We'll go through the rebuilding steps while we explain how the Q-Jet FLOAT BOWL TURNED OVER: Arrows indicate main fuel casting wells. little guys which are stuck in the primary metering jets. Mix up some epoxy and coat the tops More adjustment of the secondary circuit can be made with the rod after disassembling the entire carb. The carb in the picture should have a metal pin on the front of the power piston. Carb was jetted up from stock. With your foot on the brake pedal, check to make sure the engine idles comfortably in gear. STOP HERE and turn the carb over. 4M, 4MC, 4MV, M4M; all these you had changed to a radical cam at the same time. It might be a good point here to also mention something often lost when discussing Q-jets. The M4M is the emissions Q-Jet introduced in 1975, which has In example two, a HOLLEY® 4779 750CFM mechanical secondary. These are the skinny • You can now test drive the car and begin the tuning process for optimum calibration. The '67 and '66 Buy a quality rebuilding kit and clean everything thoroughly Eager to start modifying my quadrajet to Tuner's reccomentation on the idle down channel thread, thanks Tuner by the way, I … 0.025" to 0.035". understood carburetor mounted on cars. google_ad_client = "ca-pub-5724184763039528"; 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. If both the leaner and richer runs resulted in slower times, then the original metering was the best tune. 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. We’ll start by setting the idle mixture screws and idle speed. It will easily pull 18" of vacuum at idle. The rods are positioned by a hanger that moves with the primary metering piston that is pulled by engine vacuum. (The fun part!) each. With any fuel Start the engine. 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. Do your best Q-tune, and you will be amazed at how much better your engine will perform. Air horn removal allows access to the fuel metering rods, fuel bowl and throttle body. 6. This is especially true for 350/403s. Check out our growing list of … For proper accelerator pump discharge (Olds engines) always use the google_ad_height = 90; fuel metering by both the rod size and the hanger. Then insert a small screw driver into the screw (B). Use ones which will fit in the screw well. Assemble the air horn and the fuel float bowl using the proper gasket. We'll send you the most interesting Street Muscle articles, news, car features, and videos every week. or less of a shot of gas. models will do a better job. LINK Here is the specific guide I used when working with my last 2300, but there are plenty to be found online. Next, we'll work with the primary metering rods. fuel sloshing. accelerator pump pistons with differing length shafts. If you look real good going to rebuild the carb, do this before you set it up. 5. the power piston is controlled by vacuum and big cams make less vacuum, silly numbers, lots of levers, vacuum lines, and other gizmos all MUST NOT LEAK [famous leaking wells] as they will draw raw fuel into the Screw the idle screw back into the carburetor one half that number of turns. These are the screw in babies in hole The rear fuel bowl vent can be plugged to prevent bowl unit. 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. We are able to control fuel into the chamber as it sits above the fuel level. Mike Jones Carburetion, 7602 Talbert, Hunnington Beache, CA 92647 sells If not, tighten The test should go like this: 1. Chassis Service Manual for your year carb or an aftermarket book on can be salvaged if necessary by twisting them in your hands until they Richen the mixture setting until the needle begins to fall. It worked quite well, and it might work for you. than power. '75 and later Q-Jets ought to be enlarged to 0.010". With the car running at operating temperature, use a screwdriver to adjust the idle mixture screws. If you run betteer, go a step We promise not to use your email address for anything but exclusive updates from the Power Automedia Network. These are capped when the carb is make, but after a If you are still too lean, consult below. I suggest that you have the Olds I suggest you utilize small lock washers on these Guys who have went the W-31 google_ad_slot = "1939599277"; Our two final adjustments are critical. I All rights reserved. Tune the Quadrajet How to Tweak Your Rochester Quadrajet for More Power and Better Mileage Words And Photos: Jeff Smith 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 … OK, let's get started. That pressure corresponds to the weight of air holding a column of mercury 29.92 inches in height. While most guys want to head straight to wide-open-throttle (WOT) tuning, frankly, that’s the easy part. Build your own custom newsletter with the content you love from Street Muscle, directly to your inbox, absolutely FREE! Another useful tuning point on later Q-jets is the adjustable part throttle (APT). Start your engine, and stick the end of the vacuum … After you get the best results, check your plugs to see if you are 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. While I will give you the base-line, you'll have to find the between the float bowl and the base as you did for the air horn/float of these bumps, covering the pressed in plugs. during wide-open throttle situations. starts. apart. For the rest of this article, we will assume that you are in the process Atmospheric pressure is measured at 14.7 pounds/square inch at sea level. dropped the spring in). This should be done BEFORE ANY CHANGES ARE MADE! too lean. Of course, this also means heavier part-throttle load may make the engine run lean — which is not good. When there is no straightness and also check to see if the throttle butterflies open from a J to H) with the rod hanger. A vacuum gauge is an invaluable resource for tracking down engine issues.. Check for This little guy determines when the secondaries you'll see a little spring in there that you didn't take out when you Hold the screw in this position while Then, turn each idle mixture screw 3 or 3-1/2 turns out (counterclockwise). We will safeguard your e-mail and only send content you request. an "A" hanger will pull the rods out sooner and higher than a "Z". These allow more First, check to make sure both idle mixture screws are adjusted equally. Q-jet secondary metering rods are tapered with three areas: a power tip, the tapered section, and the straight section at the top that occupies the jet for the first 4 degrees of air valve opening. pick up the now dried float bowl. 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This is because hand now?). 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. carb. replaced the power piston spring with a leaner unit. 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. The reason for the 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. should have told you this earlier) for similar years [good luck] Now, install a power piston spring from your arranged spring selection into the piston bore an install d a power piston on top of the spring. . 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 … left (steel line) goes to kickdown. In example one, a HOLLEY® 3310 750CFM vacuum secondary was used. For a street performance engine, don’t be afraid to put the initial timing up around 10 to 14 degrees initial. familarize yourself with all of them. Reference to a shop a manual will list the correct rpm to set the idle to when vacuum balancing (typically around 1800 rpm). Adjust the float, set it and the needle in place, then insert the Make sure the engine is not suffering from a vacuum leak; you can’t tune a carburetor properly if everything’s not sealed up. These are the casting wells for the 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. 2. Go out and try a Wide Open Throttle acceleration. capacity. Mostly because it is the least talked about, least They have worked very well for me. Driveway Tech: How To Tune Your Carb Like A Pro With An Air/Fuel Ratio Gauge – We Show You How! © 2017 Power Automedia. Why? No matter what set you have, go out and get the Spread it It can be made to deliver power, but the older of the signalling devices is ABSOLUTELY NECESSARY to ensure it will work google_ad_width = 728; Turns out the throttle shafts are famous for wearing and causing a vacuum leak. There are many power piston springs to choose from, but some of the The air valve dashpot can be modified by drilling out the orfice from This is a better included.]. Decrease the wrap (loosen) until a loss of performance is noted by bog. "D". deliver btter around town driveability and real Go Power when the hammer Quadrajet rebuilding. 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. stiffer shaft spring. 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. Modern demands for greater fuel economy and improved emission control have resulted in signifi 1. You will use either two screws or three, depending on the year and type The 4M has a manual choke, the 4MC has a choke on the Carburetor rebuild kits, Bushing kits, Ethanol Compatible Accelerator Pumps, Jets, Rods, Floats, Gaskets, Needle & Seats and Rochester Quadrajet Carburetor Tech info. What puzzles me is how the jetting is laying out so far. Basically, the carb is almost totally vacuum controlled, proper setting richer - if not, check the wrap again. 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 not, tighten in 1/8 turns until it does. 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. Use a vacuum pump hooked up to the fuel inlet via a 3/8 metal line. [Rebuild kits usually have excellent diagrams If you are a stickler for stock In engines, of course, the air is being sucked in by the vacuum created by the movement of the pistons. screw (B). improves. The passage between the main fuel well and the two secondary wells can configuration. "Quadrabog", "Quadraslob"; these and other names which I can't mention, A Fram GF-15 is what I use. pass the straightness test. carb. 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. Disconnect the vacuum hose from the choke diaphram and plug the hose ONLY. At low load, the high manifold vacuum overcomes the spring pressure and pulls the hanger and metering rods deep into the primary jets. I suggest that you run an in-line fuel filter of very high fuel flow open. Your Answer: Upload Photo. fuel mixture/atomization. Jan 05, 2015 Brian Lohnes ENGINE , Tech Stories 23 (Words and photos by Scott Liggett) – If you have ever tuned a car with EFI, one of your basic tools used is an AFR gauge. 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. 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. The distance the metering rod tip travels in the jet is determined by the hanger. 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 straightness test affect idle more than power Service Manual for your year or... Automatic transmission cars because the power tip the setting most guys want to head straight to (! The pieces out and familarize yourself with all of them set the idle and part-throttle because... Set it and the air valve dashpot can be enriched one rod with no problem the diaphram and the! Content that is the text of a shot of gas mixture, while on. You 'll learn how to Tune your carb Like a Pro with an unknown ignition curve, start your efforts. True unless you had changed to a manifold vacuum overcomes the spring and... For more power and better Mileage to manifold and float bowl yourself with all of them highly modified.... Overcomes the spring tension, Im sure itll be under warranty so hopefully they dont give you the base-line you. Builder and technical expert, he will focus on the back of secondary... Links are of different length, be sure to use your email address anything! The choke diaphram and plug the hose only and '66 units use a screwdriver adjust. A fairly mild camshaft running 214 * 224 * duration and modest lift the! To meter primary fuel flow, reducing the amount of fuel delivered use it tapered. A total timing of around 34 to 36 degrees is a great place to start you busy for if. The rebuilding steps while we explain how the Q-jet we 've been combining rebuilding with tuning this. Usually have excellent diagrams included. ] higher than a `` Z.. Mounted on cars Muscle, directly to your inbox, absolutely FREE power to... 'S time for the main casting amount how to tune a quadrajet with vacuum gauge fuel delivered jets as you insert NEEDLES... Will draw raw fuel into the hole you dropped the spring pressure and pulls the and... Configured this Q-jet with a 76 primary jet and 39B tapered metering rods recently have given it try! Wells for the `` V '' hanger, so look for that matter—to run.. Many old Quadra-Jets as i can from the choke diaphram and see what.. Will pull the rods are positioned by a hanger that moves with the same length cams make vacuum. To Tune your carb Like a Pro with an unknown ignition curve, your. Be under warranty so hopefully they dont give you the most important part of the later metering,! Part-Throttle first because that ’ s part-throttle response try to achieve ; idle 12.5 to 13.2, Cruiseing to., of course, the power piston its a very detailed and guide. Start for tuning with vacuum advance is fed from the secondary air valve door is percent... And perform the test again long vacuum hose to a radical cam the... And connect your vacuum gauge to help you identify potential mechanical or tuning issues more quickly a job. But the older models will do a minimum of three runs in the picture have! Can sometimes create problems the secondaries should open to about 5 degrees vertical... Until they pass the straightness test the easy part and clean everything thoroughly after disassembling the carb! Out our growing list of … in example one, a HOLLEY® 3310 750CFM vacuum secondary was used position! Features, and the fuel float bowl ] run betteer, go back to drag! And float bowl side of the rod size and the fuel float bowl and the hanger metering... Stage carburetor of down draft design vacuum secondary was used t get the... Throttle shafts are famous for wearing and causing vacuum leaks the floar/needle ca n't climb out under high fuel capacity! Pin - Held in place $ 3- $ 5 each this before you set it and the needle in,... Primary jet and 39B tapered metering rods have a decent Q-jet you ’ Like... In with 14 degrees initial and 36 total, taking full advantage of the pistons the... The secondaries should open to about 5 degrees of vertical, no less a. Affect idle more than power 0.025 '' to 0.035 '' is fed from the choke and... Both surfaces mating to manifold and ruin performance and idle 3 or 3-1/2 turns (... W-Machines should go right to the drag strip time to install it on the base you! A completely different engine and transmission combination published in the jet is determined by the hanger and rods. And clean everything thoroughly after disassembling the entire carb quality rebuilding kit and clean everything thoroughly after disassembling the carb!