This reduces fuel flow, reducing the amount of fuel delivered. between the float bowl and the base as you did for the air horn/float hanger. to the junk yard. size first: Most carbs can be enriched one rod with no problem. //-->. We’re working on a separate story for tuning with vacuum advance, so look for that in an upcoming post on Most of the Q-Jet's bad reputation has resulted from the hot More adjustment of the secondary circuit can be made with the rod contribute to make that thing look overly complicated and impenetrable. transition is taken out. and looking at the bottom. We'll go through the rebuilding steps while we explain how the Q-Jet We’ve included a photo showing where this adjuster is located on late model carbs. This is a better Obiously, we've been combining rebuilding with tuning in this article. google_ad_width = 728; Adding an affordable vacuum gauge to your toolbox is a good idea, but you can tune your Quadrajet carburetor pretty well without one, provided the carb and the engine are in proper running condition. You will use either two screws or three, depending on the year and type An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs. slightly so that the floar/needle can't climb out under high fuel 3. Vacuum is by definition: pressure below normal atmospheric, commonly caused by a suction that is taking the air molecules away from a particular location. The engine is 9.5:1 Pontiac 400 with a fairly mild camshaft running 214* 224* duration and modest lift. Before we get started, we will assume the engine is in good condition and the entire ignition system and ignition curve is dialed in. If you are a stickler for stock Remember, turning the idle mixtures screws in (clockwise) creates a smaller passage and a leaner idle mixture setting. different, and then we can use this knowledge to extract the excellent Sounds like you already checked for that. Decrease the wrap (loosen) until a loss of performance is noted by bog. It might be a good point here to also mention something often lost when discussing Q-jets. Find a manifold vacuum port on the Quadrajet, such as the PCV port, and connect your vacuum gauge. There is an APT for early carbs, but it is located in the baseplate and more difficult to access. They ensure proper I suggest you utilize small lock washers on these Connect the diaphram and see if the bog goes away. Our mild 383 was previously dialed in with 14 degrees initial and 36 total, taking full advantage of the vacuum advance. configuration. With the car running at operating temperature, use a screwdriver to adjust the idle mixture screws. Do your best Q-tune, and you will be amazed at how much better your engine will perform. A total timing of around 34 to 36 degrees is a great place to start. 4. 4M, 4MC, 4MV, M4M; all these main fuel wells. in-line filter is that we want to remove the filter in the nose of the If the vacuum reading increases—say, from 14 inches to 14- … interchangeable as well as their 3 main parts. Remove the rubber vacuum hose from the vacuum port in the front of the carburetor between the two adjustment screws and attach a vacuum gauge to the port. IN THE JETS AS YOU INSERT THE PISTON OR YOU WILL BEND THEM! Then, make a secondary metering change and perform the test again. place. until the butterfly rotates up and contacts the air horn. If your metering rods have a long power tip, it’s not necessary to have a more aggressive hanger (like a B) since the tip will have achieved its maximum metering long before the door is fully open. Scrounge the junk yards for this piece and use it. The Q–jet’s small primary venturis combined with those highly efficient booster means very small changes in throttle will affect fuel metering, starting with a tapered primary metering rod that moves up and down within a primary jet. carb. Carburetor rebuild kits, Bushing kits, Ethanol Compatible Accelerator Pumps, Jets, Rods, Floats, Gaskets, Needle & Seats and Rochester Quadrajet Carburetor Tech info. silly numbers, lots of levers, vacuum lines, and other gizmos all First, check to make sure both idle mixture screws are adjusted equally. When tuning an engine with an unknown ignition curve, start your tuning efforts with the ignition system first. Basically, the carb is almost totally vacuum controlled, proper setting If the car is "flat" on acceleration try leaner, then richer. improves. But let’s say you have a decent Q-jet you’d like to try on a completely different engine and transmission combination. And with those small primaries, a 750 cfm Q-jet can work very well even on a tiny small-block like a 305 or even a 283. Make sure the engine is not suffering from a vacuum leak; you can’t tune a carburetor properly if everything’s not sealed up. replaced the power piston spring with a leaner unit. included.]. Tuning with your new rebuilt Quadrajet.If you have had an old worn out #quadrajet on your car for years and years, chances are you have continued to tune your engine to run as good as it can.When you put on a nice fresh Quadrajet, your engine needs to be tuned correctly again. Go out and try a Wide Open Throttle acceleration. Another useful tuning point on later Q-jets is the adjustable part throttle (APT). google_ad_slot = "6369798871"; We promise not to use your email address for anything but exclusive updates from the Power Automedia Network. If so, go up two numbers in your primary jets and one step in As you step on the throttle, load increases, vacuum decreases, and spring pressure pushes the rods out of the jets, increasing fuel flow. After you get the best results, check your plugs to see if you are We'll send you the most interesting Street Muscle articles, news, car features, and videos every week. 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