Rochester 4GC - Wild About Cars
Contents
1. adjust Dashed arrow shows position of small screw which sets air valve tension v Turn the screwdriver clockwise to free the Auxiliary Throttle Valve the butterflies should flop back and forth freely Tighten the Allen screw momentarily set the tension and wiggle the butterflies to ensure that the Auxiliary Throttle Valve is loose v Now return the screw driver to the slot then loosen the Allen Screw and turn the screwdriver counter clockwise until the Air Valve just barely closes Zero tension Note the position of the screwdriver This is your starting adjustment position Tighten the Allen screw and make a note of the position for future reference v The proper Auxiliary Throttle Setting is the additional tension BEYOND the position you just marked usually 1 4 and less than turn counter clockwise Remember to tighten the Allen Screw after each adjustment v NEVER TIGHTEN the spring screw more than 1 turn This will stretch the spring and make future adjustment impossible Step Five Test Effect of Adjustments to the Auxiliary Throttle Valve Accelerate the car WOT from a start with the wrap at 2 turn which we will call the stock position This is your baseline Now v Decrease the tension in g turn steps until bog is detected This usually occurs by only a turn If bog is felt immediately first run tighten s turn Automatic transmission cars may need more tension than manual transmission cars Aft2. with new vy Make sure that that the choke assembly is working Since it usually uses a thermostatic spring applying some heat to the spring should cause it to expand and unwind Clean all rods and levers in the choke system to make sure that they do not bind v Follow all adjustment and settings using the owner s manual for your year make and model Make any alterations to factory settings as will be shown in this Section See the Appendix for general settings and adjustments 2 Use a quality rebuilding kit preferably from one of the following sources The Carburetor Refactory 815 Harbour Way South 5 Richmond CA 94804 510 237 1277 www carbkits com The Refactory has kits for specific year and application starting with the 50 s Select the kit for the year and application Fusick Automotive Products 22 Thompson Road P O Box 655 East Windsor CT 06088 860 623 1583 Fusick has more general kits but they will work www fusick com 92 Supercars Unlimited 13980 SW T V Highway Suite 5 Beaverton Oregon 97005 503 641 1442 SU has kits for specific year and application starting with 1964 Select the kit for the year and application www supercarsunlimited com 3 Set Floats Proper float adjustment is critical to ensuring that the primary and secondary metering jets can meter sufficient fuel Set the floats according to the procedure found in the Appendix and according to the settings shown in the Chas
3. Carburetion Induction Exhaust Suggestions Section 5 5 1 Tuning Your Carb for Best Performance Rochester 4GC Rochester 4GC Carburetors Information and General Operation When designed the 4CG used as small a venturi as possible to keep air velocity high and as such allow efficient metering of the fuel Conceptually the 4CG is two 2CGs welded together back to back much like two carbs in a Tricarb setup You will find 4CG carbs on 1964 1966 V8 Oldsmobiles The Rochester 4GC Carburetor Note Rochester 2GC carbs utilize many of the 4GC s primary side parts and function in the same manner You may use the instruction and operation information here for the PRIMARY side of the 4GC for the 2GC While certainly not a performance carb by today s standards the 4GC can be made to perform quite well Obviously if stock appearance is not an issue or budget allows this carb should be replaced with an aftermarket unit mostly because very few replacement parts are available Rebuilding kits are available see the list in the Performance Tuning Section on the following pages Primary Operation Normal driving between 42 and 60 degrees of throttle is handled by the primary side of the carburetor smaller front bores called primaries Fuel metering is controlled by the Main Metering Jet s and the Power Valve that adds additional fuel as needed and which is restricted or opened by a combination of vacuum and spring press
4. Primary Jet size first until the optimum performance has been achieved without touching the secondary jets Step Four Set the Auxiliary Throttle Valve No matter what you did in the way of rebuilding or modifying if the Auxiliary Throttle Valve spring wrap is not set correctly the carburetor will either sag or bog or most likely accelerate like a lazy 2 barrel car The instructions regarding clock wise or counter clockwise assumes that the Auxiliary Throttle Valve is out of the carb and is sitting on the workbench as shown in the picture v First insert a small screwdriver into the screw slot on the side of the valve as shown See arrow on the bottom left in the picture opposite v Note the location of the screw head in relation to the Throttle v Then insert an Allen wrench in the hole on the valve throttle body and loosen the Allen lock screw while 27 For 1966 Tricarb units treat the center carb as the Primaries and the two outer carbs as the Secondaries Always make sure that the center carb is opening all the way and that you have richened the jets as much as possible before tuning the outer carbs Carburetion I nduction Exhaust Suggestions Section 5 keeping the screwdriver steady See arrow on the top right in the picture below Auxiliary Throttle Valve showing the means to adjust it The Allen wrench holds the cam that keeps the setting locked I I I Auxiliary Throttle Valve showing means to
5. ation We will note any differences as appropriate 1966 W 30s Running the 2GC The 2GC carbs in the 3 2barrel setup are strikingly similar to a stock 2GC except that the end carbs have no provision for a choke and some other small differences You may order a reprint of the Dealer Service Manual Complete Tuning Guide for 5 00 includes postage and example of the first page is found in the Appendix Step One Rebuild It Blueprinting a 4GC or any other carburetor should be started the same way as blueprinting an engine with complete disassembly and a thorough cleaning During disassembly be careful not to lose any of the tiny but important parts We tend to use either a cupcake tin or a multi separated plastic container like those that can be found in a kitchen or housewares store and place the pieces in separate bins as they are removed If you do this in proper order the pieces can be reinstalled by reversing the order of removal 1 Before you start your rebuild do the following v Check the throttle Body Assembly Base and the Float Bowl Assembly and Air Horn mating surfaces with a straight edge to ensure they are not warped v Ensure the floats are not leaking by setting then in gasoline and see if the sink If they sink they have a hole v Examine all bolts screws fasteners springs and clips for wear and loss or tension Most of the spring steel washers will have lost their tension after all these years replace them
6. er you have reached your maximum tension change your Secondary Jets Try going richer first but don t be afraid to go one step leaner too When you have base lined to the best set of Secondary Jets try readjusting the wrap in 1 8 turns Throttle valve wrap changes the point at which the secondary mixture is fed to the carb in the low and mid range of engine speed so this is why you need to reset the wrap after you ve changed the Jets If you are unsure as to the result go back to the original wrap and try one step in richer Jets and work from there 93
7. ring Jet size 4GC Air Flow The 4GC carburetor came in 3 CFM sizes measured by throttle bore 1 4375 Primary 1 4375 Secondary 486 CFM 1 4375 Primary 1 6875 Secondary 553 CFM 1 5625 Primary 1 6875 Secondary 692 CFM This is the largest airflow possible with a 4GC 2GC Air Flow The GC carburetor came in 2 CFM sizes measured by throttle bore 1 250 CFM 1 335 CFM This is the largest airflow possible with a 2GC Rochester 4GC Carburetors Performance Tuning 4GC carbs suffer from being passed over for mods during the heyday of muscle cars because the Quadrajet and Holley were better able to handle higher air flow levels If you are running one though our tips can perk them up If you would prefer to have a professional upgrade or modify your 4GC Automotion Rochester Carburetor Service located Carburetion I nduction Exhaust Suggestions Section 5 at 837 36th Ave E Great Falls MT 59404 406 453 5395 www hotrodcarbs com can provide both rebuilt 4GC carbs and parts for performance applications If you contact them give them your application and intensions Rochester 2GC Users Those of you big block owners using 2GC carbs Turnpike Cruiser with 400 and Cutlass Supremes with the 455 2 bbl engine and all small block engines through 1972 can follow along as the 2GC utilizes many of the 4GC s primary side parts and functions in the same manner Obviously there is no secondary oper
8. sis service manual for your vehicle Choose the Proper Power Valve Spring At one time there were different power valves available Today most kits supply a medium spring if any Contact AUTOMOTION Rochester Carburetor Service to see if they have an appropriate spring for your usage Set Choke Adjust the choke so that on a warm engine that has reached normal operating temperature the choke is fully open Step Two Increasing the Fuel Flow at the Primary Metering Jets The best place to start is to increase the Primary Jet size Each 0 001 size increase in jet size is equivalent to a 1 increase in WOT fuel mixture Richening the Primary Jets may give a better surge off the line but if one goes too deep in jet size changes the car will slow down For a baseline increase the Primary Jet size in one jet size then go up in jet sizes until the car slows down Step Three Increasing the Fuel Flow with the Secondary Metering Jets Secondary jets can be increased by one or two sizes Maximize the primary metering jet fuel flow before moving on to the Secondary Jets As in the primaries each 0 001 size increase in jet size is equivalent to a 17 2 2 increase in WOT fuel mixture Richening the Secondary Jets will not effect off the line feel if the throttle valve spring is set correctly however if one goes too deep in jet size changes the car will slow down when the secondaries are fully open For a baseline increase the
9. ure SECONDARY VENTURI ISTER a IDLE COMPENSATOR FACTORY INSTALLED ONDITION CARS i PRIMARY VENTURI T PUMP RETURN SPRIN 4CG Carb with the air horn removed to show iets and power valve location 91 Secondary or Full Throttle Operation At between 42 and 60 degrees of throttle opening the primary fuel delivery is controlled by the Primary Jets plus the Power Valve opening completely At this point the air passing over Auxiliary Throttle Valve draws it open it is spring loaded and sensitive to air passing through it The Auxiliary Throttle Valve is located below the Secondary Venturi Cluster and between the Float Bowl Assembly and the Throttle Body Assembly Base As it opens air draws fuel out through the Secondary Metering Jets there is no Power Valve on the Secondary side of the system The speed and opening rate is controlled by a spring located on the side of the Secondary Venturi Cluster whose tension can be adjusted Air flow opens valve Pa Calibrated spring holds valve closed AUXILIARY THROTTLE VALVE Auxiliary Throttle Valve showing the spring which holds tension against the air flow and determines when the valve will open Fuel Metering Primary fuel delivery amount is determined by the Main Metering Jet size and the spring tension of the Power Valve Secondary fuel delivery sequence and amount is determined by the Auxiliary Throttle Valve opening point and the Secondary Mete
Download Pdf Manuals
Related Search