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Post Info TOPIC: Quadrajet Overhaul


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Date: Aug 31, 2009
Quadrajet Overhaul
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Getting Started with your Rochester Quadrajet Overhaul
 The steps in a Rochester Quadrajet overhaul are: disassembly, inspection, cleaning, modification and repairing defects, reassembly and adjustment.

In this instruction manual the modification and repairing of the baseplate throttle shaft defect will be covered in full detail, along with the correct techniques on how to successfully install the bushing kit to insure maximum performance.

As shown below the only tools you will need are a drill press, a 25/64 drill bit, and the 5/16 guide shaft provided in the bushing kit. The easy out that is shown is only needed should you want to remove a bushing once it is installed. You will also need the standard tools that all mechanics have on hand such as a hammer, pliers, etc.

Bushing Kit Service Procedure
Oil, dirt, varnish deposits, water and gum deposits are some of the main causes of poor carburetor operation. Proper cleaning is essential when installing the bushing kit.

Once you have soaked the throttle body base plate in good carburetor cleaner and rinse thoroughly, (high pressure water is the best way to rinse) blow out all passages in casting with compressed air. Do not pass drills through passages. Use proper safety precautions whenever air or water pressure is used.

Disassembly of Throttle Shaft
Experienced technicians develop their own order or method of disassembly. It is probably best for a beginner to follow the methods in this manual very closely. The most important tool or asset of any carburetor repairman is a keen sense of observation.

 Before disassembling the base plate, place the base plate on a suitable stand or legs. We prefer to put a pry bar in a vise where the base plate will be at eye level.

Now install one of the rear mounting holes in the base plate on the pry bar where the base plate sets somewhat level and near your eye level.

The first step in removing the primary throttle shaft is to bend the primary lever that holds the secondary actuating rod about 45 degrees as shown in the picture below. This will allow the rod to slip off later when you're ready to remove the throttle shaft. By bending this lever now before you remove the throttle plates you insure that you won't bend the throttle shaft by accident. When the throttle plates are removed the throttle shaft bends very easy so you will need to be careful.

The next step will be to remove the four throttle plate screws. The most reliable method for doing this is to turn the screws in and break them off. The factory screws break very easy, then turn the base plate over and with either needle nose pliers or a small needle point vise grip pliers unscrew the screws from the bottom. There is always enough screw exposed from the bottom side to catch a hold of where you can easily remove the screws.

 If for some reason there isn't enough screw on the bottom side to catch a hold of then the next step will be to turn the base plate upside down and grind the ends of the throttle plate screws enough where they are not mushroomed enough to cause the screw not to travel back through the threads. Any small grinder with a small stone should work, however you must be careful not to accidentally grind any other part of the carburetor except the screw ends.

Should you break a screw while trying to back it out then you must drill the screw out and retap the threads. The stock screw size is 3-48, should the threads become enlarged, then you must tap to the next larger size and install a larger screw. In this event contact us and we will send you the larger tap and screws. However, this can all be avoided by simply turning the screws in and breaking them off and removing them from the bottom as previously described.

Remove the screw, spring, and the fast idle adjusting levers from the right side of the base plate and keep in mind their proper order. Now remove the throttle shaft. When the throttle shaft is about one inch out of the base plate the secondary actuating rod should begin to come off, however you may still need to use needle nose pliers to help separate the lever on the rod.

Once the throttle shaft is out inspect it for deep grooves. If the shaft just shows signs of normal wear then take wet 400 sandpaper and sand the shaft where the wear is until those areas are very smooth.

Drilling Base Plate and Installing Bushings
Install base plate in drill press vise with guide shaft as shown in the picture below. Tighten drill press vise with guide shaft as shown in the picture above. Tighten drill press chuck to assure alignment, then loosen chuck and remove guide shaft and install 25/64th drill bit. The base plate should now be aligned with the drill press.

Begin drilling the base plate slowly, and only drill as deep as the length of the bushing. You only get one chance so be sure not to drill too deep, and drill with only one stroke so you don't make the hole too wide. Now drill the other side the same way.

On the late model Q-Jets the right side is very thin so you will need to drill very straight so that the base plate won't crack later when you install the bushing on that side. However, if the base plate does crack, contact us and we will send you a longer bushing that will correct that problem.

Remove base plate from vise, and hold it in one hand while tapping the bushings in with the other hand with a brass hammer. The bushings should pound in without too much effort, but tight enough where they don't need loctite. Even though the bushings once installed can't go anywhere, they may need some form of loctite if they appear loose enough where you can move them with you finger.

Now you're ready to re-install the throttle shaft in the base plate. When the shaft is about two inches out begin to install the secondary actuating rod on to the shaft while installing the shaft.

 Once the shaft is all the way in the base plate make sure that the shaft turns freely, if it is too tight then remove the shaft and inspect for a tight spot. Usually in this rare occasion you may need to slightly ream the right side of the base plate housing, but not the bushing, with reamer or hone. Start with the reamer size .310 to .312. If the shaft is just a little tight but moves freely where your return spring will still return properly that's acceptable and will loosen more during normal operation.

Install one throttle plate aligning the holes as close to center as possible. Turn the shaft and make sure that the plate seats correctly and there is a very little light showing when you hold the base plate up to a light. Use the screws that are included in the kit. Now install the second throttle plate the same way, it should follow suit with the first plate. When both plates are installed correctly there should be very little light showing and the shaft should turn freely. Tighten screws tight but don't break them. Turn the base plate over and with a small ball peen hammer slightly tap on the bottom of the screws while positioning the screws head on a punch or the pry-bar. A slight tap is all that's necessary to insure that the screw can't loosen and wind up in the engine. Now turn the base plate right side up and re-tighten the screws again.

With needle nose pliers straighten up the lever holding the actuating rod to its original position and install the screw, spring and fast idle levers to their original position.

Now inspect the base plate for normal operation. The primary throttle shaft should turn without too much effort and engage the secondary throttle shaft correctly, the primary throttle plates should close correctly with very little light seen, and the fast idle lever's should spring back and work correctly.

Satisfactory performance of the Q-Jet can be obtained once all the defects have been repaired, this bushing kit will eliminate one of them which causes a great deal of the poor idling, and running response conditions


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