Curing Quadrajet Carb Woes - Tech

1 This gas was in the Q-jet when it first acted up. They say that the shiny finish seen here is from the 10-percent ethanol-alcohol mixture. Nothing was seen on the needle and seat the first time around.

1 This gas was in the Q-jet when it first acted up. They say that the shiny finish seen h

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?

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.

2 Bob Jennings' Dyno Shop has been super-tuning engines since 1969. Here, he's carefully disassembled our Quadrajet so all of it can be cleaned in EPA-approved solvent.

2 Bob Jennings' Dyno Shop has been super-tuning engines since 1969. Here, he's carefully

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.

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.

Culprit: Ethanol? (Ethyl Alcohol)
Today's pump gas contains as much as 10-percent ethanol. It is a liquid compound made from simple sugars. Refined, it is an alcohol with the chemical structure CH3-CH2-OH. It is said to be 20 to 30-percent less efficient than gasoline. Nationwide, carburetor experts I know all agreed that in my instance, ethanol did not cause, and would not cause the Q-jet's particulate contamination. I'm not sure how long it takes for ethanol gasoline to evaporate from inside a carburetor, or if oxygen could eat up or weaken the carb's diecast metal threads over 5-10 years idle-time. The crudosis was created by some means. How many 40-year-old Q-jets does a professional rebuilder see where one owner has had it for 35, of which it just sat for a decade? Not too many.

Jennings did mention that ethanol/alcohol will cause an AFB carb's leather accelerator pump to peel up over time. This was also related to me by noted Carter AFB restorer/rebuilder Dick Katter in Manassas, Virginia (703/754-7547), and renowned Quadrajet restorer/rebuilder / NCRS seminar expert Jerry Luck in St. Louis. All three also noted that a helicoil could be installed on the worn inlet threads, and that actually is a common process.

After noting that the float's brass seat felt frozen in place, thus very difficult to unscrew, Jennings decided to leave it alone. He then re-epoxied the two secondary metering rod well plugs that Hedworth had first coated 30 years ago. Age and use via 66,000 miles of engine heat had cracked apart the epoxy on both. We've been told that Q-jets made in the first decade of their production (1965-'74 more or less) have plugs that historically tend to leak fuel out of the float bowl into the intake manfold plenum. This causes a richer idle-mixture, as well as an up-and-down idle-speed characteristic, which can be mentally baffling for laymen like us.

Oh No, Not Again!
With all new gaskets, Jennings installed the Q-jet on his V-8 Chevy dyno engine to make final external air/fuel mixture and idle-speed adjustments. It ran perfectly. The next day it acted up again--just like before. I suspected there were more diecast particulates under the frozen-in-place brass seat. No fault of Jennings.

To remove it takes a very wide, long, flat blade screwdriver. Harbor Freight to the rescue! After very careful back-and-forth unscrewing (to save the diecast threads), we were aghast to find many more deposits. The threads on the brass seat were also completely packed with crudosis (see photo 8). We assume this was from many years of dryness. It became ultra hard--even sort of petrified. After cleaning and reassembly, the Q-jet ran perfectly. But what next?

Jennings had done a fine job, but were there multiple reasons for this dilemma? What would you do if this was the original carburetor from your 40-year-old Chevrolet? I decided that further use would eventually be a cause for heli-coils. Nothing wrong here, but I decided that until I can gather more facts, and the cost for professionally installed helicoils, I ought to remove the original Q-jet. I replaced it with a like-new Q-jet I bought years ago (thinking some day I might need it).

I also decided to visit eBay - where as luck would have it, I was top bidder on a used, 750 cfm AFB carburetor. From my 409 racing days in the mid-'60s, I have a ton of metering rods and jets to super-tune it for this 350. My Project Econo-Performer Monte Carlo has long since retired from drag strip parts testing and its 66,000-plus miles of continual mpg testing. So, it will now sport two efficient carbs, a nearly identical Q-jet, and a Carter AFB. We'll see if either gets eaten up. I don't plan to take either apart unless one burbles out for help.

In the world of longtime vehicle ownership, this Q-jet carb problem and other simple woes are nothing new--except you never hear about them.

We hope you have learned something new here. We sure did.