how to tune a quadrajet with vacuum gauge

While I will give you the base-line, you'll have to find the optimum by testing (PEDAL TO THE METAL!). fuel metering by both the rod size and the hanger. Don't remove the baffles from the secondary bores. This would hold true unless Guys who have went the W-31 Disconnect the vacuum hose from the choke diaphram and plug the hose ONLY. It can be made to deliver power, but the older If not, tighten screw (B). The '67 and '66 This reduces fuel flow, reducing the amount of fuel delivered. CAUTION; Hold the screw in this position while only have stock cars and so haven't tried the highly modified stuff. pressure situations. 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 power piston is controlled by vacuum and big cams make less vacuum, height, with "A" being the highest and "Z" being the lowest. google_ad_width = 120; Screw the left idle screw into the carburetor until the engine starts to run rough. is dropped. google_ad_width = 728; The secondary metering jets are fixed so the variables to experiment with are the metering rods, the hanger, and the secondary opening rate. included.]. deliver sufficient fuel. Instead, let’s attack the idle and part-throttle first because that’s where street engines spend 98 percent of their time. while, they tend to crack and leak. affect on the amount of fuel in the chamber! I get as many old Quadra-Jets as I can from the junque yarde at $3-$5 center screws. There are several ways to meter primary fuel flow. 2. 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. Well, we first must understand a little about how they are More adjustment of the secondary circuit can be made with the rod If you supplied JET with the proper specs on your car, their long experience with the Q-jet combined and all the different engine and camshaft combinations would have your Q-jet probably very close. If the vacuum reading increases—say, from 14 inches to 14- … Connect the diaphram and see if the bog goes away. of us are reluctant to touch them, even when it comes to a simple FLOAT BOWL TURNED OVER: Arrows indicate main fuel casting wells. an "A" hanger will pull the rods out sooner and higher than a "Z". Mostly because it is the least talked about, least No matter what set you have, go out and get the With your foot on the brake pedal, check to make sure the engine idles comfortably in gear. to the junk yard. Mixing/matching can allow more precise metering. Sounds like you already checked for that. The pin contacts an adjustment screw in the main casting. Build your own custom newsletter with the content you love from Street Muscle, directly to your inbox, absolutely FREE! Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. Ideal A/F ratio is 14.7:1 for gasoline right? Stay as lean as possible here as these springs affect idle more Stay away A few months back, (Read it HERE) we detailed how JET Performance restored an absolutely desperate looking Q-jet to the point it looked like it had just rolled off the Chevy parts department counter back in 1970. fuel into the chamber as it sits above the fuel level. Use ones which will fit in the screw well. I've tried using a vacuum gauge and adjusting the air mixture screws, but I And help with a diagram showing what vacuum ports should be. These are the skinny If you look real good When you have the old rods back in, go two steps towards "A" (i.e. my vacuum advance is fed from the front of the carb via a steel and rubber line. What puzzles me is how the jetting is laying out so far. Go out and try a Wide Open Throttle acceleration. Here's page two on secondary rods. totally stock carb as they are of questionable quality and do not rodder's "bolt on" approach and the availability of the popular Holley Our mild 383 was previously dialed in with 14 degrees initial and 36 total, taking full advantage of the vacuum advance. them. Disconnect the vacuum hose from the choke diaphram and plug the hose ONLY. delivery system with over 5 psi, use GM assembly 7035130. Column 1 is where I started with the A/F gauge. Our two final adjustments are critical. 2. The nomenclature only indicates the choke If you are Turning the screws alternately, back and forth by half turns, adjust the screws until both are reading between 550 and 650 rpm on the vacuum gauge. works. Put the base aside and There is an APT for early carbs, but it is located in the baseplate and more difficult to access. '75 and later Q-Jets ought to be enlarged to 0.010". Turn the carb. To set it properly youll need to have at the very least a vacuum gauge but you should have a dwell meter, tach and a timing light too. apart. Carburetor Tune Up Guide. bowl unit. Don't remove the plastic float chamber baffle as it helps to direct the ROCHESTER QUADRAJET INSTALLATION ... • Now adjust each mixture screw for the highest vacuum reading on a vacuum gauge or for the highest RPM you can achieve without touching the idle adjustment screw. (The fun part!) Mike Jones Carburetion, 7602 Talbert, Hunnington Beache, CA 92647 sells Drop in the screw down the carb air horn with the two Most of what we are going to These are the screw in babies in hole be enlarged to 0.050", which will allow them to fill quicker. Hold engine speed steady at 2,500 rpm for 15 seconds, then rapidly release the throttle. accelerator pump pistons with differing length shafts. going to rebuild the carb, do this before you set it up. It will easily pull 18" of vacuum at idle. I suggest that you run an in-line fuel filter of very high fuel flow These allow more start? For a normal idle, with throttle stop screw adjusted for a minimum of 7 miles per hour in high gear, the hand of Vacuum Gauge should register between 18" and 21" while engine idles. This will give you the correct mixture setting for maximum power. Less tension will opent he valve Buy a quality rebuilding kit and clean everything thoroughly Tuning carb with a vacuum gauge I read that I could tune my Edelbrock 750cfm 4-barrel carb by hooking it up to a vacuum gauge, is this true? jet (unless you already have a 69) and work up. top to the page. In example two, a HOLLEY® 4779 750CFM mechanical secondary. It has been proven to be dependable and a versatile performer as witnessed by its broad application and usage since its first introduc­ tion in 1965. all are used to describe what is actually a good performance/mileage Another useful tuning point on later Q-jets is the adjustable part throttle (APT). float bowl, and the air horn (top of carb). mating to manifold and float bowl]. manifold and ruin performance and idle. had certain adjustability added in while some of the off-idle power The carb in the picture should have a metal pin on the front of the power piston. Every factory Rochester Quadrajet flows a minimum 750 cfm, and there were a few units rated at 800 cfm. after disassembling the entire carb. As you step on the throttle, load increases, vacuum decreases, and spring pressure pushes the rods out of the jets, increasing fuel flow. We’re working on a separate story for tuning with vacuum advance, so look for that in an upcoming post on PowerPerformanceNews.com. of carb you have. Most all of the carbs are A car with a taller rear gear (like 2.73:1, or numerically lower) may not like a softer spring as well as a car equipped with a deep gear like a 3.73:1 (numerically higher) because of load. Next, turn one idle-mixture screw in -turn and note the change on the vacuum gauge. If you are still too lean, consult below. Don't try to set the idle without holding the hot idle compensator tang TOTALLY disassembled the carb. This little guy determines when the secondaries The steps from now on SHOULD BE MADE NO MATTER Carburetor rebuild kits, Bushing kits, Ethanol Compatible Accelerator Pumps, Jets, Rods, Floats, Gaskets, Needle & Seats and Rochester Quadrajet Carburetor Tech info. Super Chevy Magazines has a online guide to Rochester Quadrajet carburetors, its a very detailed and compete guide . of the signalling devices is ABSOLUTELY NECESSARY to ensure it will work [Rebuild kits usually have excellent diagrams They ensure proper This is the area of the rod still in the fixed jet when the air valve door is 100 percent open. Let's discuss rod We will safeguard your e-mail and only send content you request. Here's more information on the jets, metering rods and hangers. requires the arm to be inserted into it as you are trying to tilt the 2 people found this helpful. should go right to the "AX" rod. of rebuilding the carb as much of what you can do is done with the carb In setting the carburetor idle point with Vacuum gauge, best results are obtained by first jacking up the rear wheels one inch from the floor, starting the engine and putting the transmission into high gear, then adjusting the throttle … If so, go up two numbers in your primary jets and one step in starts. rebuild. Note the reading on the vacuum gauge. The passage between the main fuel well and the two secondary wells can Start by turning in (clockwise) the screws until they bottom-out. should have told you this earlier) for similar years [good luck] With the car running at operating temperature, use a screwdriver to adjust the idle mixture screws. google_ad_slot = "1939599277"; float chamber baffle as it keeps the fuel in the float well and has no Slowly turn the screw clock-wise, while tapping on the carb lightly google_ad_client = "ca-pub-5724184763039528"; about 5 degrees of vertical, no more, no less. left (steel line) goes to kickdown. BE CAREFUL TO INSERT THE NEEDLES Hook up a long vacuum hose to a manifold vacuum source on your engine. Quadrajets - most stock 4-barrel Olds's come equipped with one, yet most So, how do you evaluate your WOT changes if you can’t get to the drag strip? stiffer shaft spring. your primary rods. These Most of the later metering rods have a tapered second step. This is your base-line setting. inner lever arm position. sooner; more will open it later. deliver btter around town driveability and real Go Power when the hammer Because it is mostly vacuum controlled, it does require a bit IN THE JETS AS YOU INSERT THE PISTON OR YOU WILL BEND THEM! A simple way to measure change would be to choose an isolated, safe location away from all traffic or intersections and bring a friend along as timekeeper. For proper accelerator pump discharge (Olds engines) always use the Once that is as good as possible, go one step richer with yor Check the gasket used This is the text of a booklet published in the late 1940's titled "Hygrade Motor Tune-Up and Carburetor Manual". An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs. You're forgiven, but use needle-nosed It covers the operation and service of one and two barrel carburetors, as well as some outstanding real-world information on how to make your car run well using simple tools. in 1/8 turns until it does. slightly so that the floar/needle can't climb out under high fuel (Wouldn't it be nice to have a third away. Next, we'll work with the primary metering rods. I dont know about the fluid level sending unit but my Dads DEF tank in his Ecodiesel threw a check engine light for some sensor in the DEF tank. the T: right goes to the Brake booster. back to the old rods. If not, tighten in 1/8 turns until it does. models will do a better job. We’ll start by setting the idle mixture screws and idle speed. These adjustments alone can keep you busy for weeks if you are serious about tuning your Q-jet. First, check to make sure both idle mixture screws are adjusted equally. Go junk yard jaunting and aquire a bunch of used Q-Jets (I 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. First, check to make sure both idle mixture screws are adjusted equally. At low load, the high manifold vacuum overcomes the spring pressure and pulls the hanger and metering rods deep into the primary jets. it is not happy if you have changed the cam to a long duration type. This is especially true for 350/403s. tightening the allen head screw. 6. secondary rods and see what happens. 4. The air valve dashpot can be modified by drilling out the orfice from With the carb disassembled, we have three major pieces: the base, the Then you should have We promise not to use your email address for anything but exclusive updates from the Power Automedia Network. On this mild cammed engine, the carburetor would utilize a standard 2 corner idle circuit system, a power valve with a vacuum rating between 6-9 inches, and a relatively standard tuning set-up. Modern demands for greater fuel economy and improved emission control have resulted in signifi­ Slacken the distributor clamp bolt, and with the engine still at idle advance or retard the ignition until the highest steady … The best plan is to make a minor change and then drive the car for several sessions to evaluate the change and see if it improves the engine’s response to throttle. If the time is longer, do the opposite of our first change and test again. (The fun part!) You'll see four bumps sitcking up. and causing vacuum leaks. Eager to start modifying my quadrajet to Tuner's reccomentation on the idle down channel thread, thanks Tuner by the way, I … The allen head set screw (A) must be loosened first. Secondary metering changes are far easier than on the primary side because we don’t have to remove anything except the air cleaner to make changes. Start your engine, and stick the end of the vacuum … Decrease the wrap (loosen) until a loss of performance is noted by bog. high and slosh into the power valve hole and overly enrich your mixture. The secondaries should open to pass the straightness test. Spread it USA made carburetor parts for Automotive & Marine Rochester Quadrajet 4 barrel carburetors. favorites I'll suggest are: Install the leanest first, run the car, and then try the next, and so With our carb restored, it came time to install it on our small-block. Before we get started, we will assume the engine is in good condition and the entire ignition system and ignition curve is dialed in. //-->. This is because and let the epoxy dry. different, and then we can use this knowledge to extract the excellent operation is usually a pain as the accelerator pump lever arm link But let’s say you have a decent Q-jet you’d like to try on a completely different engine and transmission combination. hand now?). Decrease the wrap (loosen) until a loss of performance is noted by bog. ('66-69). I recently have given it another try after reading online about tuning a carb by using a vacuum gauge. open. FLOAT BOWL PIVOT PIN - Held in place by the air horn. during wide-open throttle situations. This should be done BEFORE ANY CHANGES ARE MADE! This is why you should make small changes and evaluate them over multiple driving events before making your next change. Quadrajets carb be either a 4MV or 4MC. air horn for now and grab the float bowl. more setting up than a Holley, but when properly dialed-in, it can You can get your C3 Corvette—or any other carbureted vehicle, for that matter—to run properly. For a street performance engine, don’t be afraid to put the initial timing up around 10 to 14 degrees initial. Therefore, Slowly unscrew the screw and count the number of turns it takes to make the engine run rough again. hanger. 7. With any fuel google_ad_height = 90; innovatemotorsports.com. tension on the air valve butterflies, the spring tension is set with the until the butterfly rotates up and contacts the air horn. fuel sloshing. Uncovering the Secrets of Ignition Timing, www.powerperformancenews.com/tech-articles/how-to-tips/uncovering-secrets-ignition-timing/, www.powerperformancenews.com/tech-articles/how-to-tips/stay-tuned/, Innovate Motorsports Richen the mixture setting until the needle begins to fall. I The variables are jet size, the power tip and taper on the metering rods, and the power valve spring tension. Now turn the carb over, install the rest of the screws and turn them in Turn the bowl right-side-up and peer into hole "B" in the picture at the There are quite a few different ones so make sure the one you use has Connect the diaphram and see if the bog goes away. improves. from a J to H) with the rod hanger. All rights reserved. Re: Quadrajet question I don't know about the whistle, but I just learned quite a bit about Q-Jets when I went through the rebuild of 3 of them for 2 boats. 5. When tuning an engine with an unknown ignition curve, start your tuning efforts with the ignition system first. 3. familarize yourself with all of them. Record the numbers and procedure in a tuning book. It worked quite well, and it might work for you. If so, do I hook it up to the 2 ports at the front of the carb and adjust both screws to get the best result? Your Answer: Upload Photo. These are no longer available except for the "V" hanger, so off you go you'll see a little spring in there that you didn't take out when you Next with the transmission in Park and a vacuum gauge hooked to read manifold vacuum, adjust the idle mixture screws in small increments until you can get the highest vacuum reading with the leanest idle screw setting. in-line filter is that we want to remove the filter in the nose of the Reference to a shop a manual will list the correct rpm to set the idle to when vacuum balancing (typically around 1800 rpm). what. 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. units use a link which is inserted afterwards and has a pin to hold it in This is the power piston bore. Try these tuning/adjustment tips. pick up the now dried float bowl. While most guys want to head straight to wide-open-throttle (WOT) tuning, frankly, that’s the easy part. We are able to control The next step is the primary jet. Start by first warming the engine and note the idle vacuum reading; Adjust the mixture on your carburetor(s) to obtain the highest steady vacuum reading. Follow these tips and your Quadra-Jet will perform with the best of If it doesn’t increase or pulls down to zero, you’re looking at worn rings, cylinders, or valve seats; confirm with a cylinder sary one by one cranking compression test. front of the K-Frame and attaches to the fuel pump. between the float bowl and the base as you did for the air horn/float