The JET Technique For Restoring A Quadrajet

Words and Photos: Jeff Smith

Twenty years ago, the magazines all did stories on electronic fuel injection and proclaimed “The carburetor is dead.” That prediction has yet to come to fruition. Among the legions of four-barrel carburetors still surviving on the streets, it could be argued that the Rochester Quardrajet is the least understood and most under- appreciated. Millions of GM vehicles came equipped with the Q-jet from the mid ‘60s well into the late ‘80s and it’s safe to say the Q-jet is one of the most sophisticated and tunable four-barrel carburetors ever built. Sure, it’s complex and has more individual components than any other four barrel carburetor. But it is its inherent adjustability that makes it such a great fuel mixer.

Because this carburetor is so aggressively ignored, it deserves a little attention. Among the companies well-versed in Q-jet tribal knowledge is JET Performance. They’ve been rebuilding and restoring performance carburetors like the Q-jet, Rochester 2 barrels, Carter ThermoQuads, and Holleys for over four decades. Plus they have the parts and the experience to resurrect even a long-dead Q-jet back to life while making it look and run better than new.

We dropped a long-dormant Q-jet on JET’s doorstep to get the inside story on a typical restoration. We met technician Johnnie Valdez and his crew who immediately began the process and within a couple of hours had transformed our crusty Q-jet into something that we would swear was an NOS piece that had been sitting in a sealed container somewhere since 1970. We’ll show you how JET restores a Q-jet along with a few Valdez techniques. Check it out.

Recalibration

JET offers three stages of rebuilds for the Quadrajet. Stage 1 returns the carb to its original factory calibration while Stage 2 is intended for performance street engines. Stage 3 is reserved for highly-modified circle track or drag race engines that must use Q-jets due to rules restrictions. Our carb was to be used on a modified 355ci small-block Chevy. The engine is also outfitted with a set of Edelbrock Performer RPM cylinder heads, 9.5:1 compression, and an Edelbrock Performer RPM dual plane intake. The cam is one of our a favorites – the COMP Cams Xtreme Energy 268 flat tappet hydraulic featuring 224/230 degrees of duration at 0.050-inch tappet lift with 0.477/0.480-inch valve lift and a lobe separation angle of 110 degrees.

For our 750 cfm Q-jet, Valdez chose to lightly increase the discharge opening for the idle mixture screws from 0.086 to 0.089-inch, combined with a 76 primary jet and a 39B rod. All Q-jets use a fixed secondary jet with metering adjusted with dozens of secondary metering rods identified with a two-letter stamp. Our AY rods were too lean (with a 0.57-inch tip) so Valdez replaced them with DA rods with a 0.044-inch tip along with a V hanger. The hanger determines how far the tapered metering rods are pulled out of the fixed jets. There’s a whole second story on how to tune and adjust a Q-jet but for our initial run with this engine, this will serve as a very accurate baseline.

RELATED: How to Tweak Your Rochester Quadrajet for More Power and Better Mileage

Parts List

Source

Jet Performance Products
jetchip.com