on new GM vehicles. Although the E4M sounds complex, it’s actually quite simple. The Feedback Quadrajet has two ports; a three-pin connector for the throttle position sensor and a two-pin connector for the fuel metering solenoid.

The fuel metering solenoid valve receives a rapid pulse signal from the ECM (Electronic Control Module) which pulses based on power demand and the oxygen content of the exhaust gas. This is called electronic mixture control. The goal is cleaner emissions and finite performance tuning and fuel efficiency. The throttle position sensor provides feedback of the throttle position to the ECM. An O2 sensor in the exhaust monitors the exhaust gas oxygen content. Fuel dipsticks pulse rapidly to precisely control air/fuel mixture.

If you run a CCC Quadrajet with a non-CCC engine you will end up with an overly rich fuel mixture and terrible performance as there is no way to regulate the air/fuel mixture. Fuel dipsticks stay full. The E4M/CCC solenoid-actuated system replaced the vacuum-actuated power piston used in traditional 4M Quadrajet carburetors.

Understanding the Quadrajet function

Despite the Quadrajet’s reputation for frustrating adjustability, it’s a great performer when properly assembled and tuned. Because the Quadrajet is a vacuum-actuated spreadbore carburetor, it can be difficult to tune. Too many of us give up because that carb takes time, knowledge and patience. In order to get the Quadrajet running, the air valves must be activated in perfect time with fuel delivery and engine speed.

One nice thing about the Quadrajet is the fuel economy if you keep your foot off it. Small primary bores keep fuel consumption to a minimum. Holland Tunnel-sized secondaries deliver brute power when timed correctly. Good for the interstate cruise. Handy when it’s time to pin the butterflies. In fact, the Quadrajet performs quite well under normal driving conditions. It’s time to bring it to the Quadrajet, which is struggling to deliver if we can’t get the fuel delivery and throttle movement properly timed.

The Quadrajet was never designed as a high-performance carburetor, although GM installed this carburetor in dozens of high-performance applications. General Motors developed the Quadrajet as an economy carburetor with the added twist of available power. The Q-Jet has small primaries for normal riding and large mechanical air-valve-assisted secondaries for full-throttle power and acceleration. The vacuum operated air valves above the secondaries open with a slower clip when properly adjusted to compensate for the large mechanical secondary throttle bodies which open quickly and cause the legendary bog/lag that Quadrajets are known for.

If you study the architecture of the Quadrajet, it’s obvious that it has nothing in common with Holley-based carburetor designs. It has more in common with the Carter Thermo Quad and AVS carburetors in that it uses main metering rods and a secondary air valve, making it function very differently than a Holley.

The Quadrajet has 1 3/32 or 1 7/32 inch primary throttle bores with either 750 or 800 cfm. It has been proven that you can modify the Quadrajet to flow in excess of 1,000 cfm. However, you don’t need that much flow for great street/strip performance. For most street and weekend races, you can get by with 750-800 cfm without breaking a sweat. And that has been proven in drag racing for decades.

When a Quadrajet is installed in a standard application, it operates in the environment for which it was designed. There isn’t much you need to do on the Quadrajet to make it work in a standard application. However, once you start making mods like a hot-roller cam, aftermarket induction and heads, and a host of other performance items, the Quadrajet struggles to keep up. This is where you need to modify your Q-Jet’s fuel metering and air valve function to keep up with engine upgrades.

Because the Quadrajet flows so well by design, it’s not that difficult to change fuel metering via larger nozzles and metering rods to start supertuning. One mistake performance enthusiasts make is not matching the fuel curve to the powerband. Fuel metering must follow the engine’s power curve, maintaining torque and power.

Because GM’s Rochester Carburetor Division has produced so many Quadrajets in 25 years, there’s still a generous inventory of cores to choose from. The trick is to find a good core to build on. The downside of older Quadrajet cores is abuse and core drift. With Core Shift come altered and closed passages.

It’s recommended that you find a virgin core that was never rebuilt, or even a new old storage unit if you can find it. There are still Quadrajets in their factory packaging. Check the eBay, Craigslist, and Racing Junk websites for options. Swap meets are another excellent place to find a good core. You’ll want to find a newer, more refined core, typically dating from the 1970s and early 1980s. The beauty of the newer Q-Jets is the “APT” (Adjustable Part Throttle) feature, introduced in 1968, which makes tuning easier. The easiest way to identify a later model Q-Jet is by the part number stamped on the casting. Older Quadrajet part numbers start with 702 and 704. Newer castings start with 170.

Another important issue with Quadrajets is carburetor sizing. Small blocks up to 350 ci are happiest at 750 cfm. Big blocks want 800 cfm. Of course, this also depends on the aggressiveness of your small block. The best approach to a Quadrajet conversion is to open up all the passages and make sure they are clear. Lead plugs must be carefully melted and removed, then the passages inspected and cleared. Passage integrity can be verified by blowing WD-40 or carburetor cleaner through these passages and observing fluid flow. The passages can then be sealed with high temperature epoxy or lead.

While you’re at it, it’s highly recommended that you inspect the throttle shafts and bushings for excessive wear that can lead to vacuum leaks and drivability issues. Vacuum leaks can also occur elsewhere between the mating surfaces of the carburetor body and the vacuum ports. Vacuum leaks can exist off the engine, e.g. B. on the brake booster, the air conditioning and more. Any vacuum leak anywhere will cause drivability problems.

We decided to visit Ted’s Carburetor Service in Lancaster, California, north of Los Angeles, who showed us how to properly tune GM’s spreadbore carburetor.

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01. This is a Rochester Quadrajet “4MV” carburetor from a Chevrolet 427ci, 390hp big block. The basic design of the Quadrajet did not change significantly during its 25-year production life, with the exception of the introduction of APT (Adjustable Part Throttle) in 1968 and later electronic mixture control. Fuel metering was electronically controlled in 1980.

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02. Disassembly of this Q-Jet begins with disconnecting the accelerator pump arm, choke assembly and all air horn screws. A Quadrajet conversion is actually quite simple. Follow the carburetor kit instructions and take lots of photos during the disassembly.

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03. Removing the air horn allows access to the fuel dipsticks, fuel bowl and throttle body. The Quadrajet has mechanical secondaries that open instantly when you crush the gas. The secondary air valves, located in the air horn, allow a smooth transition into secondary air via manifold vacuum at full throttle. This design is not unique to the Quadrajet. Carter nailed it with the Thermo Quad and AVS. Ford did it via the 4300/4300D/4350 carburetors. All use an air valve transition into the secondaries.

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04. Next the float assembly is removed. The Quadrajet fuel bowl is a large continuous reservoir for all four wells. Because the fuel cup is central, it keeps fuel more stable under various driving conditions, with less chance of fuel shortage.

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The Rochester Products Division (RPD) was a division of General Motors that manufactured carburetors and related components, including emission control devices and cruise control systems, in Rochester, New York. In 1995 Rochester became part of Delphi, which in turn became a separate company four years later[1] and continues to manufacture fuel injection systems in Rochester, now part of General Motors Automotive Components Holdings-Rochester Operations.

history [edit]

The company began as the Rochester Coil Company, founded by Edward A. Halbleib[2] in 1908, and became the North East Electric Company the following year. In 1916 the company was located at 348 Whitney Street, Rochester.[3]

In 1929 Alfred P. Sloan announced the takeover of the company on behalf of General Motors.[4] “For several years this company has been a premier manufacturer of starters, ignition systems and other electrical equipment.”[5] “It was consolidated with GM’s former Delco-Light Company in 1930 and later renamed the Delco Appliance Division.” In 1937 Rochester Products was formed, planned as a second plant for Delco Appliance, but achieved divisional status in 1939. A 1953 ad in Life stated, “Rochester makes original equipment carburetors for 1950 to present Chevrolet, 1949 to present Oldsmobile, and 1951 to present Cadillac. Rochester also supplies replacement carburetors for 1932 to present Chevrolet.” [7] Rochester also supplied Pontiac while using the Power Jet name in the replacement market.[8]

In 1952, The Oregonian reported: “Automotive lighters manufactured by General Motors’ Rochester Automotive Products Division are tested to reach a temperature of 1400 degrees in no less than 10 and no more than 12 seconds.”[9]

The 2G carburetor (later 2GC and 2GV), commonly referred to as the 2 Jet, was introduced in 1955 and continued to be used on GM V8s until at least 1969. In total it was used in at least 125 applications, including the Brockway’s straight-six.[11] In 1957, Chevrolet introduced its first fuel-injected engine, the high-performance Rochester Ramjet option for Corvette and passenger cars for $484. In 1956, Oldsmobile also experimented with Rochester fuel injection at GM’s desert proving ground near Phoenix, but offered the Rochester triple-carburetor J2 option for 1957.

The company is best known [according to who?] for the Quadrajet carburetor, which was originally developed in the 1960s and remained in production into the 1980s with modifications to meet increasingly stringent emissions standards. The Quadrajet was computer controlled in California in 1980 and in the rest of the States in 1981; its final application was 1990 Cadillac Brougham and 1990 GM full-size station wagons with the Olds 307 engine. [citation needed] RPD pioneered fuel injection systems in road cars in the 1980s. [citation needed] Rochester also manufactured various emission control devices, such as canisters and EGR valves,[15] which were used in both GM and non-GM vehicles. [citation needed] Other products manufactured at this plant were locks and keys[citation needed] and steel tubing for vehicular and non-vehicle applications. [citation needed] Rochester’s last major carburetor design was the Varajet II, [citation needed] essentially a longitudinally bisected Quadrajet [citation needed] and was one of the few successful two-barrel progressive carburetors. [according to whom?] It was installed on 4- and 6-cylinder engines from 1979 to 1986. [citation required]

In 1981, Rochester Products and Diesel Equipment Division merged in a move publicly described as cutting costs. At this point RPD had about 7,000 employees and DED about 3,300 employees.[16] DED had facilities in Grand Rapids, Michigan. [17] Headquarters remained in Rochester.

In 1988 the diesel fuel injection business was sold to Penske Transportation and Rochester Products and AC Spark Plug merged. [19]

In 1994, AC Rochester’s Grand Rapids operations were spun off.[20]

See also[edit]

We get to the basics of how to tune a carburetor. We are on the final round of this 3 part series for a Rochester Quadrajet carburetor rebuild. This is definitely the easiest part, but probably one of the most overlooked for anyone who still owns a carburetor.

Disassembly – Part 1 to rebuild a Rochester Quadrajet carburetor

Assembly – Part 2 to rebuild a carburetor

Steps to tune a carburetor

Specifically, we’re talking about how to tune a Quadrajet carburetor, but really, these steps can be applied to most carburetors, including the Holley. There are really only 3 steps that need to be addressed. Of course, this assumes your rebuild is solid and you don’t have any intake manifold issues.

Step 1 – Choke

The first step in tuning a carburetor is dialing in the choke. The choke not only helps with cold starts, but also limits or enables the operation of the secondaries (4-cylinder). If you’ve ever not heard that 4bbl howl when you put your foot on the ground, it could be the choke keeping the secondaries closed.

Loosen the 3 small straight slotted screws on the plastic choke cover, then rotate the cover until the choke blade is barely open (almost closed). This assumes the Quadrajet is cold. Once the engine has reached temperature (160 degrees F), the choke blade should be straight and vertical and the secondaries should be free of binding. If necessary, loosen the 3 screws again and adjust rich/lean until the choke blade is vertical.

Step 2 – Idle Air/Fuel Mixture

Using a vacuum gauge, adjust the idle air/fuel mixture screws on each side until maximum vacuum is obtained. Locate a manifold vacuum connection on the Quadrajet, e.g. B. the PCV connector and connect your vacuum gauge. When the vehicle is warm, use a screwdriver to adjust the idle mixture screws. Begin by turning the screws in (clockwise) until they bottom. Then back out each idle mixture screw 3 or 3-1/2 turns (counterclockwise). This will ensure that both screws are initially set the same.

Begin by adjusting one of the idle mixture screws, but do not change them until you are finished with that screw. First, try turning the screw inward (clockwise) 1/4 turn. As the vacuum increases, turn in an additional 1/4 turn. If it decreases, go the other way. Find the maximum vacuum, then move to the other screw and repeat the steps. Once maximum vacuum is achieved for both screws, you are ready for the final step.

Final step in tuning a carburetor – fast idle

Finally, all we have to do is adjust the fast idle screw. This screw is spring loaded and is located on the linkage where it connects to your throttle cable or arm. You probably want to set the idle somewhere between 750 and 950 rpm. Turn the screw in (clockwise) to increase speed or out to decrease speed.

We bought our Quadrajet Rebuild Kit from Mikes Carburetor Parts.