Open the Door
Q-jets don't use secondary accelerator pumps, yet they sport amechanical- secondary operation. The key to making the secondaries workis the spring-loaded secondary air valve. Once you hit WOT, air rushingthrough the primaries hits the air valve and overcomes the springtension to eventually push the air valve open. You can adjust the springtension by loosening the locking Allen screw on the rear passenger sideof the carb and then adjusting the spring tension with a small straightscrewdriver. Loosening the tension will open the valve sooner, but tooloose and the engine will hesitate or bog when you mash the throttle. Toset this properly, reduce the air-valve spring tension until you feel ahesitation on hard acceleration. Then tighten the tension until thetransition to WOT is seamless.
If you watch how the air valve operates, you'll notice that a hangersits on an eccentric that lifts the hanger as the air valve opens. Thishanger supports the main metering rods. As the air valve opens, thehanger pulls the metering rods out of the fixed orifices. To fine-tunethe position of the metering rods in those fixed orifices, differenthangers can be used. Edelbrock offers hangers in five height sizes, andits spec defines the distance from the hole to the top of the hanger.The larger the distance, the less the hanger will pull the secondarymetering rods out of the fixed orifice. The B hanger will yank the rodsout the most, and the V hanger will pull the rods out the least as theair valve opens.
The secondary metering rods taper down to a power-tip diameter spec'd inthe accompanying box. The larger the power-tip diameter, the leaner themixture because it displaces more area in the fixed orifice in the carbbody. Therefore, the CC rods will produce the richest air/fuel ratiowhile the CL rods will create the leanest mixture. Changing secondarymetering rods in a Q-jet is incredibly simple--just remove the hangerand pull it out with the rods.
|Component ||PN ||Price |
|B hanger, (0.520) ||1960 ||$3 |
|Ghanger, (0.545) ||1961 ||$3 |
|K hanger,(0.565) ||1962 ||$3 |
|P hanger,(0.590) ||1963 ||$3 |
|V hanger,(0.615) ||1964 ||$3 |
|Secondary rods, CC(0.300) ||1950 ||$7 |
|Secondary rods, CE(0.410) ||1951 ||$7 |
|Secondary rods, CK(0.527) ||1952 ||$7 |
|Secondary rods, AY(0.567) ||1953 ||$7 |
|Secondary rods, CL(0.667) ||1954 ||$7 |
The Q-jet has a reputation for being somewhat complex. But if you workwith it awhile, you'll learn that it's not that difficult. If you'd liketo learn more about this very streetable and flexible carburetor, thebest place to start is with H.P. Books' Rochester Carburetors by DougRoe and Bill Fisher. The book is filled with rebuilding tips andinvaluable tuning advice that will make you a Q-jet expert in no time.
We learned this trick from Edelbrock's Kurt Hooker many years ago. Tomake his large collection of Q-jet metering rods quickly accessible,Kurt slides them into a rectangular piece of cardboard, from rich tolean, marking their stampings on the cardboard. This makes them easy tofind when he needs metering rods.
A common ailment with older Q-jets is leaking from drilled and plugged orifices in the bottom of the main carb body. The quick fix is to thoroughly clean the areas shown in the photo and then mix up some two-part epoxy or JB Weld and coat them thoroughly. This will prevent fuel from leaking into the intake manifold. Keep in mind that even with this modification, older original Q-jets have a problem with porosity that allows fuel to leak out of the bowl through the casting. When that happens, it's time to replace it with a new carburetor.