Car Care
Quadra-Jet Restoration And Modification
Massaging Rochester's Q-Jet for ideal street performance
Created by Jim McGowanMost people, even gearheads, would rather fly a copper kite in an electrical storm than tackle the innards of their carburetor. Not to worry. There are alternatives to doing it yourself. Not only can you get your carb completely rebuilt (or modified) to your specifications, it can be recolored and come back to you ready for concours show-judging.
The Rochester Quadra-Jet shown here sits atop a 400-horsepower/400 CID Pontiac big-block. The car is street driven regularly, with an occasional zoom through the gears at the strip. Hoping for a smooth idle despite the factory high-lift cam, plenty of low-end torque (and vacuum) and real throttle response, we were asking a lot. Also, since this car will see its share of show time, we figured the carb should look like the centerpiece at a White House dinner.
Be sure that the carb shop you're dealing with knows its business. Some guys have gone so far as to have a Q-Jet X-rayed to study all the invisible passages and see how the bowels of the carb function. Keep in mind the range of modifications that can be made to a Quadra-Jet—this stock 750-cfm carb can easily be bumped up to 791 cfm to flow enough air for 500-plus hp. More dramatic mods can increase the cfm further to produce up to 600 hp.
DIY
If you have some experience with carburetors and want to improve yours at home, or you feel the need to learn how, try a carb kit. Many kits on the market include all the parts and drill bits you'll need. Look for complete how-to instructions in each kit.
X-Raying
Using an X-ray machine, carburetor shops can recognize whether the restrictor-orifice well is cracked. That's not something you can see with the naked eye. If it's cracked, gas will leak from the fuel bowl directly into the intake manifold, leaving no fuel in the bowl for start-up. Carb sensitivity goes to the dogs, you can't adjust the very rich mixture. All bad stuff! Our recommendation: have your Q-Jet X-rayed before rebuilding it.
Step 1
The air horn is removed first and examined. This carb has an additional air-bleed adjustment screw under the bowl vent retainer, which allows air into the idle circuit. This screw is found on Chevy and Poncho carbs and is valuable for additional adjustability in high-altitude situations.
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Step 2
Next, the power piston assembly, float, and anti-slosh baffle are removed. If the car's owner complained of specific carb problems, each item pertaining to that problem is minutely examined to determine what caused the failure.
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Step 3
A common problem area with older Q-Jet castings: The twin arms that support the bowl float rot from constant exposure to gasoline. They deteriorate and occasionally break off. Both arms are subsequently ground down to short stubs. This will not only prevent any future breakage but will create a firm base for the permanent fix.
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Step 4
A new float pin retainer replacement is tightly fitted to the stubs of the original arms. The new retainer will secure the float, maintaining the correct level setting and resisting corrosion for another quarter century.
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Step 5
The check-ball retaining screw is removed, and the old check ball is tapped with a drift punch to create a new seat in the casing for the replacement check ball. Little touches like this make a huge difference.
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Step 6
The gas inlet needle and seat threads are checked for damage; if necessary, a Heli-Coil is inserted to form new threads. This is another common problem with the Q-Jet. Over the years, the inlet threads are stripped out, and it is impossible to get a tight, leak-proof seal.
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Step 7
Some Q-Jets have this rectangular area at the rear, which is the hot idle air-compensator valve. It allows heat to dissipate from the interior of the carb after shutting down the engine. It's something like a pressure-release valve and is very handy in hot weather.
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Step 8
The primary throttle-plate screws are removed to install the new bushings. If the primary bores are to be left stock, these plates are checked for wear along the sealing edges and replaced if necessary. If the plate-to-bore seal is not tight, fuel will be pulled through the secondary side of the idle circuit, creating idle adjustment problems.
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Step 9
The base plate is now drilled oversize to accept the new throttle-plate rod bushings. This drill bit should be included in the bushing-installation kit. Worn bushings will allow the plates to move, creating a poor seal. Once the holes are enlarged, the new bushings are tapped into place with red Loctite added to secure them.
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Step 10
All the various metal carb parts are subjected to an agitation bath in carburetor cleaning solution, including the stripped and polished air horn. This method completely cleans all the small orifices and passages as well as removing all the surface-area gunk.
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Step 11
For our buildup, we replaced the factory secondary jets with larger screw-in units. The originals are small press-in types, so the orifices are drilled out and tapped. These jets can now be changed at will with any compatible Q-Jet jet kit. We are increasing these jets 0.003 inch in diameter.
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Step 12
The factory idle tubes are drilled out to be replaced with screw-in type tubes. This will allow adjustments to be made to the carb's idle characteristics. The factory channel orifices, which are part of the off-idle circuit, are drilled out, which provides the ability to adjust the entire air-fuel mixture. These available-in-kit-form modifications will create an infinitely adjustable carburetor for street or strip applications.
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Step 13
After all the modifications to the throttle body and air horn are completed, they are bead-blasted, thoroughly cleaned and blown out, and then dipped in the coloration solution to apply the factory-correct look.
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Step 14
Epoxy is exactingly applied to seal the primary and secondary well plugs in order to prevent any leaks in these areas. Do not be hasty with this step.
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Step 15
The primary throttle-shaft screw holes are drilled out and tapped to 6/32 inch for better holding power and to attach bigger mounting screws, which will hold the larger-than-stock primary throttle blades.
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Step 16
The 1 7/16-inch blades are secured to the throttle shaft, and the shaft is placed in a milling machine for modification. The shaft is trimmed to increase air flow by providing a thinner profile in the venturi, impeding less air as it flows over the shaft.
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Step 17
The primary bores are opened in small increments and checked constantly during the machining process. The bores are milled 0.062 inch to accommodate the larger throttle plates. This is exacting work—the plates must fit perfectly in the bore to eliminate the possibility of leaks.
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Step 18
All the small parts are bead-blasted and then plated with either dichromate, zinc or cad as per the factory original. All the parts that will be identically plated are wired together and run through the process. It takes a while to plate all the small parts on a restored carb, but the end result is impressive.
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Step 19
The primary throttle shaft is now installed in the carb base and the larger blades secured. Constant examinations, including holding the base plate up to a strong light to check for leaks, are needed during this rebuild. The final product should be as close to perfect as your shop can achieve.
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Step 20
With the base completed, the throttle body is gasketed and attached to the base. All the throttle linkage is now attached and the primary and secondary venturi checked for exact clearances, from totally closed to wide open throttle. The linkage must be smooth and tight with snappy and complete throttle opening.
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Step 21
Now the top side is rebuilt. All bolts are treated to a bath in Tool Black to restore the original black finish. Here, oversized secondary jets are installed: 0.138-inch, three sizes up from the factory 0.135 inch. The primary jets come up from a stock 0.073-inch to 0.076. The metering rods remain the stock 48B variety.
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Step 22
The idle tubes are opened from a stock 0.032 inch to 0.042, which requires less vacuum signal to deliver fuel. The idle tube channel restrictors are opened up from 0.032 to 0.040. These modifications will supply more fuel on demand than the stock sizes, and that's more horsepower.
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Step 23
The new float is now attached to its mounting, and its level is set for this carburetor application. The new float-pin bracket will keep the float in the correct position and setting for a long time.
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Step 24
The new 48B primary metering rods and power piston are now installed. The stock primary-size metering rods in this carb are sufficient to handle the modifications performed and do not need to be changed. The secondary metering rods are richened up slightly from code AU to AX.
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Step 25
The anti-slosh valve is now installed. This is a very important part—it prevents fuel from sloshing out of the carb and leaking through the top gasket. Without this item, your carb will be a mess in a very short time.
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Step 26
We changed the neoprene-rubber accelerator pump cup with a new silicone unit, which is gasoline- and alcohol-proof. Gas will attack the neoprene rubber over time and it will be less efficient on the power side.
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Step 27
The replated choke blade and secondary air-door blades are installed in the air horn and checked for smooth, tight operation. A new choke canister is also installed and the choke vacuum tested.
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Step 28
The carb is now completely rebuilt and screwed back together. At this point, test all working mechanisms and set the preliminary air/fuel setting. We started at 4 1/2 turns out from closed for initial fire-up. Each engine is different, and fine tuning will be necessary on your engine.
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Step 29
Some carb shops have a test engine to check their work before sending it back to you. Our guys checked all the operative functions and set a smooth idle on a Chevy small-block. This allows problems to be diagnosed and fixed prior to bolting this trick, restored carburetor onto your engine.
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