How to rebuild a Rochester Quadrajet 4MV carburetor
This article is written with the intention of informing, guiding and assisting the first-time builder who has never dealt with a carburetor before. It merges detailed, step-by-step directions, complete with images, links to online and print resources, and a multitude of helpful tips and pointers collected from the members of Hotrodders forum.
The article is also meant to complement the rebuild guide books available, shedding extra light on any topic or procedure that may require further explanation. In the future, as time allows for updates, the article will cite sections and passages from related books to help the reader follow along with ease.
Although the article specifically deals with the Rochester Quadrajet (aka "Q-jet, qjet, etc.) 4MV model, the information found here is useful for working on other Rochester Q-jet models (including the Dualjet (which is the front half of a Quadrajet) as well.
At times, the reader may encounter differing viewpoints, especially when dealing with certain repair options. It is up to the reader to decide which method works best. If confusion persists or further advice is needed, it is highly recommended that the reader visit the Hotrodders forum and ask for assistance from fellow members.
 Planning and organization
 A. Becoming familiar with the carburetor
If you have never dealt with a carburetor in your life, it is important to familiarize yourself with the various components. Please refer to the Books/ Guides/ Websites /Multimedia section immediately below for helpful materials.
- How to Rebuild and Modify Rochester Quadrajet Carburetors by Cliff Ruggles is a frequent recommendation by the members of Hotrodders forum. It is a highly-reviewed book and the author is considered a leading authority on Quadrajet carburetors.
- In addition to his book, Cliff Ruggles also maintains a website (Cliff's High Performance), with a forum dedicated to discussing carburetors.
- Another highly reviewed title is Rochester Carburetors by Doug Roe.
- Other titles devoted to Rochester Quadrajets are also available from retailers such as Barnes and Noble, Amazon, eBay and its sister site Half.com.
- If you are a more visual learner, please check out Video series by alpheus1975 on YouTube. Newer, improved videos may be available by other authors as well so be sure to check for current material.
 Carburetor identification: Locations and decoding
- Once it has been determined that your Rochester Quadrajet carburetor requires a rebuild, the first thing to do is to locate the identification number. The identification number will help with ordering the correct parts or a rebuild kit. The number is usually found as a stamping in the body of the carb on the driver side of the carburetor, running vertically up and down. Very early model Q-jets used a round disc attached to the drive side of the carb. These carbs have the less desirable needle and seat assembly and float fulcrum location, so unless the carb is being used on a restored or period correct build, the later carbs- especially those with the adjustable part throttle adjustment or 'APT' located on the airhorn- will be a better choice for a performance application.
For decoding the carb number, please see the following article for further information and assistance: Rochester Quadrajet carburetors: Identification and decoding
 Other considerations
- When getting a carb from another vehicle, whenever possible, get the factory cable bracket. Even if you're using an aftermarket intake, it might come in handy for making up your own bracket. And regardless if using an aftermarket intake or not, the bracket can be used to measure the correct cable/carb relationship for setting up whatever bracket that's used. The throttle linkage are specific to the manufacturer (GM products including 1971 Ford Mustangs with the 429 Cobra Jet and mid-1980s Mopars have used the Quadrajet).
- Make note of the engine displacement/engine VIN code. These carbs were used on 4.3L V6 through 7.4L V8 engines (besides its use with GM products the Quadrajet was found in some obscure applications e.g. 1971 Ford Mustangs with the 429 Cobra Jet and 1983-88 Chrysler vehicles), so knowing what it came from gives a leg up on what may potentially need changed.
- If the Q-jet is to be used with an automatic transmission that uses a detent (kick down) or TV (throttle valve) cable, be sure the throttle arm is correct (below, right). Many manual transmission-equipped vehicles are missing the lower part of the throttle arm (below, left); this is where the detent/TV cable ordinarily attaches.
 B. Parts
 Carburetor rebuild kits
Available through various manufacturers. For user experiences, please check out the Hotrodders forum thread, Q-jet rebuild kit.
 Individual parts
Please refer to the Quadrajet tuning and repair parts section below.
 C. Tools
 Parts organizer
- The carburetor is held together and in place with fasteners of various sizes. There are also a lot of small parts on the inside of the carburetor body. It is very important to use some sort of a parts organizer to separate everything. It does not have to be anything fancy. You can use things usually found around the house. Egg cartons work well, and the top can be closed to keep the parts in place. Old ice cube trays can also be used.
- Other ideas suggested by Hotrodders forum members:
"The muffin pans work great. As a technique, I use two pans and label each "bin" and place the old part in one bin and at the same time, I place the new part in the other pan. Most kits are 'generic' so this is the best time to pick out the part that matches the removed part and set the residual parts aside. " ^3- trees, forum member.
"a cupcake pan works really well for organizing parts ... i also use an old silverware tray for parts" ^- matts37chev, forum member.
There are a few tools that will make rebuilding the Q-jet easier:
- A 1/16" punch for removing the accelerator pump arm roll pin
- An adjustable T-square for setting float level
- A "pin vice" and a set of small numbered drill bits for measuring and making (or enlarging) small diameter holes. They are bought typically in sets numbered from #61 (0.0390") to #80 (0.0135"). Numbered bits are also available larger than #61- all the way to #1 (0.2280"). At this point, lettered bits take over. Interspersed throughout these are also the fractional and metric sizes.
Having a complete 115 piece drill bit index is a very handy tool to have- it allows making, measuring and gauging holes as well as setting Holley and other float levels where the bit can be fit under the float, or above it when setting the float drop.
- Socket wrench set
- Open-end wrench set
- Screwdrivers - various sizes
- Parts cleaning tray - Keeps the parts in place and also keeps any chemicals from spilling in undesired places.
- Parts cleaning brush - Used for cleaning off most of the grime and gunk off the carburetor parts after dipping them.
- Wire brush - Used for scrubbing off the tough spots after dipping the parts.
- #7 drill bit ^4
- Shop towels / rags
- Compressed air - For blow-drying carburetor parts after cleaning and washing them.
"I built a stand to hold the carb body so that I could functionally check all the linkage and clearances on the butterflies. It also makes assembly of the metering rods much easier. You can buy a stand, but hard to justify unless you plan on rebuilding lots of carbs. My Q-jet book told me how to build the stand and it has been invaluable over the years." - trees, Hotroddersforum member.
- Parts cleaner dip - 1 gallon can is sufficient for the job. This is a powerful chemical with strong fumes. Care must be taken to protect the skin and eyes, and provide proper ventilation. The use of latex gloves and a full-sleeved shirt is highly recommended when working with this chemical.
- Carburetor cleaner spray - A few cans should be kept on hand.
 Taking photos
"Disassembly tips? Take lots of close up, well lit pictures of the linkages." ^ - cyclopsblown34, Hotrodders forum member.
- A digital camera with a good flash will come in very handy. There is no need for a top of the line camera. Even today's cellphones with 5 megapixel or higher rating are more than sufficient. Become familiar with the camera you have and use it to photograph everything, from every angle, every step of the way. If you ever get lost during the rebuild process, you only need to flip through a few pictures to get back on track.
- The things you will need to keep an eye on are protecting the camera from grease and dip, and the battery life. Battery life varies from camera to camera, as well as the age. Make sure to keep the camera fully charged and as often as possible. If you keep the charger plugged into a socket close to where you are working, charging the battery during down time is a breeze.
 Step-by-step documentation tips
- Remember, with a digital camera, there are no limitations on how many pictures you can take because you are not using film. And when you upload your pictures, be sure to organize them into folders and sub-folders with useful categories for titles, i.e. Removal, Dis-assembly, etc. Extra effort here will save a lot of time and headache in the future when you have to go through the photos to match up the right parts or bolts.
- As you organize photos, be sure to delete an "extras" or unusable shots. You can do this either as you are taking pictures or when you upload them to your computer. This will further eliminate clutter and you will only have to go through pictures that are useful. It is also a good idea to use a photo editor for processing images. Quick cropping, sharpening and brightening of photos will turn many mediocre shots into useful images. For a free and easy to use photo editor, Photoscape is recommended. GIMP is another one, but with more sophisticated features.
- If you prefer, keep written notes as you go along. A picture is a thousand words, but a handy set of notes can be quite valuable too. If you have printed out this article, use the empty spaces to jot notes as needed. This way, everything you need will always be in on place.
 D. Safety
Safety is always a top priority in any project, especially those which require tools and chemicals. Be sure to work in a place that can keep out children and animals. When using chemicals, it is a good idea to warn anyone nearby of fumes. Please refer to Health and safety in the shop or garage for helpful tips and ideas.
- Ventilation is very important when working with chemicals. For the rebuild, you will be using the Chem-Dip, which has strong fumes. If working inside the garage or other indoor places, be sure to keep the windows and doors fully open. If available, employ a large exhaust fan to suck out the fumes.
- Carburetor rebuilds deal with small parts, precise measurements and various tools and chemicals. Proper lighting is key to getting the job done safely without errors or hazardous mistakes.
- Latex Gloves
The Procedures section is currently under construction. Please refer to the threads and links under Recommended Resources, which will serve as the basis for this section
 A. Before you begin: Preparation
The following tips will save you time, avoid any unnecessary stress, and keep you working safe.
- Make sure you are now familiar with carburetor components, as well as the overall process of rebuilding a carburetor. Do not proceed until you are ready.
- Have all new parts, tools, chemicals, and safety gear on hand and ready to go.
- Set aside at least one to two days where you will not be interrupted. The more gaps during various steps, the more likely you are to forget things.
- Prepare your work area by cleaning it. Chemicals do spill and small parts do fall. Clear off and remove any obstructions now. #Organize your tools for easy access. Need help? Check out "Quick organization of tools in your garage pays off!"
- Print out the Procedures section of this article and keep it at your workspace. This way, you won't get any grease on the books or the computer.
- If you are a visual learner and can keep a laptop or other computer device with internet access nearby, you can follow along with the video series by alpheus1975 on YouTube, or any other video/multimedia resource you find helpful.
 B. Prior to removal: Documenting and disconnecting
- Begin by taking pictures of the carburetor as its sits on the engine. Photograph from every possible angle, including:
- Driver-side front
- Driver-side full
- Driver-side rear/firewall
- Passenger-side rear/firewall
- Passenger-side full
- passenger-side front
- Top of the carburetor
- All linkages
- All hoses, including where they originate from if possible (i.e. hose from vacuum advance canister to carburetor).
- Once the pictures have been taken, using masking tape and a pen or marker, label every hose. Take pictures of every hose, with the label clearly marked and showing in the images.
- At this point, disconnect all hoses and linkages and carefully move them out of the way. Some of the common connections include:
- Fuel line/hose - Be especially careful with the fuel hose as it will spill some out.
- Vacuum advance hose
- PCV hose
- Brake booster hose
- Throttle linkage/cable: Held in place with a clip.
- Throttle return cable: Held in place with a small bolt and nut.
- Throttle return spring
- Divorced-choke coil- Unscrew from the intake manifold if applicable.
- The carburetor is held in place by two long bolts visible on top of the air horn and either two nuts or two bolts on the back of the carburetor on top of the throttle cable bracket. Loosen these up, remove them, and set them aside in the parts organizer.
 C. Removal
Try lifting the carburetor up slowly, making sure nothing else needs to be disconnected or removed. Once it clears the intake manifold, take it out of the engine bay and turn it upside down over a container to catch the fuel that will fall out. Skipping this step can result in a large puddle of fuel on the workspace.
Place the carburetor on a stand if you have one. Keep a couple of shop towels or rags around it in case any more fuel spills out during dis-assembly.
 D. Dis-assembly
Note: Be sure to have your parts organizer near by at all times. Also, take the time to match up the new parts with the old ones as soon as you remove them. If something doesn't match up, return it for the right part before beginning re-assembly.
 Accelerator pump actuator
"Something you'll need to know for disassembly, on the driver side front, the accelerator pump, there is a fulcrum the actuator arm rocks on, there are I think two holes in it, note where the throttle linkage attaches to it. The instruction sheet will probably tell you where to attach it depending on which engine too. To remove the lever, just use a small diameter punch and push the roll pin out of the fulcrum point and lift the lever off. I did several Q-jet rebuilds, all the while cursing the way I had to twist the top of the carb to remove it, once I realized there was a removable pin, it made disassembly and reassembly a breeze." ^ - cyclopsblown34, Hotrodders forum member.
Caution: It is very important to take good, well-lit, close-up pictures of the linkages before dis-assembly. The choke linkage has several parts that must line up in a specific way during re-assembly or they will not work.
 Air Horn and Metering Rods
 Float/fuel bowl
- Accelerator pump and spring
- Power piston pump, spring and rods
- Check ball
- Float assembly
- Needle and seat
- Fuel Filter Housing
 Needle and seat
Be sure the metal seal under the old needle seat has been removed before a new seal is installed.
Be sure to hook the needle clip as shown below:
 Base/throttle body
- Idle mixture screws
 Trouble spots
Please keep an eye on the following areas during dis-assembly and rebuild.
 Accelerator pump assembly
If the accelerator pump wasn't working correctly, be sure to attach the linkage to the pump arm hole nearest the pivot to get the biggest pump shot. You can try the smaller shot and if that works well, use it.
There are several different accelerator pump rod lengths used depending on the carb, if it's the wrong length this can cause the pump to work poorly.
If the springs are reversed and the return spring that should be under the pump assembly is used as the override spring on the shaft, and the override spring is used under the assembly, that can cause it to work poorly. Sometimes the fuel filter spring gets mixed up with the accelerator pump springs as well.
If the piston seal is wrong it can have a hard time pushing fuel and/or allowing the pump to refill.
There's a ball that acts as a check valve under a screw in the float bowl, driver side. Be sure the ball is in place and sealing as it should. The check ball seat can be re-formed by carefully tapping an old check ball against the seat using a small punch and light mallet/hammer.
Sometimes the accelerator pump bore gets glazed where it doesn't seal against the piston very well. The bore can be lightly honed using 600 grit wet or dry. I use a wooden dowel w/a piece of cloth wrapped around the dowel and the emery paper wrapped around the cloth. Then make like a boy scout starting a fire or like you're lapping a valve. You don't want to remove the finish, just break the glaze ever so lightly and to smooth the bore.
 Stripped or loose/damaged fuel inlet threads
This is an all-too-common problem on the Q-jet. Because the casting is relatively thin and the threads are so large (7/8-20 for most 1971-back carbs, 1"-20 for later carbs), using an oversize fuel inlet to cut new threads is to be avoided if at all possible. The better solution is to use a helicoil-type thread insert. For best results this requires a mill and a fixture for holding the carb body perfectly in alignment for the tap to cut the threads. The tap has to be modified to also cut the seat at the bottom of the threads for the inlet gasket to seal on.
 Loose power piston retainer
According to Hotrodders forum member TommyK:
"It is not uncommon for the retainer to fail to positively secure the PP in its hole in and of itself. The solution is to take a chisel and gently stake the edge of the hole in one or two spots against the retainer." ^
Be sure that when it's all said and done that there's positively no friction from the retainer on the power piston shaft. It has to be absolutely free to move without any hindrance.
 Leaking float bowl well plugs
A common problem (mostly) on the early (1968-back) Q-jets especially, is leaky wells. The early leak-prone plugs can be identified by being brass cup-type plugs as opposed to aluminum. The late-type plugs are shown below, left- front plugs arrows, rear plugs circled. The 'cure' for this has long been to use JB Weld, etc. to seal them over. This is just a stop-gap measure, the epoxy will degrade and quit sealing.
The well plug stop-leak pads that come in many rebuild kits are a joke (below, center and right). They will seal the leaking rear plugs for a few heat cycles before shrinking down to a fraction of their former thickness. The result is the carb will initially run as it should, quickly followed by the same symptoms of a leaky plug returning. Do not waste your time with them. If you have a leaking plug, repair it right.
The tell-tail signs of leaking well plugs is if the carb float bowl empties overnight. This will usually leave a small hit of fuel in the bottom of the accelerator pump well, but unless the engine starts and runs well when cold (or has an electric fuel pump), the engine will often stall after starting, then will take a lot of cranking for the mechanical pump to refill the bowl and the engine to restart.
On the post ~1968 Q-jets, the [B]secondary[/B] plugs are spun in then peened, so to speak. The [I]main[/I] plugs are pressed and sealed w/a soft metal around them- and they can and do leak on some carbs, regardless of the year.
Epoxy is not the way to fix a leaking main well plug- tapping them for a machine screw works much longer- as in forever. Epoxy will eventually delaminate from the metal and the leak will resume.Ruggles mentions using Marine Tex epoxy to seal the threads of replacement well plugs, but he states that using epoxy alone to repair a leaking well plug is akin to "cutting your arm off and putting a Band-Aid on it".
FWIW, there is an epoxy by Devcon (made in the USA, look for "Devcon Home"). It comes in the older 2 tube packages and the newer double plunger syringe type. On the back it clearly states it *RESISTS UNLEADED FUEL*, salt, gasoline, oil etc. Full set time 8-12 hrs/2500 psi. However, be aware there is no empirical evidence that this epoxy will work for the life of the carb without any delamination or degradation.
So, if a permanent fix is wanted, the wells can be resealed with new plugs that are peened over to hold them in place, or the wells can be drilled and tapped for plugs that will seal them for good. The primary wells seldom leak, the secondary wells on the early (1968-back) carbs do leak much more often than the later carbs.
Also please see post #48 on page 4 on the thread "Rochester Quadrajet 4MV Carburetor: Removal, Disassembly, Rebuild (Rookie Level")page 6 of the thread "1977 Chevy K20: Engine Suffers from Hesitation/Power Loss When In Drive"
 Throttle shaft bore wear
The primary bore of the aluminum throttle plate is where the majority of wear is going to be. The shaft has an anti friction coating (the green colored material seen on the shaft and some linkages) but even so, the shaft wears directly on the aluminum of the throttle plate, so the primary shaft will also show some wear- but being steel on aluminum, the shaft wear is going to be much less than seen in the bore. There are bushing kits to fix the throttle shaft bores that use a brass oilite-type bushing if the bore is worn too much.
How much wear is too much? There will be some play in ALL Q-jets (even new ones); that's why determining how much is too much is a little tricky. Basically if it idles good and the idle is steady even when the throttle shaft is moved fore and aft, it's good to go. But if you get an erratic or unsteady "hunting" idle, or if the shaft moves fore and aft a considerable amount- especially if there's enough wear that the throttle blade can contact the throttle bore- the throttle shaft bore is obviously worn out and needs bushed or the entire throttle plate can be replaced. Wear on the throttle shaft bore is mainly caused by the tension the throttle return spring exerts on the shaft/bore, especially if the return spring is installed wrong (more on this below).
Wear on the primary shaft bores of the throttle plate can be avoided by properly orientating the return spring. While theoretically the best way is to use a bellcrank ("Best", below), using a throttle return spring in front of, and in line with, the throttle cable or linkage ("Better", below) is simpler and very nearly as good as a bellcrank assembly:
Throttle return springs used as shown below should be avoided:
The secondary throttle shaft rarely ever need bushings; even brand new there's quite a bit of clearance between the shaft and throttle plate. If the secondary shaft/throttle plate bore so bad that bushings are needed, the rest of the carb is likely worn very badly; another core or at a least another throttle plate should be considered.
 No drilling/Teflon bushing fix
No Drilling / Teflon Bushing Article shows how to re-bush the primary shaft with no drilling by cutting a teflon sheet into small strips, as shown below. The teflon sheets are available from smallparts.com as well as many other sources online. If this is attempted, be sure that the amount of wear is measured so the right thickness of teflon sheet is used.
This fix leaves the body original, however a correctly done repair will not detract from the value of a carb in most cases, in fact it adds to it due to the permanent nature of fixing it with a brass/oilite-type bushing as opposed to using a material like teflon that will wear out much sooner than a bushing.
 Throttle blade screw removal
If primary throttle shaft bushings are to be installed, the throttle shaft has to first be removed. After removing all the linkages and choke parts from the end of the shaft, the throttle blades will need to be removed.
First, the ends of the original screws have to be ground away because they're "staked" at the factory (see image below, left). You may notice the ends are split a slight amount or rounded over to make them impossible to loosen, back out and fall into the intake. All the screws that hold the throttle blades, air valve, and choke flap are staked in this manner.
Should a screw get broken off in the shaft, it can be removed by whatever method available; often drilling is done but be careful to get centered on the broken screw. A drill press is the way to do this best, with the throttle plate clamped down. There's no need to worry about matching the screw sizes side-to-side, in other words, if only one screw broke off, use the other original threaded hole along with the larger hole made for the broken screw. Just use the next larger size and use loctite as well as "peening" the ends of the screws. The original screw thread size is 3-48, however the holes can be opened up enough to use a larger screw like a 6-32 thread if need be.
 Throttle blade screw replacement
As mentioned above, the screws can be enlarged to 6-32 if need be. The factory stakes or peens the screws to retain them from falling into the engine, you should do the same (the end of the screw has to protrude past the shaft to do this).
To do the staking, you can clamp a piece of square stock into a vice to use as an anvil to support the screw head on, then peen the ends of the screws with a small jeweler's hammer or ball peen. You don't need to simulate the factory way of staking; the main idea is to disrupt the threads on the end of the screw so they cannot fall into the engine. The loctite will do the rest; the blue formula works good. Be careful to not hit the ends of the screws unless the head of the screw is supported against something solid. You don't want to bend the shaft.
Alternatively, you can use a pair of clamping vise grips to get the jaws on both ends of the screw. This can disrupt the ends of the screws enough to keep them from falling into the engine.
If the ends of the screws look too long, they can be ground down with a dremel or small file to the approximate length of the stock screws before peening them.
 Cleaning orifices
- Please see posts numbers 23 through 28 on page 2 in the thread Rochester Quadrajet 4MV Carburetor: Removal, Disassembly, Rebuild (Rookie Level).
One area that needs special attention are the secondary well fill orifices. Most first-time rebuilders don't even realize they exist. The location is shown below.
Do not enlarge the orifices unless you know what you're getting yourself into. If these orifices are made too large the secondaries can run too rich.
 Throttle blades
Check the base plate to be sure the throttle blades are exactly 90° when wide open. There are some who believe having the secondary throttle blades go "over center" helps with A/F distribution in some cases using certain intake manifolds. While this may be true, until you have a good grasp on the engine tune, and unless the plug readings indicate a need for this, keep them at 90°.
The linkage adjustment is fairly easy to do, it only takes a pair of pliers and a bit of patience to get them set perfectly.
 Airhorn warpage
The warpage of the air horn is discussed in Ruggles' book. For additional discussion, please see posts 49 through 53 in the thread, "Rochester Quadrajet 4MV Carburetor: Removal, Disassembly, Rebuild (Rookie Level)"
 Cleaning idle tubes
The procedure for removal, cleaning and re-installation is covered in Ruggles' book (chapter 5, "Rebuilding the Carb", pages 77-78). It is also discussed on the thread "1977 Chevy K20: Engine Suffers from Hesitation/Power Loss When In Drive" (page4, post # 53, 58, 59).
 E. Cleaning
 Soaking the parts
Caution: According to Hotrodders forum member trees:
"Soaking all metallic parts is the way to go but using small brushes lightly is a good idea. Do not soak non-metallic items." ^
 Cleaning with brushes and wires
Caution: According to Hotrodders forum member trees:
"You also need to use the correct size wire to run through all the jets and orifices. Be careful not to force the wire and score the surface." ^
 "White stuff" oxidation deposit build up
For information on the "white stuff" that can build up on carburetor castings, please see the discussions on the Hotrodders threads:
 Drying the parts
Caution: According to Hotrodders forum member trees:
"One last rinse with the cleaner and blow dry everything. Make sure you use eye protection for this because the solvent can do serious damage!" ^
 F. Rebuilding
Before beginning the rebuild, be sure all potential issues listed in the Trouble Spots section have been inspected and addressed.
 Base/throttle plate
Note: Sometimes the Q-jet base plate may be missing a screw or two (there are only two on most carbs; three at the most). While the base plate is "helped" being held on by the two long front intake manifold mounting bolts and the two long screws at the rear of the carb, it's still a good idea to replace the missing screws. Just be sure to not over tighten these long bolts. If they're over tightened, it can warp the carb permanently.
 Float/fuel bowl
- Seat and Needle
- Float Assembly
- Power Piston and Rods
- Accelerator Pump
Note: When removing the accelerator pump arm roll pin, place a ~0.050" feeler gauge (or similar thickness material) behind the accelerator pump lever pin, so when you drive the pin towards the air horn lip it doesn't butt tightly against it; instead there's room to get a thin straight blade screwdriver in behind it to lever the pin back into position.
 Air horn
- Metering Rods
- Choke linkage
- Installing the "three hole" key will prove challenging. "Fishing" method works best. be sure that the accelerator pump has already been installed before installing the linkage and key.
- Throttle linkage
 G. Re-installation
 Carb gaskets
 Early factory intake with exposed heat crossover
On early (about 1969-back) Chevy BBC and SBC Q-jet intakes, the manifold was equipped with an exposed heat crossover below the primary side of the carb. This intake manifold requires a specific gasket and heat shield (below left) be used to prevent vacuum leaks and to resist the heat.
The heat crossover can be blocked either at the intake gasket, or by blocking off the ports that feed heat to the "U"-shaped channel in the intake with 1/4" pipe plugs. This mod will prevent the base plate from possibly warping and should help provide a denser air/fuel mixture due to the cooler temperature. Be aware that this can mean longer warm up times, and may require some changes to the calibrations.
Caution: Tighten the carb to the thick insulator-type gasket evenly but not too tightly. Because the gasket of that type has a lot of give to it, it can cause the base plate to warp- sometimes enough to cause the throttle shafts/blades to bind. In severe enough cases the baseplate can even crack.
 Later factory and aftermarket intakes
In most cases a thick fiber insulator-type gasket will provide good sealing and help to keep the carb temperature down. Heed the caution above regarding not overtightening the carb.
 Baseplate vacuum fittings
Not all carbs have a tapped vacuum port in the rear of the baseplate. When it is present, this port can be used for the power brake booster. Don't use it for the PCV system; that port is located in the front of the baseplate (primary side). The port is threaded with pipe threads, either 1/8" or 1/4".
Seal the plug or fitting with teflon tape or a good thread sealant. Either of these will also act as a lubricant, making overtightening a lot easier than if the threads were dry, so be careful.
Note: Do not use teflon tape on any fuel line fittings. The filaments of teflon tape that is left over after a fitting is removed is hard to completely remove from the threads, and it will clog the needle and seat and/or jets if it finds its way into the carb.
Some basic tips and links, from from 73-87.com.
 Air fuel ratio
The a/f ratio requirements vary with the demand; richer for high demand (low vacuum) situations, leaner for light throttle cruise conditions, and in between these two when accelerating at less than WOT.
 Air/fuel ratio meter
Using an air/fuel ratio meter makes tuning a lot easier. They are available from several manufacturers, this is an example from FAST, there are several others: Innovative, Edelbrock, Auto Meter, etc.
There are also instructions available online for making an A/F meter, an example can be seen here, from scirocco.org.
 Basic carb adjustments
Rochester 4MC, 4MV, M4MC, and E4ME Carburetors from vetteworks.tripod.com
 Idle circuit
- Q-jet Idle Problems from 73-87.com
 Idle discharge ports
The idle discharge ports that are controlled by the idle mixture screws can be anywhere from around 0.050" to 0.080" diameter. If you find a lean idle condition that doesn't seem to respond to turning the screws CCW, you can gauge the port size using your small numbered drill bit index, then enlarge it one size at a time until you regain control with the screws. Usually about 0.090" is sufficient.
This is providing that the throttle plates are not open too far due to a large cam- which will over-expose the transfer slot, just like on a Holley. The cure for that is the same as you'd do with a Holley- start by using more ignition advance. This allows the primary throttle blades to be closed down some to lower the idle speed, which increases due to the advanced timing. Often, this is all that's needed, so be sure to do this FIRST, then enlarge the ports only if still needed. Normally, going larger than 0.0100" isn't needed and if the idle screws are still not responsive at this size, the problem lies elsewhere- like needing idle bypass air (or more bypass air).
 Idle bypass air
Similar to drilling the primary throttle blades, the idle air bypass orifices introduce air below the primary throttle blades. These orifices can be enlarged/reduced to help some engines idle better.
The idle air bypass orifices are almost always in the base plate. There are holes above the baseplate connected to the orifices, but they are usually much larger than the orifices so the orifices in the baseplate are all that need to be enlarged or made smaller.
There are different locations for the orifices, but they're all adjacent to the primary throttle bores of the baseplate. The sizes vary from no opening at all to around 1/8". Measure them using a drill bit.
Carbs from bulk rebuilders will often have the bypass channels or orifices blocked off with lead shot tapped into the casting channels/orifices, to make the carb "generic" as opposed to being tuned for a specific, exact application. Generally speaking, there's no harm in doing this as long as it has been done correctly. Unfortunately, trying the carb on your running engine is about the only way you can know for sure, unless the desired orifice sizes are known ahead of time. Fortunately if need be, the lead can be fairly easily removed, or drilled to give bypass air.
The rebuilders block them in various places: sometimes the baseplate, other times in the carb body. If this ever comes up, follow the channels from the baseplate into the body and you'll find these bypass holes. Often these rebuilt carbs will use cheap unmarked, unplated brass metering rods and might even have unmarked jets.
The older 'tunnel' type bypass air channels (circles):
The later 'tab' type bypass air orifices, most look like the one shown below: one orifice is in the left tab, the other orifice is just inside the bore (arrow on right). Some have two orifices like on the left (right tab would be where the circle is).
 Power piston
Testing a power piston
You can easily test your Rochester Quadrajet power valve spring to see if it is too strong. At idle, the engine vacuum needs to keep the power valve fully seated (the fully down position is the leanest position). Take a normal plastic drinking straw and place it in the top vent of the airhorn, by leaning the straw on a slant you should be able to feel the power piston as you press the piston down and up. With the engine shut off, place a mark with a marker on the straw showing to top height of the vent. When you start the engine, the straw should pull down and stay down. If you see the straw move up and down, you know that the power piston valve spring is too strong for the vacuum produced by the engine at idle and needs to be replaced with a lighter (weaker) spring. Also if the power piston is moving up and down at idle, the engine rpm will also be unstable and will rise and fall.
This will also tell you if the power piston is stuck in the up position if you suspect it might be, like after a rebuild.
 Power piston springs
A power piston spring allows the power piston (aka "PP") to behave like a Holley power valve (or "PV"), that is, the enrichening system is controlled by the amount of vacuum the engine is producing- which is also an indication of how much load the engine is under. More load = less vacuum. Less vacuum = more enrichment, all else being equal. PP springs aren't always mentioned when Q-jet tuning is being discussed, but need to be addressed just like if it were a Holley PV. The same principals apply, as far as tuning for low vacuum cams, etc.
At high vacuum, the vacuum exceeds the PP spring rate and the primary metering rods are pulled down into the main jets, leaning the fuel/air mixture. Conversely, when vacuum drops, like when under a load or the accelerator pedal is whacked WFO, the PP spring rate exceeds the pull from the engine vacuum, which allows the spring to lift the primary metering rods up to their smaller diameter, or "rich" position.
If a “medium” PP spring = a Holley 6.5 in/Hg, a "soft" PP spring (allows enrichening to come in at a lower vacuum) would be like a 3.5 Holley PV; a “stiff” PP spring = a Holley PV of, say, 8.5 in/Hg. Longer duration cams will use a softer rate PP spring.
Below, left is an image showing a variety of different springs. The springs vary in length, coils per inch, and wire diameter (0.012"- 0.020" in the collection below, as an example).
The image to the right shows three different types of power piston. The power piston on the left with the rod extending from the bottom is used with the baseplate mounted APT adjustment.
Edelbrock has four different PP springs: gold (4"/Hg), orange (5"/Hg), black (6"/Hg), and yellow (8"/Hg) in their p/n 1994 spring kit. These springs are also available in their "Race Calibration Kit" that contains a selection of main jets and rods, secondary rods, hangars, PP springs, high flow needle and seat and an accelerator pump. There are also other suppliers of PP springs and other various tune up and repair parts; see the links below this section.
- GM power piston spring p/n 7037305 is set for a tip in point of ~6 in/Hg, and would work well with a high vacuum, smooth idling and/or wide LSA type cam.
- GM PP spring p/n 7036019 has a tip in of about 8 in/Hg.
- GM part #7029922 is a rather weak spring that was OE for HO Pontiac engines from the early ‘70s. It delays enrichment until vacuum drops to ~3 in/Hg. This spring will work with durations >/= 230 degrees @ 0.050” lift.
GM/Delco PP springs come in packs of 10 springs unless you can talk your way into buying less.
 Adjustable part throttle (APT) setting
Another adjustment that can be made to a Q-Jet is the position of the tips of the primary metering rods in the jets. By setting the position of the power piston up or down using the adjustable part throttle adjustment, the primary metering rods can be made to sit higher or lower in the jets at max vacuum. This brings in enrichment sooner or later, as needed. This is another tuning aid when dealing with long duration or overlap cams. This adjustment is referred to as the APT or "adjustable part throttle" setting.
 Airhorn APT adjustment location
There are two common types of APT adjustment locations: later carbs use a plugged hole in the airhorn, earlier carbs use a plugged hole in the baseplate. The later style adjuster is shown with a red arrow pointing to it, below. The yellow arrow points to the hole where the power piston is located.
 APT adjustment
This adjustment is called the "adjustable part throttle" setting, or just "APT" for short. Using those terms will help you find additional info on the subject should you want it. The setting procedures will be for a Quadrajet 4-barrel- but they are basically the same for the Rochester Dualjet 2-barrel as well, which is the primary side of a Q-jet carb, anyway.
The following is with the carb disassembled:
If you don't have the tool that fits the adjuster screw, carefully slot the screw yourself so you can use a small straight blade screwdriver to adjust it.
Before removing the adjustment screw from the carb body (airhorn-adjusted APT), count the turns it takes to lightly bottom the adjustment screw and write it down before removing the screw- if it needs to be removed; often it can just be left in place. Use this setting as a baseline at reassembly.
You may read that 2 to as much as 3-1/2 turns CCW from lightly bottoming the adjustment screw is a starting point. What may also be done for a starting point is for the top of the adjustment screw to be just a small amount above the carb body casting without a gasket in place. The carb [I]may[/I] be richer than needed in that position (or not, depending. But in that position the engine can be run safely and further fine tuning can be done, and with the following modification to the airhorn to allow easy access to the adjustment, it can be quickly and easily adjusted.
Now, you can use the choke or your hand hand to partially block some of the air entering the carb. Set the APT to where there is a slight increase in RPM as the carb is partially choked, while running at about 2000 RPM. Don't choke it so much that it stumbles or tries to stall, just enough to see if- and how- the RPM changes. The slight increase in rpm indicates the ATP is set just at the edge of being lean. Further adjustments can be made as needed, be sure the engine is always fully warmed up and that the idle mixture screws are spot on before adjusting the APT, and readjust the idle mixture screws after each APT adjustment as well.
Another method (from SMI, IIRC): "Simply turn the screw DOWN (CW) 1/2 turn at a time until you experience a "lean surge". A lean surge will feel like someone is moving the throttle, or you get a hesitation on light throttle acceleration). Then turn the screw UP (CCW), until the lean surge goes away.
If you are experiencing an "off-idle stumble", you can turn the screw up 1/4-1/2 turn at a time until it is eliminated. But be sure to adjust your idle mixture screws properly first, as they can also cause an off idle stumble.
 Baseplate APT adjustment location
Earlier Q-jet carbs have a baseplate APT adjustment location. A hole centered between the idle mixture screws (plugged at the factory) has an adjustment screw that raises/lowers the power piston to set the APT adjustment.
These baseplate-located APT adjustment screws and wire assemblies are subject to being heavily carboned up and stuck in place, If the carb has been in service long, the baseplate needs to be removed and the APT assembly removed and thoroughly cleaned before being adjusted. Be sure to measure how far from the baseplate the wire spring protrudes BEFORE removing it so it can be returned to the same position as a baseline setting.
 Aneroid or spool location
There were also additional Q-jet designs used mainly in 1975-'76, that used an auxiliary enrichment system in addition to (or in some cases instead of) the primary PP system. One has an accordion-shaped aneroid "bellows" that lifted/lowered a single metering rod in a jet; it used no vacuum or power piston to operate it. The aneroid bellows changed the air/fuel ratio in accordance with the barometric pressure. Another was identical to the aneroid except the aneroid was deleted and was replaced by a filler spool that did not respond to barometric pressure; instead it was adjusted to give the correct air/fuel ratio according to ambient conditions only. These systems was found to be lacking, so were discontinued.
The other system uses a vacuum operated PP like the primary PP, but it differs by using only one metering rod and one fixed jet to feed both sides of the primaries. These were abandoned soon after they were released; it was found the primary PP system could be tailored to meet the requirements without the added complexity of the auxiliary system.
 Secondary tuning
- Performance Tune Your Q-jet Secondaries, Part 1 from 73-87.com
- Performance Tune Your Q-jet Secondaries, Part 2 from 73-87.com
 Secondary air valve
If the engine exhibits a flattening out at midrange rpm and everything seems to be in order, check to see that the secondary air valve spring tension isn't too tight, causing the air valve to not open fully. The secondary air valve only opens when the engine is under a load and the primary throttle blades are opened about half way or more. If the AV is adjusted so the spring pressure is too light, it can open too much, too soon- and the result is a bog.
The AV shouldn't open when you jazz the throttle in PARK/neutral. Only when under a load, and at sufficient RPM do you want the AV to open. Also, there shouldn't be a noticeable transition into the secondaries. Many guys think the carb should "bang" into the secondaries, but what's happening is a bog when the secondaries open, followed by the engine 'catching up' to the carb- this gives the impression of acceleration. A timing slip will show otherwise.
You can get a 'feel' for how it's adjusted by pushing the AV open (engine OFF) with a finger. It should shut without hesitation when you let off pressure. After you've been working with the Q-jet for a while, you will get a feel for how the AV is adjusted just by doing this.
There is an adjustment for the AV spring. Should you want to try different adjustments on the air valve, use a 3/32" allen wrench to loosen the lock screw under the AV spring adjustment. 3/4 of a turn from first contact of the spring is the base setting, changing the setting 1/8 to 1/4 turn at a time will get it dialed in.
The next thing is to check to see that you're getting full opening of the secondary air valve. Some carbs had the amount of the air valve opening set to a lot less than it's capable of. The fix is easy, just look where the red arrow (left) is pointing, that's the stop. It can be carefully bent to set the opening amount.
I will use a Dremel to slot the linkage to make bending it easier- I prefer that over grinding the stop in case it needs further adjustment to lessen the opening amount, because the idea is to increase the opening until you either reach the max it will open or to where it doesn't help performance any more- in which case you'd decrease the opening back to where it was best. The linkage can be bent several times without breaking as long as the cut isn't made so there's not enough metal, and it's not bent too many times.
Conversely, if the carb is going into the secondaries smoothly, yet the engine seems to lay down even though everything is as it should be, the air valve may be opening too far; that is, the engine doesn't need all the flow the Q-jet can supply, at WOT. If that is the case, carefully adjust the stop to provide less AV opening until it accelerates cleanly and strongly to redline.
 Pulloff orifice
Once that's squared away you can go about quickening the opening rate by enlarging the pull off orifice. One way to do this to go very large on the factory-installed orifice, then solder the opening shut. From there, enlarging the orifice is as easy as using a small numbered bit in a pin vise.
If you go too large, a touch w/a soldering iron will close it back up. And plan on going too large during the tuning of the orifice size, because you will keep enlarging it until you go too far and get a bog, then you back up to where it was best (just like the opening amount of the air valve).
Some pulloff diaphragms have a 90 degree bend in the vacuum tube. If there is enough length before the bend, the 90 degree can be cut off in order to reach the orifice. If not, the tube can be carefully ground down at the 90 degree bend enough to access the orifice with a drill bit, then enlarge the orifice oversize as explained above. Reseal the tube and tune it by changing the orifice size at the soldered over tip of the vacuum tube.
 Casting smoothing
Generally speaking, reworking the castings of a carb without any empirical way of measuring changes is like porting a cylinder head without a flow bench. What looks good may in fact be the exact opposite of what the carb needs, and without knowing what is happening as you make changes, the results can be worse than if you had done nothing at all, so if you decide to do anything, do not change the shapes, angles of attack, or any other parameter that was a designed-in feature of the carb. Keep things to a minimum- smoothing obvious flaws and mismatches, and leave the rest alone.
If you are a "burr freak", Cratex points can be carefully used in a Dremel tool. Using them will provide the very highest finish of any "Dremel-type" points, especially on curved and/or intricate castings. Some come mounted, others can be mounted on a small arbor w/screw (you know the arbors- like used for the small cutting discs).
I would avoid using mounted stones altogether- they will leave a rougher than stock finish, and generally are not as precise as I believe is needed for that type of work.
There are paper sanding discs (400-600 grit and finer) that are fine enough; backing them with a round piece of plastic cut from a coffee can lid can give enough backing to allow smoothing and polishing, when mounted on a small arbor.
On the primary side, the booster rings and the venturi can be carefully smoothed where there may be a casting mismatch or casting flash or et cetera. If there's a mismatch between the baseplate and main body, that can be taken care of- but this is not generally a problem, nor has the Q-jet historically had any significant amount of casting irregularities.
There's no point in doing anything (other than for personal satisfaction) to the secondary side casting unless there's some huge gaping mismatch, etc.The fit of the base plate gasket can be more of a flow problem than any casting irregularity; if the gasket intrudes into the throttle bore, removing THAT will help more than anything.
Also, everywhere the casting is ground on or polished removes the protective finish from the zinc of the carb body, and that can lead to the dreaded attack of the "white stuff" that's mentioned anytime old carbs are talked about.
 Carb gaskets
The heat crossover can be blocked either at the intake gasket, or by blocking off the ports that feed heat to the "U"-shaped channel in the intake with 1/4" pipe plugs. This mod will prevent the base plate from possibly warping and should help provide a denser air/fuel mixture due to the cooler temperature. Be aware that this can mean longer warm up times, and may require some changes to the calibrations.
 Quadrajet tuning and repair parts
- Return to Individual parts, above.
 Recommended Resources
- Many good links to various tuning and rebuilding info across the internet
- How to rebuild automotive carburetors
- Wiki article intended to provide information on rebuilding a carburetor. (Article remains under construction as of June 22, 2011).
- Wiki articles that provides background information, history, and variants of the Quadrajet carburetors, as well as the pros and cons of using one.
- Carburetor Articles
- Selection of articles linked to on Hotrodders' Knowledge Base.
 B. Hotrodders forum threads
- Rochester Quadrajet 4MV Carburetor: Removal, Dis-Assembly, Rebuild (Rookie Level)
- The thread discusses the removal, complete tear-down, cleaning, rebuild kits, rebuild process, re-installation, and tuning of a Quadrajet 4MV model. The discussion includes detailed, step-by-step pictures, various links to online resources, and lots of helpful advice from forum members.
- Restoring and tuning a quadrajet
- Covers base plate-mounted APT adjustment. Also see article below.
- Quadrajet flooding - BAD
- Discusses sudden flooding and float issues.
- Q-jet Power Piston Springs
- Thread offers information on power piston springs in relation to tuning a Rochester Quadrajet. Information provided on spring ratings, manufacturers and part numbers*.
- Rochester Quadrajet Tuning Tips
- Covers adjusting carburetor and timing settings for optimal performance.
- Quadrajet, high idle with warm engine
- Throttle return springs and choke unloader issues.
 C. Other online resources and digital media
- Delco Carburetor Models 4M, 4MC, 4MV Service Manual
- Complete factory service manual.
- Rebuilding and adjusting the Quadra-Jet Carburetor by Ken Styer.
- A step-by-step article with photos on rebuilding a Quadrajet. Covers electric chokes and tamper-resistant plugs.
- Rochester 4MC, 4MV, M4MC, and E4ME Carburetors
- Article provides information on rebuild process and carburetor adjustments.
- Quadrajet Info by Lars
- A detailed document written by Lars Grimsrud on troubleshooting and tuning Quadrajet carburetors.
- 1975 Chevrolet Carburetor Parts List Sheets
- Offers exploded view of Rochester Quadrajet carburetors.
- Quadrajet Air Flow Ratings by GRZ
- Discusses the Quadrajet's air flow ratings from the factory, and tips on modifying or replacing the air valve shaft, the secondary metering rods and hangar, and the cam follower for maximum air flow.
- How Well Do You Know Your Rochester Quadrajet by Rocky Rotella
- Explains fuel metering in Quadrajets. Covers the relationship between jets and metering rods.
- Video series by alpheus1975 on YouTube
- Gives a detailed, step-by-step demonstration of disassembling and rebuild a Quadrajet carburetor. Although the carburetor he uses is a marine application, the procedure is the same.
 Online sources for identification and decoding
- Carburetor Designation/Identification - Covers casting designations and identification numbers. From Cliff Ruggles' Cliff's High Performance.
- Recarbo Fuel Systems - "This guide is just that: a guide. It is not set in stone, because it is a documented fact that Rochester didn't follow these rules 100%."
*Manufacturers and/or part numbers may no longer be current. Members are urged to add updated information if and when they come across it.
 Photos to be used if and where needed