Our '85 Monte was in need...
Our '85 Monte was in need of improved part-throttle performance. The carb came off so Sean Murphy could have his way with it.
If there were a category titled Good Carb, Bad Reputation, the Rochester Quadrajet would be at the top of that hit parade. It's doubtful there is a domestic fuel mixer more universally misunderstood and scorned than the Q-jet. And yet, for those who speak fluent carburetor, the opinion is that this is an excellent piece when properly installed and tuned. Perhaps it is the Q-jet's complexity that makes it an easy target. Within the Q-jet family, the electric or feedback Q-jet is even more despised. Rather than just jump on the blame train, we decided to take one on-or more accurately-apart. Our '85 Monte Carlo rejuvenation project is afflicted with one of these unresponsive feedback Q-jets, making it a perfect candidate for a rebuild.
Since the electric Q-jet dialect was unfamiliar, we sought out a reliable knowledge base, which brought us to Sean Murphy's door at Sean Murphy Induction (SMI). Murphy's expertise spans the breadth of most domestic carburetors, so he was intimately familiar with our carburetor's eccentric notions. The Q-jet's electronic solenoid circuit is tied directly into the primary main metering circuit and is commanded by the computer that sends out part-throttle signals based on input from a narrow-band oxygen sensor. The solenoid circuit constantly adjusts the air/fuel mixture based on these signals from the computer. The primary metering circuit still functions like an original Q-jet except that the primary metering rods now move much more rapidly to adjust the air/fuel mixture.
We yanked the Q-jet off the Monte's L69 305 engine and shuttled the carb down to SMI, where Murphy could more closely inspect its inner workings. We're only going to cover the main items on the rebuild list, since a step-by-step Q-jet rebuild session would entail many more pages to do correctly. Most of the modifications Murphy performed on this carb can be replicated on nonfeedback Q-jets, so there's much to learn about these fuel mixers. If you'd rather send your carb to SMI, the company can perform all the tricks and return it to you looking like new.
It's a little-known fact that the original name of Joan Jett's band was Joan Q-Jett and the Blackheart Exhaust. Honest...
Most Q-jets require driving out this roll pin to remove the accelerator pump arm. Only push in the pin far enough to remove the pump arm. This will give you enough room between the choke housing and the pin to lever it back in place during reassembly.
Using needle-nose pliers, remove this throttle position sensor (TPS) plunger. Don't lose it; GM doesn't service this part anymore, so it's scarce.
Using a flat-blade screwdriver, remove this idle air bypass valve. Now remove all 12 lid screws using a Torx T-25 male socket. Don't forget the two tapered screws hiding inside the choke housing. Carefully remove the lid and disconnect it from the secondary air valve arm.
Note the orientation of the different springs above and below the flat plate. You'll need a special double D tool. Murphy made his out of a length of mildly flattened 1/4-inch tubing, but Sears offers an OTC tool as well. The assembly unscrews out of the base. OTC also makes a special tool to remove the primary metering jets out of the carb.