Quadrajet
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[[File:Qjet clean.jpg|right]] | [[File:Qjet clean.jpg|right]] | ||
==Overview== | ==Overview== | ||
| − | 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 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 '80s. | + | 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 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 Chrysler (Mopar) on their 360 truck engines in the '80s. |
The Quadrajet has been described as one of the best carburetors made for passenger vehicles, offering the best compromise of fuel economy, performance, driveability, and emissions control. | The Quadrajet has been described as one of the best carburetors made for passenger vehicles, offering the best compromise of fuel economy, performance, driveability, and emissions control. | ||
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: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. 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 (heat and fuel contact) encountered 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. 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 (heat and fuel contact) encountered for long. | ||
* The small float bowl 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 even off road performance use it provides excellent control of the fuel level. | * The small float bowl 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 even off road performance use it provides excellent control of the 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. | + | * The fuel inlet/fuel filter housing threads tend to be very fragile, possibly exacerbated by exposure to ethanol since the mid-2000s. 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 often 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. Without the inserts the base gasket can compress, warping the castings. | + | * Almost all Quadrajets today have some amount of warpage of the castings. The root cause of this warpage is often 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. Without the inserts the base gasket can compress, warping the castings. A warped casting and/or airhorn can sometimes be unwarped using a special tool or sent to a professional service. |
* 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 can result in poor idle quality and unfiltered air to be pulled into the engine. The aftermarket has responded; several vendors are supplying bushing repair kits for the Q-jet throttle body. | * 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 can result in poor idle quality and unfiltered air to be pulled into the engine. The aftermarket has responded; several vendors are supplying bushing repair kits for the Q-jet throttle body. | ||
| + | * The idle tubes (which are located in the main casting) are usually sized at .031" or smaller (especially if it was used with a Chevrolet 305 (including the E4M and dual discharge pump models). These idle tubes are retained with a press fit collar. The bottom of the idle tubes may become plugged up with debris so care should be taken to clean this area. Idle tubes can be fine-tuned and opened up with numbered drill bits. | ||
==Q-jet variants== | ==Q-jet variants== | ||
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You need to pay attention to a few important things when choosing a carb to rebuild/use on your engine: | You need to pay attention to a few important things when choosing a carb to rebuild/use on your engine: | ||
| − | *Fuel inlet (straight or 90º), so it doesn't interfere | + | *Fuel inlet (straight or 90º), so it doesn't interfere with any of the accessories or water neck, etc. 90º fuel inlets are usually exclusive to Chevrolet (small block V8, V6-90 (1985/'86 and marine/industrial), 1968-'85 Cadillac V8 (472, 500, 425, 368), and 1983-'89 Mopar LA V8s (Dodge Ram truck and van applications are non computer controlled while the passenger cars (M platform) had electronic controls). |
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*Choke type. Most prefer an electric. A hot air type choke can be converted to electric easily. A divorced choke Q-jet is able to be converted to electric, however the kit is rather expensive. | *Choke type. Most prefer an electric. A hot air type choke can be converted to electric easily. A divorced choke Q-jet is able to be converted to electric, however the kit is rather expensive. | ||
| + | *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. | ||
| + | *Whenever possible, make note of the engine displacement/engine VIN code. These carbs were used on 4.3L V6 through 7.4L V8 engines, so knowing what it came from gives a leg up on what may potentially need changed. | ||
| + | *Throttle arm- whether or not it can accommodate the trans you're using. This is important if using a TH350 that needs a "below-the-shaft" hook-up point to pull the detent (kick down) cable, or when using a GM OD automatic (700R4/4L60 with hydraulic controls) that uses a TV (throttle valve) cable (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. | ||
| + | {| | ||
| + | |[[File:TYPICAL MANUAL TRANS Q-JET THROTTLE ARM1.jpg|250px|Manual trans throttle arm]] | ||
| + | |[[File:LINKAGE TH350 CARB WITH AT.jpg|215px|Automatic trans throttle arm]] | ||
| + | |}<br> | ||
| − | Beyond that, you basically just work | + | Beyond that, you basically just work with what you have. The Q-jet is very versatile and will work fine on any number of engines. |
| − | 1976-up carbs are often preferred due to less wear than the older carbs, often an | + | 1976-up carbs are often preferred due to less wear than the older carbs, often an integrated choke assembly (hot air or electric) will be present, and the float and needle and seat assembly setup is better. Also because there were improvements made throughout the run of Q-jets, newer is better. The '76-up carbs will also have APT (adjustable part throttle, a way to fine tune the carb). |
| − | + | Cars (not necessarily trucks/vans) from '81-up are going to have feedback carbs. No good for anything other than computer use (CCC), or possibly as an all-out race carb where it is set up/jetted to run without primary metering rods. Some 1985/'86 Chevrolet/GMC trucks and vans with the base 305 will have a dual capacity accelerator pump assembly (which resembles the feedback carb used in passenger cars). These carbs are often overlooked for performance use. However, they can be readily utilized once they are understood. More on them can be found at the link located at the end of this section. | |
| − | + | Interestingly, one of the best/easiest way to find a good carb is to look for one off of a truck or van. Up until 1986 truck carbs were often non feedback (not an "electronic" carb), and they were nearly all 800 cfm units- even those found on 4.3L V6 engines! These engines can also have the large coil-in-cap, non-feedback HEI distributors. | |
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| + | It's also a good idea to acquaint yourself with the [http://www.crankshaftcoalition.com/wiki/Q-jet_with_dual_capacity_accelerator_pump '''"dual capacity" accelerator pump Q-jets'''], in case you run across one. They can make a good performance carb. | ||
==Tuning== | ==Tuning== | ||
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*[http://www.carburetion.com/index.htm Carbs Unlimited] | *[http://www.carburetion.com/index.htm Carbs Unlimited] | ||
*[http://www.carbkitsource.com/ The Carburetor Doctor] | *[http://www.carbkitsource.com/ The Carburetor Doctor] | ||
| + | *[http://www.smicarburetor.com/ SMI Sean Murphy Induction] | ||
==References== | ==References== | ||
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