Setting Up a Quadrajet

 

Setting Up a Quadrajet

 

 

 Many people that aren't concerned with originality quickly give up on a GM Rochester Quadrajet carburetor and replace it with an aftermarket piece. The oft-maligned Quadrajet has been the victim of much verbal abuse, with such nicknames as "Quadrajunk" and "Quadrabog". Truth be known, though a little complicated, they are one of the most precisely metered and best designed carburetors ever made. Millions were installed on many different GM engines in the 60's, 70's, and 80's, as well as some non-GM vehicles. The small primary bores gave good fuel economy, throttle response, and emissions control, while the huge secondaries when called upon would boost air flow to 750 CFM for most versions and 800CFM for a few others. Many 600+ hp Super Stock drag race cars utilize them (stock carb as required by rule) and go deep into the 9 second range in the quarter mile.


Usually the biggest problem in getting a Qjet to perform properly is getting the secondary air valve to open at the proper rate. Secondary jetting is easily changed by changing metering rods, which doesn't require any carb disassembly. There are no replaceable secondary jets, just rods. The primaries have both replaceable jets and metering rods but the top of the carb must be removed to get to them. Due to the small size of the primaries and effort required to change primary jetting, and the relatively small effect it would have on performance, it's usually not worth fooling with unless there is a severe lean condition. The Qjet is a somewhat complex piece as carbs go, but once one is right, it usually stays right for a long time. "Set and forget".


To set up a Qjet for good street performance, first a basic rebuild is recommended if the carb is old and hasn't had one lately. You can do it yourself with an approx $15 rebuild kit, basic tools, some carb cleaner, a tooth brush (not your wife's), and some patience. Blow out all the tiny passages with compressed air if you can. Before reassembling, seal the bottom of the secondary fuel wells with epoxy or JB Weld. They are notorious for leaking and should be pointed out in the rebuild instructions. When the carb is assembled and adjustments made to spec, turn your attention to the secondary air valve rate of opening. This is the big flapper on the very top of the secondary throttle bores. There is an adjustable tension spring and linkage rod connecting it to a vacuum diaphragm on the passenger side. Most stock Qjets open the air valve too slow. To correct this you can loosen the spring tension by loosening the small allen-head set screw (on the bottom of the air valve pivot) and use a small screwdriver to adjust the screw (in-line with the pivot) controlling the tension. Then tighten the set screw. If you have a bog caused by the secondaries opening too fast, tighten the spring tension in small steps til the bog goes away.


If loosening the spring tension can't produce even a slight bog, you'll have to do more. You can drill and enlarge the tiny vacuum orifice that's down inside the vacuum hose connection of the plastic diaphragm housing to allow the linkage rod to move faster. The orifice is about .010 diameter and should be drilled to .015-.020. But this requires tiny bits and a pin vise. The simplest and easiest thing to do is remove the air valve linkage rod completely and tighten up the spring tension til the bog almost goes away. I say almost because a slight bog is usually unavoidable with this modification. But boy will it wake an engine up! A caution is that on some models the linkage rod also doubles as the choke unloader so a choke adjustment may be required.


Now go play with your newfound power. Caution: It can somehow mysteriously increase the rate of rear tire wear.