Curing Quadrajet Carb Woes - Tech

Here Are Some Interesting 21st Century Fixes For Your 20th Century Rochester Carb.

Doug Marion Apr 27, 2011 0 Comment(s)
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Back in the '60s and on up, I was adept at removing, cleaning, and modifying performance Chevy carbs, including the '65-and-newer Quadrajets. The thought of getting beat at the drags or losing miles per gallon due to something I overlooked, or mechanically did not know, spurred me on to learn all the rebuilding and hop-up tricks I could. Remember venturi wall polishing? Plenum spacers? Metering rod spring stretching? Larger (illegal) base assemblies?

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This story is pretty much a result or culmination of all that. How? Somehow I got to be obsessive on the little things that can sometimes help make you a winner. This was and still is a positive for me, but in this story it made my actions a negative--all because of my compulsive behavior towards wringing the best out of my Q-jet.

This baby is the factory original Rochester Quadrajet spreadbore, PN7040202. It's been doing its thing for 40 years on my '70 L48 350 Monte Carlo--forever known as Project Econo-Performer (Super Chevy, circa 1979-1990). In 1979, my Q-jet was professionally blueprinted and jetted by top Chevy S/S record-holder and NHRA national event S/S eliminator champion Val Hedworth at Hedworth Racing in Macon, Missouri. The miles per gallon and performance both jumped as a result. From the late '80s on through the early '90s, Bob Jennings increased the primary and secondary carb jetting to help the engine retain its previous maximum horsepower (245 at 4,500 rpm at the rear wheels) on reduced quality, 91 octane pump gas. From 1996 to 2009, my elderly parental care responsibilities caused the Monte to be garaged. Well, enter 2010. Read on.

Long story short: the Q-jet's needle and seat stuck open, causing the engine to load up with gas dumping out onto the intake manifold. The inline, high-performance glass fuel filter displayed no sediment or foreign matter. This has happened a few times in the past, so I removed the Q-jet, plugged the metal fuel inlet line to keep the fuel in the bowl, then carefully removed the top cover on my workbench. The 10-percent-max mixture ethanol gasoline had a shiny hue to it, but the bottom of the bowl area and the needle and its seat assembly were visually clean. Not particularly knowing what else to check, I headed over to carb and ignition specialist, Bob Jennings at Bob Jennings Dyno Shop, North Hills, California. This shop and Jennings personally have worked with most major Southern California car magazines over the last 30 or more years. Besides having a Clayton chassis dyno, an engine dyno for carb testing, and a vintage Sun Distributor machine (for mail-order work worldwide), the guys there know engines and everything there is with mostly carburetors.

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After disassembling the entire carb and putting all of its small parts in a metal screen basket for emersion into a cleaning solution, Jennings took his narrow beam flashlight and shined it up into the fuel inlet fitting area. He then casually moaned and asked, "How many times have you had your Q-jet apart?" I replied, "Mmm, in the last 35 years, probably 20." After seeing that I had not over-tightened the airhorn (causing warping), he explained that because Quadrajets are made of diecast metal, long-in-use examples that have been constantly fiddled-with, like mine, develop certain problems.

Removing the fuel inlet fitting causes the diecast threads into a mode of what I call "molecular disintegration." The flow area just inside these threads looked like it was packed full of diecast metallic flashing. But then Bob gently tapped the Q-jet at the inlet and a bunch of crudosis (my term for diecast metal particulates suspended in ethanol gasoline fluid) spewed onto his metal workbench. Where did this junk come from? The carb inlet's internal threads were being eaten up--literally--from my 20 times of screwing and unscrewing the inlet fitting since '75.

A telltale problem herein is that there are few of you who, like me, have owned a Chevy for 40 years and have a running journal on its miles-driven--down to its carburetor.




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