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The Rochester Quadrajet was last installed onto a Pontiac engine during the '81 model year, and most of those used during the final years of production were computer assisted. So it's unlikely that they'll be found in performance applications today, but what about the myriad conventional castings Pontiac used on several engines for the '67 model year and then exclusively from '68 forward?

Still relatively plentiful, virtually any common casting can make an excellent high-performance unit once correctly modified. If you read "Quadrajet Quotient" in the Dec. Increasing air-valve angle can maximize total airflow, subsequently improving full-throttle performance in applications where an engine is airflow-limited, but what happens to idle quality when placing a stock ci casting onto a heavily modified ? An original casting is calibrated to supply a sufficient range of idle fuel for its intended application, but even the simplest modifications to an engine can change those requirements.

Enlarging a few small restrictions by drilling can increase available idle fuel and transform the Quadrajet into an excellent, low-buck alternative that can satisfy the needs of most any combination.

Quadrajet Secondary Metering Rod Hangers

So follow along as we improve the idle quality and off-idle and low-speed street manners of a '67 GTO. You may find that a small amount of tuning can pay off in a big way for your Pontiac, too. Carburetor Basics The carburetor plays a critical role in engine operation. It must provide the correct mixture of atomized fuel and air for maximum performance in every operating condition, and it uses three specific fuel circuits to accomplish this; idle, primary or mainand secondary.

The primary and secondary fuel circuits control moderate-to-heavy part-throttle and full-throttle performance, respectively. A Quadrajet's idle circuit was designed to maximize idle and low-speed operation, but the circuit continues to operate as engine speed increases, also affecting the primary and secondary calibrations.

The circuit consists of a series of fixed restrictions within the main body and a pair of mixture screws located on the throttle body. Under normal operation, an engine draws fuel and air through the primary barrels, but at idle and low-speed, air velocity isn't great enough to initiate the main fuel circuit. Sufficient fueling from the idle circuit is required. During such conditions, fuel is drawn from the main fuel well through calibrated idle tubes in the main body.

As the fuel travels towards the engine, it mixes with a small amount of air drawn from calibrated upper air-bleeds found in either the air horn or main body. Adjusting the mixture screws can increase or decrease the amount of available idle fuel entering the engine.

An adjustment screw on the main body controls actual idle speed by changing throttle-blade angle, subsequently changing the amount of air passing though the primary barrels. It is this delicate balancing act that produces a strong, stable idle that allows the engine to operate solely off of the idle circuit.

This can sometimes require excessive throttle angle, which allows high-velocity air to pass through the primary barrels and can lead to a number of issues, including erratic idle quality with little control or even premature actuation of the main fuel circuit, often referred to as nozzle drip. To combat said situation, Rochester incorporated a fixed bypass that allows a specific amount of filtered air to enter the engine without passing through the primary barrels.

However, no matter how much air an engine ingests at idle speed, the air must be combined with sufficient fueling to attain proper idle quality.

What many hobbyists fail to consider is that each carburetor was calibrated for its original application, and the amount of available idle fuel is directly related to the range of requirements for that combination. Simply stated, installing a unit originally designed for a small-displacement, smog-era application onto a heavily modified performance engine may not contain sufficient idle fuel to always yield positive results, but a few calculated modifications found in a newly released book may no longer make that a great concern.

Modifying The Quadrajet In the past, hobbyists have referred to the Quadrajet by a number of disparaging names, and most of these can be attributed to poor idle and off-idle characteristics.This article is about my experience with calibrating a M4MC Rochester Quadrajet to work properly on the slant six in my Barracuda. The Rochester Quadrajet spreadbore 4bbl carburetor has a much smaller, centrally located fuel bowl that seems to work well for off-road vehicles.

Since I also had a surplus Rochester Quadrajet that I removed my Pontiac in November of for a propane conversion I completed in January ofI thought I would give it a try in August of Just bolting-on the carb with its Chevrolet calibration after sitting dry for 7 years, I found that it ran better than my AFB. After a disc brake upgrade that dragged on way too long several yearsI found that my Quadrajet's fuel inlet wouldn't seal and the carb would overflow, likely from a rusty gas tank.

After replacing a rusty gas tank inI ran the AFB again to ensure that any residual rusty silt didn't settle out in my newly rebuilt Quadrajet. On November 16,I had enough of chokeless cold starts and reinstalled the Quadrajet.

After a few tankfuls of gas, I found that, even though I now have a working choke, it was difficult to start in cold weather and it has noticeable lean surge at part throttle. The Quadrajet's primary venturis are in use at all times and it is crucial that the primary metering circuit is properly calibrated for all engines. Although the primary jets interchange between all years of manufacture, there was design change in and the primary metering rods from PNs starting with 70 are 0.

Since the Quadrajet has been out of production for decades now and it is not as popular as modern squarebore aftermarket carburetors for performance applications, performance parts aren't as available as they used to be.

quadrajet secondary metering rod adjustment

Only the newer M-series metering rods ie, 48M, 49M, The part throttle cruise section of the metering rod comes in a variety of diameters and tapers transition between between lean and rich. Lars Grimsrud LG is very knowledgeable about Quadrajets and has a Quadrajet tuning paper that is available upon request. He may be reached via the Corvette Forum and I would ask him for his paper before trying to tune your Quadrajet. Assuming you are starting with a freshly rebuilt carburetor, LG's Quadrajet tuning procedure has the following steps:.

The best way to determine the optimum jet diameter is on a chassis dynamometer. This involved multiple timed runs from mph.

Rochester Quadrajet Carburetor Secondary Metering Rods

While a drag strip would have been better, I found a nearby straight length of road with very light traffic and got my wife to take several measurements with a stopwatch for each jet. To keep the times as consistent as possible, I left the transmission in Drive where it shifted from around RPM.

Rochester Quadrajet 4 Barrel Secondary Setting

The APT is an adjustable lower stop for the metering rods, located beside the power piston on newer Quadrajets. A pin sticks out of the side of the piston, which hits the APT stop at high manifold vacuum. Having the metering rods sit a bit higher above the jet effectively enriches the part-throttle fuel mixture as the APT is turned out from the bottom. The APT has a rectangular head, which requires a special tool. SC made me a tool in their shop.

There is plug in the Quadrajet's cover that is removed to access the APT without having to disassemble the carb. SC drilled a cone into this plug so that it can be removed with an ordinary wood screw.

I've also seen a kit for replacing this press-fit plug with pipe plug but I think that this is way more work than necessary for the few times the APT needs to be accessed. According the above documentthe top of the economy step of the metering rods should line up with the bottom "lip" of the jet. Using my digital vernier caliper, I measured the length of my 52K metering rods to be 2. The length of the non-metering portion of the metering rod was 2.

With the power piston pulled down all the way into the jet economy zonethe length from the top of the metering rod to the top of the jet should be approximately 1.

quadrajet secondary metering rod adjustment

After screwing in my APT all the way, I found that the top of the metering rods were about 1. This confirmed my expectation the factory used 0 turns of the APT adjustment starting point. Since no specification exists for the number of APT turns of any carburetor, I presume that the factory set each carburetor individually on a chassis dynamometer when each car reached the end of the end of the assembly line.

My understanding is that Specialty Carburetors sets each Quadrajet they rebuilt to 3. If turned out this far, the metering rods would be sitting very high out of the jet so that the economy fuel mixture would be fairly close to the power fuel mixture. With the popularity of aftermarket carburetors and fuel injection, the supply of Quadrajet tuning parts has dramatically shrunk. It is impossible to exchange early metering rods with late metering rods.

Only truck rods M-series are now available new so all other series rods must be sourced from the used market.Search this site. Price is often the biggest sticking point in an auto repair job. Some customers just have no feel for what certain services or parts cost, others have perhaps been treated poorly in the past or are cheapand some just flat out do not have the money Would you hop on the roof of a moving car?

The brake system does not differ greatly from the previous platform, but it does have a few quirks. Rear Brake Pad Wear The rear pads and rotors will typically wear out The parking or emergency brake has to perform two distinct jobs.

First, it must be able to hold the vehicle on an incline. Second, it must be able to stop a vehicle under a specific distance if the hydraulic brakes have failed. Measuring the distance from the top of the bowl vent to the float and then subtracting the dimension from the bowl vent to the cover gasket can also determine the float level.

The metering rod travel can be checked by inserting a screwdriver or punch into the bowl vent hole in front of the choke valve to see if the metering piston spring returns the metering rods to a raised position with the engine off. Spraying some penetrating oil into the bowl vent and tapping on the metering rod hanger with a screwdriver or punch can often loosen the metering rods.

Quadrajet Adjustments The Quadrajet has a number of critical adjustments that make it perform better. The initial adjustment for idle mixture screws on most Q-jets, for example, should be lightly seated and then backed out three turns. Initial idle speed can be obtained by seating the primary throttle plates and then turning the idle speed screw in about two turns. Primary jetting is most easily corrected by increasing the jet size in two-step increments.

To remove the primary metering rods, lightly tap the metering rod hanger with a screwdriver handle. Spring pressure will pop the plastic retainer from the main body casting. Before installing any Q-jet, make sure that the secondary throttle plates are seated and that the secondary throttle linkage is adjusted to specification. A tab on the primary throttle shaft actuates the secondary linkage. This tab determines the open position of the secondary plates.Quadrajet Secondary Metering Rod Hangers.

Questions about this item? Ask here. See 3 Questions. Availability: In Stock. Quadrajet Secondary Hangers.

quadrajet secondary metering rod adjustment

Quadrajet Secondary Hangers B. Quantity Add to Cart. Write a Review. Email a friend. Changing your hanger size will change the time the secondary rod moves to a richer or leaner setting. A shorter hanger will make a quicker richer setting. There are 5 different hangers. Please measure your existing hanger and match it to the list, or find the stamped letter on your old hanger.

Select your hanger by clicking on the option box. Hard to find parts at a fair price and fast and inexpensive shipping. Tried Summit racing and Jegs with no success. Mikes carb parts was the best deal I could find. Did you find this helpful? Submit an answer. I have found a Delco but cannot find it cross referenced to any "letter" anywhere? We can't cross it by carb number. There should be a letter stamping on the existing hanger. The rod hanger is marked M, do you know it's measurement? We only know the ones listed in the item description.

Recently Viewed Items. Can't find what you need?Power Piston Metering Adjustment Screw allows adjustment of height of primary metering rods within the primary jets. Note: Spring C is required under adjustment screw. Sold separately. Before removing original power piston adjustment screw, turn screw in clockwise, counting the number of turns, to lightly seated position. Then return screw to original setting.

If original setting is unknown, set adjustment screw to approximately 3. Toggle Navigation QuadrajetParts. Add to Cart. Specifications: Length: 1. Sold separately Before removing original power piston adjustment screw, turn screw in clockwise, counting the number of turns, to lightly seated position. Note: Not for use in electronic feedback carburetors with mixture control solenoid.

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Modifying A Rochester Carburetor To Improve Your Pontiac's Performance - Calibration Celebration

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Quadrajet Base Gasket.No author mentioned. Quadrajets - most stock 4-barrel Olds's come equipped with one, yet most of us are reluctant to touch them, even when it comes to a simple rebuild. Mostly because it is the least talked about, least understood carburetor mounted on cars.

Most of the Q-Jet's bad reputation has resulted from the hot rodder's "bolt on" approach and the availability of the popular Holley carb. So if the Q-Jet can be made into a good performance carb, how do we start? Well, we first must understand a little about how they are different, and then we can use this knowledge to extract the excellent performance and driveability that the carb is capable of.

APT Adjustment

Quadrajets carb be either a 4MV or 4MC. The nomenclature only indicates the choke configuration. The 4M has a manual choke, the 4MC has a choke on the carb '70 and later Olds and the 4MV has the choke housing on the intake ' The M4M is the emissions Q-Jet introduced inwhich has had certain adjustability added in while some of the off-idle power transition is taken out.

It can be made to deliver power, but the older models will do a better job. If you are going to rebuild the carb, do this before you set it up. For the rest of this article, we will assume that you are in the process of rebuilding the carb as much of what you can do is done with the carb apart.

Buy a quality rebuilding kit and clean everything thoroughly after disassembling the entire carb. Lay all the pieces out and familarize yourself with all of them. I suggest that you have the Olds Chassis Service Manual for your year carb or an aftermarket book on Quadrajet rebuilding.

This way, you'll learn how to fix it as well as what affects what. OK, let's get started. With the carb disassembled, we have three major pieces: the base, the float bowl, and the air horn top of carb. Put aside the base and the air horn for now and grab the float bowl. Most of what we are going to do is going to happen in this baby, so start by turning it upside-down and looking at the bottom. You'll see four bumps sitcking up.Until throttle body fuel injection started taking over in the s, if you had a General Motors product with a V-8 engine, you could just about guarantee a Quadrajet manufactured by Rochester would be sitting on top of the intake manifold.

And while Quadrajet-style carburetors aren't nearly as popular these days-especially when it comes to stock car racing-as the iconic Holley-style four-barrels, you'll still find them on both the street and the racetrack. This is particularly true in the Pure Stock and some IMCA Modified classes that require racers run the same carburetor that originally came installed on the engine.

Of course, racing with a Rochester Quadrajet isn't exactly a death sentence to poor performance and on-track frustration. When properly prepared and tuned correctly, these carburetors actually are very good performers.

The trouble is working with a Quadrajet is becoming a bit of a lost art. Sean Murphy of SMI Sean Murphy Induction is one of the few carburetor builders and tuners around who is still trying to advance the art of racing with a Quadrajet. We recently spent some time with him and he graciously shared several tips for getting the most out of these carburetors. What follows isn't your typical rebuild story-if you want that, there are plenty of books out there-instead, Murphy gave us the real lowdown on what it takes to turn these carbs, which can be rather pedestrian in stock form, into real screamers.

The tips were so good we decided to pass them right along to you in his own words. How Big is It? There are basically three different sizes Rochester produced for the Quadrajets. Image 1 is a stock cfm carb. If you look at Image 2, that's an cfm. The main difference with these two is the bump on the venturi wall is significantly larger than the others.

That bump was to enhance the venturi effect, which increases airspeed across the booster and, in turn, increases the signal at the booster. But it comes at the cost of decreased overall flow. Image 3 is the cfm carburetor. It's easy to identify because it doesn't have the secondary booster ring. That's how it gets such a larger airflow. These carbs are more rare because the only place you'll find them is on '71 and '72 high-output applications on Pontiacs outfitted with engines.

Maximizing Flow By removing that bump on the venturi wall, you can really increase the amount of cfms these carburetors will flow, but you have to be careful. You don't want to get too happy with the Dremel and mess up the shape of the venturi. As an example, you can look at Image 4 to see a before and after.

On the left is a venturi that we've modified for maximum flow, while on the right the venturi is still the way it came from the factory. When we go in and modify these carburetors, we can get as much as cfm out of the And on the we can get it all the way up to cfm, but it really depends on what your engine needs. You can over-do it with the carburetor size. Remember that many of these engines are limited by the rules, so if you get too large you will lose responsiveness off the corner.

In circle track applications the most popular carburetor to start with is the Flow Quality We also look at the quality of the airflow through the carburetor. In the stock carburetor Image 5you can see casting flash in the secondary venturis. To get a little more airflow and also decrease the turbulence as the air moves through that area, we will polish the secondaries Image 6.

It doesn't look like much, but by decreasing the turbulence you can increase the overall airflow. Secondary Main Wells The fuel comes through the secondary main wells then comes back up and through the jet. You can see in Image 7 how restrictive this is.