versions. I was a bit surprised to see that my Quadrajet idle speed responded so linearly to APT enrichment. Quadrajet Idle tube replacement. Specializing in website content and design for the automobile enthusiast, he also has many years of experience in the auto repair industry. Many Quadrajets are computer-controlled, and operating temperature must be reached before the idle mixture can be accurately adjusted. Turn the engine off. Rochester 4 Barrel Quadrajet Carburetor MCM /LX Rough Idle or Can’t Adjust Idle Mixture Screws Possible cause could be air leak through carburetor mounting gasket. IDLE MIXTURE SCREW LOCATIONS - (Q-JET, CARTER) The idle mixture screws are located on the bottom front of the carburetor. .032"-.037" is what I find in higher perf. Carburetors - Quadrajet Vacuum ports - Anyone know the purpose of port N, Well, I have diagrams from 84 and 86 saved on my computer and went and. Using my misguided approach, I ended up running from one side of the car to the other like a madman, losing track of how much I had turned the previous mixture screw and in which direction. Test drive the vehicle to verify that the engine doesn't stall or load up at stops. The idle screws on a Rochester Quadrajet carburetor fine tune the air-fuel ratio going into the engine during idle. I always adjust with a vacuum gauge, but I did adjust for high RPM years ago. If you have any questions or comments regarding my project, feel free to email me. Quadrajet Parts Page: M4MC,M4ME Heat or Electric choke Model : Electronic Computerized Model: Description: 4MV , 4MC Diagram: 1-Choke Shaft and Lever Assy.2- Clip - Choke Rod(Horseshoe) 3- Screw - Choke Valve The hot idle compensator is located on the primary side of the float bowl and looks like a bi metal flap. I run a 454 Chevy in a 50 Mercury. Adjustments to fine tune the idle quality should be done in quarter-turn increments, and should be done to both idle screws equally to maintain the balance of the idle circuit. The analogy would be hot water with fuel, cold water with air, and volume with the idle mixture screw. This is the air fuel mixture screw. Idle Adjustment. Find a manifold vacuum port on the Quadrajet, such as the PCV port, and connect your vacuum gauge. Locate the curb idle speed screw and the idle mixture screw; both can be found at the base of the two-barrel Holley carburetor. Use this plug for the APT access hole in the air horn on Quadrajets with internal Adjustable Part Throttle mixture screw. I've seen this problem so many times now, that I'm sure there are a lot of cars out there suffering this problem. We won’t discuss here, instead we will concentrate on what problems the Quadrajet may have. Analogy. Unscrew the screws two and one half turns. Screw the left idle screw into the carburetor until the engine starts to run rough. Start the engine and allow it to run until the thermostat opens and upper radiator hose becomes hot. Bench adjust the mixture screws by using the double "D" tool to screw the mixture screws, located on the front lower edge of the carburetor, into the carburetor until they lightly seat. Idle mixture screw stuck and other carb questions. 4: Rochester Quadrajet idle mixture and speed adjustment screw locations; Fig. Turn the screws carefully with a screwdriver in a counter-clockwise direction, until the screws bottom out and you feel resistance. Quadrajet Adjustments. The early carbs all have a factory spec of ¼â€. He has written Web content for eHow, and designed the DIY-Auto-Repair.com website. See above for the ‘rough idle’ article. Remanufactured Rochester Quadrajet Carburetor, Quadrajet Tuning: Replacing the Idle Mixture Screws, Actron CP7677 Automotive Digital Multimeter and Engine Analyzer, Dorman 55144 Fuel Filter Spring (Pack of 2), Dorman 55143 Carburetor Inlet Gasket Assortment, 8-Mile-Lake Fuel Pump & Carburetor Vacuum Tester, 1975-1985 Rochester Quadrajet Hot-Air Choke Remanufactured Carburetor, Installing the '78 steering column (Final Chapter). Tried to get the idle mixture screw out and it isn't budging. Quadrajet Kit Installation Instructions. Start the engine. This is the metering area of this rod/jet combination with the rod fully inserted in the jet. 5. Lee Sallings is a freelance writer from Fort Worth, Texas. My engine has a mild cam and 15 in. Turn each mixture screw in (lean) slowly in 1/8th of a revolution increments until the engine speed starts to drop. 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