1974 Chevy Camaro Z28 Quadrajet - Special Q

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.