“Quadrabog,” “Quadrajunk,” and a host of other time proven nicknames define GM’s most notorious four-barrel carburetor. Enthusiasts either love it or hate it. However, the Quadrajet is easily 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 produced by the Rochester Carburetor Division of General Motors from 1965-’90. It was produced in many forms, including electronic-control from 1980-’90, for a wide variety of GM applications. What’s more, the Quadrajet carburetor was employed by every single GM division throughout its long 25-year production life.
In its most basic form the Quadrajet was produced as the “4MV” (automatic choke/manifold choke stove), “4MC” (automatic choke, integral choke heat coil, heat tube) and the “4M” (manual choke). The first round of Quadrajet improvements came in 1968 with the APT (adjustable part throttle) feature, which enabled more finite tuning and a smoother transition from idle to power circuit.
Computer Command Control
Before we go any further, the “CCC” (Computer Command Control) electronic control Quadrajet, known as the “E4M”, cannot be used on a non-CCC vehicle. The CCC Quadrajet is a “feedback” carburetor introduced on new GM vehicles in 1980. Although the E4M sounds complex it’s actually quite simple. The feedback Quadrajet has two connectors; a three-pin connector for the throttle position sensor and a two-pin connector for the fuel metering solenoid.
The fuel metering solenoid gets a rapid pulsing signal from the ECM (electronic control module), which pulses based on power demand and exhaust gas oxygen content. This is known as electronic mixture control. The objective is cleaner emissions and finite power tuning and fuel efficiency. The throttle position sensor provides throttle position feedback to the ECM. An O2 sensor in the exhaust monitors exhaust oxygen content. Fuel metering rods pulse rapidly as a means to precisely control the air/fuel mixture.
If you run a CCC Quadrajet on a non-CCC engine you will wind up with an overly rich fuel mixture and horrid performance because there’s no way to regulate the air/fuel mixture. Fuel metering rods stay at full rich. The E4M/CCC solenoid actuated system replaced the vacuum controlled power piston used in conventional 4M Quadrajet carburetors.
Understanding Quadrajet Function
Despite the Quadrajet’s reputation for frustrating tunability, it is a great performer when it is properly assembled and tuned. Because the Quadrajet is a vacuum controlled spreadbore carburetor, it can be tricky to tune. Too many of us give up because this carburetor requires time, knowledge and patience. Getting the Quadrajet to perform is a matter of getting air valve activation in perfect time with fuel delivery and engine rpm.
One nice thing about the Quadrajet is fuel economy if you keep your foot out of it. Petite primary bores keep fuel consumption to a minimum. Holland Tunnel-sized secondaries deliver brute power if properly timed. Good for the interstate cruise. Convenient when it’s time to pin the butterflies. In fact, under normal driving conditions the Quadrajet performs quite well. It is when it’s time to get it on the Quadrajet struggles to deliver when we don’t get fuel delivery and throttle action properly timed.
The Quadrajet was never developed to be a high-performance carburetor to begin with though 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 performance. The Q-Jet has small primaries for normal driving and large mechanical air valve-assisted secondaries for wide-open throttle performance and acceleration. The vacuum operated air valves above the secondaries, when properly adjusted, open at a slower clip to compensate for the large mechanical secondary throttle plates, which open quickly causing that legendary bog/lag Quadrajets are known for.
When you study the Quadrajet’s architecture it’s apparent it has nothing in common with Holley-based carburetor designs. It has more in common with the Carter Thermo Quad and AVS carburetors because it employs main metering rods and a secondary air valve, which makes it function very different 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 you can modify the Quadrajet to flow in excess of 1,000 cfm. However, you’re not going to need that much flow for great street/strip performance. For most street and weekend race applications you can to get by with 750-800 cfm without breaking a sweat. And this has been proven in drag racing for decades.
When a Quadrajet is installed in a stock application it’s operating in the environment it was designed for. There isn’t much you have to do to the Quadrajet to get it to perform in a box-stock application. However, when you start making modifications such a hot roller camshaft, aftermarket induction and heads, and a host of other performance elements the Quadrajet struggles to keep up. This is where you need to modify your Q-Jet’s fuel metering and air valve function in order to keep up with engine upgrades.
Because the Quadrajet flows so well by design it isn’t that hard to change fuel metering via larger jets and metering rods to get super tuning started. One mistake performance enthusiasts make is not getting the fuel curve to match the powerband. Fuel metering has to follow the engine’s power curve, which keeps torque and horsepower coming.
Because GM’s Rochester Carburetor Division produced so many Quadrajets in 25 years there’s still a generous inventory of cores out there to choose from. The trick is finding a good core you can build from. The downside to older Quadrajet cores is abuse and core shift. With core shift comes altered and closed off passages.
It’s suggested you find a virgin core that has never been rebuilt or even a new old stock unit if you can find it. There are still Quadrajets in their factory packaging out there. Check 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, which is typically from the 1970s and early 1980s. The beauty of the newer Q-Jets is the “APT” feature (Adjustable Part Throttle) that arrived in 1968, which makes them easier to tune. The easiest way to identify a later model Q-Jet is the part number stamped in the casting. Older Quadrajet part numbers begin with 702 and 704. Newer castings begin with 170.
Another important issue with Quadrajets is carburetor sizing. Small-blocks through 350 ci are happiest with 750 cfm. Big-blocks want 800 cfm. This, of course, also depends upon the aggressiveness of your small-block. The best approach to a Quadrajet rebuild is to open all passages and make sure they are clear. Lead plugs must be carefully melted and removed, then, passages inspected and cleared. Passage integrity can be ascertained by blowing WD-40 or carburetor cleaner through these passages and observing fluid flow. Then, passages can be capped off with high-temp epoxy or lead.
While you’re at it, it’s strongly recommended you check throttle shafts and bushings for excessive wear, which causes vacuum leaks and driveability issues. Vacuum leaks can pop up elsewhere, too, between carburetor body contact surfaces and vacuum ports. Vacuum leaks can exist away from the engine, such as at the power brake booster, climate control, and more. Any vacuum leak anywhere will cause driveability issues.
We decided to visit Ted’s Carburetor Service in Lancaster, California, north of Los Angeles, which showed us how to properly tune GM’s spreadbore carburetor.
01. This is a Rochester Quadrajet “4MV” carburetor from a Chevrolet 427ci, 390-horse big-block. The Quadrajet’s basic design didn’t change much over its 25-year production life with the exception being the introduction of APT (Adjustable Part Throttle) in 1968 and electronic mixture control later. Fuel metering became electronically controlled in 1980.
02. Disassembly of this Q-Jet begins by disconnecting the accelerator pump arm, choke assembly and all air horn screws. A Quadrajet rebuild is actually quite simple. Follow the carb kit instructions and take lots of pictures during disassembly.
03. Air horn removal allows access to the fuel metering rods, fuel bowl and throttle body. The Quadrajet has mechanical secondaries, which open promptly when you mash the gas. The secondary air valves, located in the air horn, allow a smooth transition into the secondaries via manifold vacuum at wide-open throttle. This design is not unique to the Quadrajet. Carter did it with the Thermo Quad and AVS. Ford did it via the 4300/4300D/4350 carburetors. All employ air valve transition into the secondaries.
04. The float assembly is removed next. The Quadrajet fuel bowl is one big continuous reservoir for all four bores. Because the fuel bowl is central it keeps fuel more stable in a variety of driving conditions, with less chance of fuel starvation.
05. Ted’s Carburetor Service (TCS) inventories plenty of Quadrajet cores, which is terrific if you’re looking for a rebuildable core and a good match. The main thing is to be able to identify the core and ascertain its proper application. Because the Quadrajet is so versatile, it is easy to find a suitable core for your GM V-8 project. Did you know Ford ran the Quadrajet on its 1970-’71 429ci big-blocks?
06. Any good carburetor builder will mill the Quadrajet’s mating surfaces to prevent leakage. TCS runs each main part across this sanding belt to true the surface. Carburetors can warp from heat cycling, which calls for contact surface work. Warping causes vacuum leaks, irregular fuel delivery and other performance issues.
07. Our Quadrajet has been boiled and blown out with all passages free and clear. The Quadrajet consists of three main pieces: baseplate, main body and air horn. Threads are chased and any marginal hardware replaced. If you examine this Q-Jet’s primary boosters you will see they are dual boosters. It can be endlessly debated if a dual booster was even necessary. In fact, not all GM Quadrajet applications had dual boosters. Pontiac, as one example, went with a single stage booster, which improved airflow.
08. Assembly begins with main metering jets. One cool thing about the Quadrajet is these main metering jets and rods, which advance fuel flow as the throttle is opened. The aftermarket offers a wide variety of main metering jets and rods. The trick is to know how to size these rods and jets for your application.
09. The main body to base plate gasket is next. Use a mild thread locker on these screws. Make sure all passages line up and are clear. Note the fuel wells, which have been opened, cleaned out, and recapped with an epoxy resin. The factory lead in these locations tends to crack and leak over time and use.
10. The main body and base plate are assembled. Assembly of the main body is next. Before assembly begins, check all passages with carburetor cleaner and compressed air. Don’t forget to wear eye and ear protection for this step.
11. This is the accelerator pump check ball and retaining screw. The ball unseats when fuel is drawn into the pump chamber. It seats when the throttle is opened and primaries get a shot of fuel.
12. The Q-Jet’s accelerator pump system feeds the primary bores only. The secondaries get a wet fuel charge from a fuel bleed above each secondary air valve plate. Airflow through the secondaries draws fuel from the secondary main metering circuit, which acts like an accelerator pump. If you’re struggling with throttle lag at “pedal to the metal,” chances are you have insufficient accelerator pump shot. To fix this you may want to try to slowly and carefully drill out the primary accelerator pump nozzles one small step at a time. Go too large and you wind up with poor fuel atomization. Before you do any accelerator pump nozzle mods make sure you have the accelerator pump properly adjusted, including the correct duration spring for the job.
13. Accelerator pump duration spring is installed before the pump piston.
14. The accelerator pump piston and spring have been installed. The float valve and float assembly are installed next.
15. Float adjustment is critical to proper fuel delivery and safety. Follow the float height instructions in your carburetor kit or Chevrolet service manual. Float adjustment boils down to application and driving agenda. Measure mid-float and tip of the float as shown. The engineering genius of the Q-Jet is evident here. Float is dead center in a generous fuel bowl for great stability regardless of vehicle attitude.
16. The Quadrajet’s power piston does the same job as a Holley power valve, only in this case the power piston modulates the main fuel metering rods. When vacuum comes on strong at wide-open throttle the spring-loaded power piston increases fuel delivery.
17. The power piston and fuel metering rods are installed next. The power piston works like a Holley power valve (spring pressure against manifold vacuum), allowing higher fuel flow through the power circuit as the throttle is opened. Metering rod sizing is what makes or breaks Quadrajet performance.
18. The choke assembly is installed next. Choke heat comes from a choke stove in the intake manifold with the 4MV. The 4MC has an integral choke with a manifold heat tube.
19. The float bowl baffle is next; inserted above the float valve. This baffle contains fuel splash.
20. A close examination of this Quadrajet air horn shows the absence of the APT feature common to 1968-up Quadrajets. APT (adjustable part throttle) control enables a more finite means to mixture control as the throttle is opened. In your search for a Quadrajet core, you’re going to want the APT feature, which is adjustable without having to disturb the air horn.
21. The air horn screws should be snugged in crisscross fashion to prevent warpage, then, tightened firmly.
22. The main fuel metering rods and linkage are installed as shown, then, properly adjusted. This is where the Quadrajet becomes challenging. If you’re installing a Quadrajet in a stock application atop the engine it was designed for, very little tuning is required. However, as you ramp up performance, precision tuning via primary and secondary main metering rods and jet sizing becomes critical. Always err to the side of rich. The aftermarket offers a wide variety of jet sizes and metering rods. It is suggested you have a variety of rod and jet sizes on hand.
23. The idle mixture screws are installed next. Never tighten these needle tip screws. Seat them, then back out 1 1/2 turns for initial start-up and adjustment. When you fire the engine, get the idle mixture as lean as you can while maintaining the most intake manifold vacuum possible. If there is surging, back the idle mixture needles out until the idle stabilizes.
24. The accelerator pump lever is installed and adjusted. Begin adjustment where the accelerator pump arm was to begin with, then take your tuning from there. If you experience a flat spot as the throttle is aggressively opened, ramp the accelerator pump shot tuning up from there. A bog or flat spot does not always mean a weak accelerator pump shot. It can also be air valve timing or main metering issues.
25. This is the Quadrajet’s vacuum actuated choke pull-off for reduced cold-start emissions. Here, TCS checks the choke pull-off for vacuum leakage.
26. This is the fuel bowl vent valve, which was a predecessor to evaporative emissions control common from 1970-up. When the throttle is opened, this vent valve allows air into the fuel bowl, but doesn’t let fuel vapors out.
27. Quadrajet carburetors are easily identified by the GM part number stamped into the main body casting (arrow). If you see 702 or 704, you’ve found an older Quadrajet.
28. There are two basic types of Quadrajet fuel filters: paper and sintered bronze. Sintered bronze is exclusive to early Quadrajets while paper came later.
29. We dyno tested the 800-cfm Quadrajet on a hydraulic roller cam-equipped 427 with remarkable results for a street engine. We achieved 450 horsepower and 450-475 lb-ft of torque at 4,300 rpm on a hot, triple-digit day in Southern California. Gregg Jacobson of PHD Speedcenter also dyno-tested this 427 roller with Holley/Edelbrock Air-Gap induction and managed 514 horsepower and 538 lb-ft of twist. Had we had the element of time, Gregg could have pushed the numbers closer to 550.