July - Aug. 1996 Lane Racing And Rodding Article By Jim Kaekel, Jr.
Since its introduction in 1965, the Rochester Quadrajet has never received the respect it deserves. Few people realize that the Q-Jet has a strong racing heritage, particularly in N.H.R.A. Stock and Super Stock drag racing. Record-holding Super Stocks have run in the nine second bracket with Q-Jets atop their manifolds.
While the Holley may be the best carburetor for all out drag and circle track racing, a well prepared Q-Jet can certainly give it a run for the money.
Well known Competition Eliminator drag racer/engine builder, and recent author, John Lingenfelter, was one of the first to turn the Q-Jet into a record-setter on his Super Stock Chevys in the early '70's. Knowledgeable Q-Jet racers know that there are not necessarily any tricks to perform, but rather cures for ailments, according to Lingenfelter.
Two basic sizes of Q-Jets were produced; 750 and 800 cfm versions. The 800 version (identified by a slightly larger primary venturi) was relatively rare, found only on 1971-73 Buicks and 1973 Pontiacs.
To prepare a Q-Jet for competition, or for a performance street machine, there are several fixes and a few special parts you'll need to acquire. As the fuel enters the carburetor, it must pass through the inlet valve, commonly referred to as the needle and seat assembly. Rochester produced three different sized inlet valves, a .110", .125" and a .135". Edelbrock also offers a .149" high flow assembly (Part #1980) for thirsty machines. The Rochester .135" or Edelbrock valve will work well in drag or circle track applications. Primary jets are screw-in replaceable type ranging from .066" to .078". It is best to change jets rather than primary metering rods when tuning. Primary jet size is commonly in the .070" to .075"range.
One common Q-Jet problem encountered is getting your carb to idle properly after installing a long duration, high lift camshaft. Engine vacuum is dramatically reduced, and often the power system will begin to actuate at idle. This problem is usually remedied by installing a different power valve spring. Edelbrock offers an assortment (Part #1994) of four different springs in a package designed for low vacuum applications. Float level should be left the same as the factory setting, which is usually around 1/4".
The accelerator pump should be replaced with a high performance type (Edelbrock Part #1982) to give more pump shot, especially important on drag cars. One of the most common ailments is a "bog" after the throttle is winged open under load. Most often this is caused by an improperly adjusted secondary air valve. As the secondary throttle valve opens, the air valve also begins to open. If the air valve opens too quickly, the engine will bog. If the valve opens too slowly, the engine will not accelerate to its potential. The valve is controlled by return spring tension In all and a secondary diaphragm, which through engine vacuum, slows the opening. The valve can be adjusted by first backing off the Allen head lock screw (located on right side of carb) and then either turning the adjusting screw clockwise to tighten, or counter-clockwise to loosen the tension.
As the secondary air valve opens, the secondary metering rods are raised out of a pair of orifices, rather than replaceable jets. Secondary fuel metering can therefore be altered by changing these rods. Going to a rod with a smaller "power tip", for example will richen the mixture. Most performance applications utilize rods with "power tips" in the .030" to .050" range. Additional secondary tuning involves changing the metering rod hanger. The hanger, which holds the metering rods, can also be changed to alter the fuel curve. Hangers are identified by a letter stamped on them. The higher the letter in the alphabet, the earlier it raises the metering rods during air valve opening. Be sure to check that the air valve opens to a full 90 degrees. Simply file the stop tab on the valve to correct if it doesn't.
Fuel sloshing out of the float bowl is another problem that needs to be dealt with, particularly on drag cars with lots of front end travel. Raw fuel will slosh out around the secondary metering rods. This can be easily cured by putting a piece of black electrical tape on the air horn gasket and carefully using a razor blade to make two small "X"shaped slits where the metering rods pass through. During a rebuild, it would also be wise to epoxy over the fuel bowl plugs which are located on the bottom of the main body. These plugs are notorious for leaking since they are just press-fit from the factory.
Following these procedures and doing a thorough rebuild will wake up your Q-Jet. H-P Books offers a Rochester (HP 014) carburetor book for those in need of additional information on quickening their Quadrajets. Edelbrock also offers a pair of race calibration kits (#1985 for '74 and earlier, #1984 for '75-up models) for the serious tuner. These kits include several sets of primary and secondary metering rods, jets, secondary hangers, power valve springs, high performance accelerator pump and a high flow needle and seat assembly. For those of you who would rather toss your current carburetor, you can really step up your performance with an
Edelbrock Performer RPM Q-Jet. In all, Edelbrock offers five Q-Jet models. Four are excellent replacement carbs for various GM cars and trucks originally equipped with Rochester Quadrajets. The Edelbrock #1910 is 850 cfm, larger than any Rochester model ever produced, and is fully capable of handling the needs of up to a 500 hp "fire breather".