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1974 Chevy Camaro Quadrajet Rebuild - Special Q
A Well-Built Quadrajet Is Just the Thing for CHP's Z28
Apr 1, 2003
Sean Murphy Induction (SMI)
Huntington Beach, CA
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1974 Chevy Camaro Quadrajet Rebuild - Special Q
In this photo, Murphy has already removed the idle-speed solenoid, as well as the accelerator pump rod and lever. Next, the choke rod must be removed...
...as well as the vacuum break assembly (which must actually be pried off) and its rod.
This Q-jet was dirty but still working well enough when we rolled our '74 Z28 into the CHP shop. With a stouter engine in the works, however, we wanted this mixer in tip-top shape. Sean Murphy begins the teardown by removing the secondary rod hanger; the rods themselves pull right out with it.
Lift the lid, and our Q-jet's internals are in plain view. The top of the power piston assembly protrudes through the air-horn gasket, so be careful when peeling it off. Once the gasket is out of the way, the power piston and the attached primary metering rods pull right out. Don't forget the power piston assembly spring underneath; it might be easier to get at once the fuel-bowl insert and float-and-needle assembly are removed.
The intermediate choke lever disconnects from inside the carb after the intermediate choke shaft and the fast-idle cam are pulled from the side.
Here, the float-bowl needle seat has been removed, as has the pump-discharge ball screw...
...Next to come free are the pump-discharge ball (previous) and the unscrewed main jets.
The main body and the throttle-body can then be separated. The idle needle screws are removed from the throttle-body, as are the old gaskets and the fuel filter in the main body, then both bodies are chemical-dipped and air-blown clean.
Murphy then disassembles the throttle-body assembly. In this shot, the fast-idle cam follower has already been removed, as have the secondary butterfly valves. The secondary blades had never been removed, so the flattened ends of the factory retaining screws had to be ground down before disassembly
The primaries, however, had already been out. Look closely, and you'll see that the throttle shaft has bushings installed to guard against leaks. This is a normal part of SMI's Q-jet rebuild and tuning service.
At this point, the main body is worked over in the media-blast cabinet and then recolored (along with the disassembled air-horn assembly). The throttle-body gets a very light once-over on the sanding wheel, just to make sure its mating surface is true.
Now everything looks like new, right down to the casting plugs.
With the main body and throttle-body back together, the tuning fun starts. The mill that will rest under our fresh Q-jet will have a much more aggressive cam than our Z28's stock 245-horse 350. That means less vacuum, especially at idle...
...Murphy carefully measured the mixer's idle discharge ports; they came in at 0.066 inch. Murphy enlarged both ports to 0.086. Why? The pressure drop as air moves past them will be less with a hotter cam, so the larger holes will be better able to pick up the appropriate vacuum signal.
Along the same lines, Murphy also enlarged the idle downtubes (the brass piece being drilled here). Stock sizes vary from 0.026 inch to 0.042; ours were drilled out to 0.038, which will help our new mill get the fuel it needs at idle. The carb's channel-restriction orifices were also spec'd out. These openings intersect the mixture screws, ultimately controlling how much fuel gets through during idle. The orifices measured 0.049 inch, which was acceptable for our purposes.
Our Q-jet came with 0.075-inch main jets and 43B primary metering rods. In the interests of keeping our new, thirstier powerplant satisfied, our carb guru bumped us up to 0.076 mains (arrows) but kept the straight-taper 43B rods. As we stated before, that means we'll have a richer mixture across the rpm range.
Here, the float-and-needle assembly have already been replaced, though the needle seat now measures 0.135 inch as opposed to a stock seat's 0.90-0.120-inch diameter. Again, this is done in the interest of making sure our healthy little small-block has enough to drink. The orange power-piston spring has a lower vacuum rating, ensuring that our Q-jet's power piston--and therefore its primary metering rods--will work well with our new combo.
Murphy also replaced our original accelerator-pump-stem assembly with this updated version. Two-and-a-half coils were cut from the spring before it was installed. With the pump cup sitting higher in its bowl, our carb will get a bigger shot of fuel when we step on it, which we like.
It's about time to put the lid back on, so Murphy sets a new power piston with the 43B rods into place, making sure the rod tips contact the main jets.
The air-horn top plate was also recolored, and the secondary pickup tubes tapped back into place.
With the air-horn gasket in place (remember to carefully fit it over the protruding top of the power piston assembly), the accelerator pump can be set into place and the lid put back on.
The air-door tension must then be properly adjusted. If it's too tight, the door won't open under load and the engine will strain. If it's too loose and the door opens too quickly--you guessed it, Quadrabog! If you feel some light-to-moderate resistance when pushing the door open, the adjustment should be about right.
At this point, our man drops in the secondary-metering-rod assembly using a stock "G" hanger, which he thought was rich but left alone...
...and DA metering rods, which Murphy said would help give our impending powerplant a healthy midrange.
Speaking of tuning ... Murphy laid out, from top to bottom, a small variety of jets, primary metering rods, power piston springs, and secondary metering rods...
...In this shot, we've got just a few of the many available secondary-metering-rod holders. The variety of rod shapes and sizes is mind-boggling, and the combinations nearly endless.
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