Compu-Q
"It was a crutch," says our Q-jet guru, Sean Murphy, proprietor of Sean Murphy Induction. "It wasn't optimal, but it was what GM had at the time, while they were transitioning from carburetors to fuel injection." As Murphy explains, the difficulty with extracting performance from one of these crossbred beasts lies not in the carb, but in the speed of the computer. "Anytime you make changes to the engine, the computer has a hard time compensating," he tells us. These setups are very sensitive, especially to mods that affect engine vacuum-such as a heads-and-cam swap. With this mind, Murphy concentrates his efforts on improving performance during hard acceleration, without affecting emissions during mid-throttle operation. As you'll see, he also pays attention to transition areas; e.g., idle to part-throttle. In large part, however, dealing with a computer-controlled Q-jet is the same as dealing with any other carb; i.e., making sure everything works right. "And use the highest-quality components," Murphy advises. "Cheap accelerator pumps and gaskets won't last."
Could've, Should've, But Didn't
Right off the bat, Tom Woodside of GMCOPO told us that, with our Camaro's stock fuel delivery system in place, we were running out of gas before we even got through a quarter-mile. The fix he prescribed is found in the manual for the smog-legal 350 H.O. swap GM used to offer. The car's in-tank pump is replaced with a later-model version from an F.I. car; a pressure regulator, set at 6 psi, takes the place of the mechanical pump also found on these models. Changing out a third-gen Camaro's in-tank fuel pump gets a bit involved, since the tank itself must be dropped to accomplish the job. Nonetheless, we got all the parts from Scoggin Dickey Parts Center and planned to make the change-right up until we put a pressure gauge on our stock system and measured a steady 6-7 psi. All the same, our Z ran out of fuel at high rpm. "Q-jets hate pressure, but love volume," says our on-site tuner, Sean Murphy. And the bottom line is that although we may have had enough pressure going, we weren't pumping enough volume to keep things going at the upper end. We may not have listened the first time, but this critical change will be made before we proceed.
The little things can make a big difference. Common replacement floats can actually interfere with the carburetor casting, preventing the float from dropping all the way, and therefore how high the float needle can come out of the seat. Murphy made a simple modification (arrow) to eliminate this interference. | 
With stock-size 0.076 main jets in place and float height properly set, Murphy installed the mixture control solenoid, which manages the primary circuit in this type of Q-jet. The primary metering rods actually hang from the spring-loaded carrier. Setting the correct amount of travel is critical to maintaining a proper mixture; 3.5 turns up from fully bottomed should be about right. | 
The mixture control solenoid is further adjusted via the rich-lean stop, located in the air-horn plate. This limits solenoid plunger travel; correct adjustment can be achieved by backing it all the way out, then turning the stop in 1.5 turns. |