Quadrajet Fixes and Tricks

In simplicity the Rochester Quadrajet is made up of many parts but it all resides in 3 main castings. One, the Air Horn which by the way looks like a horn on top and is the uppermost casting of the 3 pieces. It is basically a cover for the components within the float bowl and has the four upper throttle plates and two shafts that are controlled by the choke and throttle linkages. Additionally the air horn is also responsible for flowing the amount of air into the engine. The second part, the float bowl contains the fuel reserve, meeting devices and for the most part many passages throughout the carb for fuel and air. The last and lower most component is the throttle plate. This unit houses the two lower throttle shafts and plates that allow for air and fuel ratios to mix and enter into the manifold. Other that all the parts that are affixed on the carb that primarily it! Now the big question is are the 3 parts interchangeable with other parts from different numbered models. The answer is yes and no. The question is how do you decipher this data? An important note is what generation Quadrajet you have! As such in one of the best historical understanding of Quadrajet Design & History at http://www.cliffshighperformance.com/tech_qjet_history_1.html, summarized from the book: "How to Build and Modify Rochester Quadrajet carburetors" By Cliff Ruggles. Cliffs is an excellent source of information, tools and parts for Quadrajets and more. They also offer rebuilt units, and services.

 

 

Bent or Warped Castings
As previously noted: the castings especially the air horn can become warped by overnighting the unit to the manifold, however contrary to some beliefs this can take place in manufacturing and assembly. There are write ups that specify that you place the air horn onto the float bowl and press down on the edges and look for any rocking. However manufacturing features like the rib, baffles, passages are in fact raised and will in itself because rocking. In the plant as a quick check, we would often place the proper gasket in place and then test for rocking. Once this is completed and if there is an issue, look for the spacing, so to speak the degree of warpage by placing a straight edge across the air horn and float bowl as to look for spaces and to what degree the warpage represents, a small 6 inch steel ruler works well for this. Keep in mind that small deflections on the two mounting holes to the manifold can be lightly filed, but never file across the centers of the castings. A note, if the gap is twice the thickness of the stock gasket then a thicker gasket can be used to elevate the issue. A stock gasket is primarily .030" and a "Premium", extra thick is .062". I use the Premiums and keep a handful on hand at all times. (SEE QUADRA_JET RESOURCES) In the worst case there are jigs to straighten out your castings available that are well over $300.00 or you can opt to send your castings to a shop that will straighten out your castings for a fee. In any event researching different configurations and buying used carbs on places like "E Bay" can allow one to get the parts you need at a reasonable cost. One can build a really nice set-up and obtain a nice spare parts collection.

 
Internal Gold:
In respect to the internal components, there are many variations that make up the overall performance of any given carb. Obviously the first step is to get fuel to the float bowl. As you can see there are many articles on how to increase performance from a lot of good sources. However some simple understanding can get you a long way with certain set-ups. If you look at many aftermarket Rochester Quadrajet carburetors you will see that they are all basically based ion the same 3 castings "Air Horn", "Float Bowl" & "Throttle Plate" with the exceptions mentioned above. Thus the understanding that the Quadrajet is a very universal unit that with a few changes can accommodate a wide range of applications. As understood in a more basic way the "Float Bowl" is the integral workings of a carburetor, not unlike the passages and valves in the heart the "Float Bowl" maintains fuel at a sufficient level to deliver performance and overall drive ability. Thus too much fuel causes flooding and poor performance, too little starves the demand and stalls the motor. Under extremes like tight turns especially in marine applications, rapid acceleration, high angles and alike are dependent on fuel level and the ability to take on fuel in an expeditious manner. Thus two functions are important in these cases, one many carbs incorporate baffles or a device that looks like a fixed float that keeps fuel from sloshing around the fuel bowl. The other is the ability to deliver fuel in rapid fashion. This is accomplished by a slot in the floats needle seat valve. This allows for fuel to enter into the float bowl rapidly due to the fact the fuel does not have to travel all the way to the top of the seat housing. 

"Float System" is used to describe a typical carburetor's operation. Fuel flows through the inlet fitting "Nut" and through a seat "Needle & Seat", then past the end of the needle and into the float bowl. The needle is important because as the bowl fills up, the float pushes the needle into the needle seat, cutting off the fuel. Thus controlling a constant fixed level of fuel.

Throttle Shaft
Throttle shaft leakage can occur in units that have experienced a lot of usage i.e. high mileage, hours on unit etc. The fix is to either purchase a throttle shaft repair kit from a good source or sent the unit out for repair, in either case, quality repair kits run as high as $75.00 that includes a reamer, bushings and instructions. Service for throttle shaft repairs from Cliff’s service includes rebuilding the entire baseplate "Throttle Plate" by cleaning, re-bushing the throttle shafts with bronze bushings, straightening bent shafts, indexing the plates and setting them for correct full opening. For just $45.00 plus shipping, this makes buying the kits and doing the work yourself almost a no brainier. Keep in mind when you do this work yourself it’s advisable you use a quality bench drill, setting up the piece accurately vertical in a vice. Any other way can result in a messy injury if the bit grabs the piece. Additionally you will have to remove the screws holding the throttle plates in place (Most applications the screws are staked on the ends to prevent loosing and being sucked into the intake) by grinding the ends off prior to removing the screws. If you do not you can strip out the threads and will have to re-tap to a larger size. In production every screw was Loctite into position, this prevented screws from entering into the manifold resulting in destruction of the head, piston and valves. There are a lot of cheap tricks on the internet that look practical but for $45.00, to get this service professionally done and to boot saves you about two hours or more of your time! It’s almost a no brainer, especially if you have a very expensive quad.