Quadrajet

The '''Quadrajet''' is a 4 barrel [[carburetor]] made by the [[Rochester Products Division]] of GM that was first used on the Chevrolet 396 Mark IV engine in 1965 then widely used on [[General Motors]] motor vehicles until 1990.  Its last application was on the [[Oldsmobile V8 engine#307|Oldsmobile 307]] V8 engine, which was last used in the [[Cadillac Brougham]] and full size [[station wagons]] made by [[Chevrolet]], [[Pontiac]], [[Oldsmobile]], and [[Buick]]. Not widely known, the Q-jet was used briefly by Ford on the 1971 429 Cobra Jet and was also used by Mopar on their 360 truck engines in the '80's.  

The Quadrajet has been described as one of the best carburetors made for passenger vehicles, offering the best compromise of [[Fuel economy in automobiles|fuel economy]], performance, driveability, and [[automobile emissions control]].{{Fact|date=July 2007}}

[[File:Q-JET 750 TO 800 CFM VENTURI COMPARISON.jpg|thumb|100px|Comparison of 750 cfm and 800 cfm Q-jet primary venturi/throttle bores.]]

The Quadrajet is a "spread bore" carburetor; the primary [[venturi]]s are much smaller than the secondary venturis, and the center to center spacing of the throttle bores are different, primary to secondary. By comparison, a "square bore" carburetor may have primary and secondary venturis of similar if not exactly the same size and the center to center spacing of the primaries and secondaries are the same, thus the name "squarebore".   

Most Quadrajets made prior to 1976 were capable of 750 [[cubic feet per minute]] (CFM) maximum air flow measured at a 1.5"/Hg pressure drop, but some were manufactured as 800 (and larger, in isolated cases like the '71 Pontiac H.O., more below) CFM. The increase in airflow was due to a larger primary venturi. This was first done in 1971 by the Pontiac and Buick division of GM for use on their 455 cid/high performance engines.  

[[File:ANEROID OR FILLER SPOOL DETAILS.jpg|thumb|300px|Early APT, ca. 1975]]

Most Quadrajets use a vacuum operated piston (the "power piston" or "PP") to move the primary metering rods to control the air/fuel ratio, allowing the mixture to be lean under low load/high vacuum conditions and rich during high load/low vacuum conditions.  A less-common, early version uses a linkage driven off the primary throttle shaft to mechanically move the power piston.  There were also two Q-jet designs used mainly in 1975 only, that used an auxiliary enrichment system in addition to the primary PP system. There are two types of auxiliary systems: one has an aneroid "bellows" that responds to barometric pressure changes; it used no vacuum or power piston to operate it. The other uses a vacuum operated PP like the primary PP, but it differs by using only one metering rod and one 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. Other changes came on-line in that same era, info on this can be seen at [[Quadrajet#Variants|Variants]].  

# Its leaky fuel bowl. As in nearly all carburetors, the Quadrajet bowl had pressed-in plugs used to seal holes left after drilling fuel passages during the manufacturing of the carburetor.  These plugs in the Q-jet sometimes leaked fuel causing: (a) a cold engine being hard to start, (b) erratic idling, (c) poor fuel mileage, and (d) excessive emissions.  Many Quadrajets have their fuel bowl plugs sealed with [[epoxy]] when rebuilt to prevent leaks. This may be good for a short time but epoxy will not stand up to the extreme conditions encounterd for long. Fortunately, leaking well plugs is primarily a problem only on the first few years of production when pressed in cup plugs were used instead of the later (and better) spun-in aluminum plugs that were swaged in place.  

# The very small float bowl/fuel chamber can result in fuel starvation in extreme high-performance situations, but can usually be traced to a fuel delivery problem to the carburetor, such as a worn fuel pump, pump push rod, or camshaft eccentric. In normal driving and mild performance use it provides excellent control of fuel level.

# The fuel inlet/fuel filter housing threads tend to be very fragile.  It is common for the fuel inlet threads in the main casting to strip.  There are several "fixes" available in the aftermarket:  New oversize self-tapping [[fuel filter]] housings; new fuel filter housings that seal with [[O-rings]]; and [[Heli-Coil]] rethreading kits.

# Almost all Quadrajets today have some amount of warpage of the castings.  The root cause of this warpage is over-tightening the front two carburetor mounting bolts, often in combination with a base gasket that doesn't have hard [[nylon]] inserts for the bolt holes.

# The steel primary throttle shaft will tend to wear the aluminum casting material.  This results in an air leak and in extreme cases could cause the primary throttle blades to not close properly. This results in poor idle quality.  The [[aftermarket]] has responded; several vendors are supplying bushing repair kits for the Q-jet throttle body.

==Variants==

A major change to the Quadrajet was implemented for the 1975 model year.  These newer carburetors are considered "Modified Quadrajets" or "Mod Quads".  In addition to the casting revisions that result in a physically larger carburetor, the primary metering rod length is different from '74 and older Q-Jets.  They were also equipped with a self contained choke mechanism that no longer relied on an intake manifold mounted choke, and a number "1" was added to the beginning of their identification numbers.  

Quadrajet carburetors were also built under contract by [[Carter]].  This seems to have happened at times when Rochester's facility could not keep up with demand.  Carter-built Quadrajets will have the name "Carter" cast into them, but are functionally identical to the Rochester-built equivalent.  The "newest" Q-Jets to be made were built for, and sold by [[Edelbrock]].  There were several versions made, for both stock replacement and "performance" applications.  One version was specifically intended as a replacement for Carter Thermoquad carburetors.  The Edelbrock Q-Jet carbs have been discontinued, although at this time Edelbrock still supplies some tuning and replacement parts. [[Quadrajet#Power enrichment|Return to top]]

== Choosing a Carb ==

*Choke type. Most prefer an electric. A hot air type choke can be converted to electric easily.

 

[http://www.cliffshighperformance.com/index.html Cliff's High Performance] <br  />[http://quadrajetparts.com/index.php QuadrajetParts.com] <br  />[http://www.carburetion.com/index.htm Carbs Unlimited] <br  />[http://www.carbkitsource.com/ The Carburetor Doctor]

Hotrodders Knowledge Base: [http://www.hotrodders.com/kb/carburetor-articles Carburetor articles] - Contains links to various useful carburetor websites and threads.