When the engine in your Cadillac is running, air/fuel mixture from the carburetor is being drawn into the engine by a partial vacuum which is created by the downward movement of the pistons on the intake stroke of the four-stroke cycle of the engine. The amount of air/fuel mixture that enters the engine is controlled by throttle plates in the bottom of the carburetor. When the engine is not running, the throttle plates are closed, completely blocking off the bottom of the carburetor from the inside of the engine. The throttle plates are connected, through the throttle linkage, to the gas pedal. What you are actually doing when you depress the gas pedal is opening up the throttle plates in the carburetor to admit more of the fuel/air mixture to the engine. The further you open the throttle plates in the carburetor, the higher the engine speed becomes.
As previously stated, when the engine is not running, the throttle plates in the carburetor remain closed. When the engine is idling, it is necessary to open the throttle plates slightly. To prevent having to keep your foot on the gas pedal when the engine is idling, an idle speed adjusting screw was added to the carburetor. This screw has the same effect as keeping your foot slightly depressed on the gas pedal. The idle speed adjusting screw contacts a lever, or, on most late model cars, a solenoid on the outside of the carburetor. When the screw is turned in, it opens the throttle plate or plates on the carburetor, raising the idle speed of the engine. This screw is called the curb idle adjusting screw and the procedures in this section will tell you how to adjust it.
Since it is difficult for the engine to draw the air/fuel mixture from the carburetor with the small amount of throttle plate opening that is present when the engine is idling, an idle mixture passage is provided in the carburetor. This passage delivers air/fuel mixture to the engine from a hole which is located in the bottom of the carburetor below the throttle plates. This idle mixture passage contains an adjusting screw which restricts the amount of air/fuel mixture that enters the engine at idle. The idle mixture screws are capped on late model cars due to emission control regulations.
General Motors Rochester carburetors are identified by their model code. The first number indicates the number of barrels, while one of the last letters indicates the type of choke used. These are V for the manifold mounted choke coil, C for the choke coil mounted in the carburetor body, and E for electric choke, also mounted on the carburetor. Model codes ending in A indicate an altitude-compensating carburetor. Four different carburetors have been used on Cadillacs since 1967: the Rochester 4MV 4 bbl 1967-74; Rochester M4ME 4 bbl 1975-80; the Rochester E4ME 4 bbl 1980-84; and the E4MC in 1986.
REMOVAL & INSTALLATIONAll Carburetors
- Remove the air cleaner and its gasket.
- Disconnect the fuel and vacuum lines from the carburetor.
- Disconnect the choke coil rod, heated air line tube, or electrical connector.
- Disconnect the throttle linkage.
- Disconnect the throttle valve linkage if so equipped.
- Disconnect the EGR line, if so equipped.
- Remove the idle stop solenoid, if so equipped.
- Remove the carburetor attaching nuts and/or bolts, gasket or insulator, and remove the carburetor.
- Install the carburetor using the reverse of the removal procedure. Use a new gasket and fill the float bowl with gasoline to ease starting the engine.
See Figures 1 and 2
Efficient carburetion depends greatly on careful cleaning and inspection during overhaul, since dirt, gum, water, or varnish in or on the carburetor parts are often responsible for poor performance.
Overhaul your carburetor in a clean, dust-free area. Carefully disassemble the carburetor, referring often to the directions packaged with the rebuilding kit. Keep all similar and look-alike parts segregated during disassembly and cleaning to avoid accidental interchange during assembly. Make a note of all jet sizes.
When the carburetor is disassembled, wash all parts (except diaphragms, electric choke units, pump plunger, and any other plastic, leather, fiber, or rubber parts) in clean carburetor solvent. Do not leave parts in the solvent any longer than is necessary to sufficiently loosen the deposits. Excessive cleaning may remove the special finish from the float bowl and choke valve bodies, leaving these parts unfit for service. Rinse all parts in clean solvent and blow them dry with compressed air or allow them to air dry. Wipe clean all cork, plastic, leather, and fiber parts with a clean, lint-free cloth.
Blow out all passages and jets with compressed air and be sure that there are no restrictions or blockages. Never use wire or similar tools to clean jets, fuel passages, or air bleeds. Clean all jets and valves separately to avoid accidental interchange.
Check all parts for wear or damage. If wear or damage is found, replace the defective parts. Especially check the following.
- Check the float needle and seat for wear. If wear is found, replace the complete assembly.
- Check the float hinge pin for wear and the float(s) for dents or distortion. Replace the float if fuel has leaked into it.
- Check the throttle and choke shaft bores for wear or an out-of-round condition. Damage or wear to throttle arm, shaft, or shaft bore will often require replacement of the throttle body. These parts require a close tolerance of fit; wear may allow air leakage, which could affect starting and idling.
Throttle shafts and bushings are not included in overhaul kits. They can be purchased separately.
- Inspect the idle mixture adjusting needles for burrs or grooves. Any such condition requires replacement of the needle, since you will not be able to obtain a satisfactory idle.
- Test the accelerator pump check valves. They should pass air one way but not the other. Test for proper seating by blowing and sucking on the valve. Replace the valve check ball and spring as necessary. If the valve is satisfactory, wash the valve parts again to remove breath moisture.
- Check the bowl cover for warped surfaces with a straightedge.
- Closely inspect the accelerator pump plunger for wear and damage, replacing as necessary.
- After the carburetor is assembled, check the choke valve for freedom of operation.
Carburetor overhaul kits are recommended for each overhaul. These kits contain all gaskets and new parts to replace those which deteriorate most rapidly. Failure to replace all parts supplied with the kit (especially gaskets) can result in poor performance later.
Some carburetor manufacturers supply overhaul kits of three basic types: minor repair; major repair; and gasket kits. Basically, they contain the following:Minor Repair Kits:
- All gaskets
- Float needle valve
- All diagrams
- Spring for the pump diaphragm
- All jets and gaskets
- All diaphragms
- Float needle valve
- Pump ball valve
- Complete intermediate rod
- Intermediate pump lever
- Some cover hold-down screws and washers
- All gaskets
After cleaning and checking all components, reassemble the carburetor, using new parts. When reassembling, make sure that all screws and jets are tight in their seats, but do not overtighten as the tips will be distorted. Tighten all screws gradually, in rotation. Do not tighten needle valves into their seats; uneven jetting will result. Always use new gaskets. Be sure to adjust the float level when reassembling.
The following should be observed before attempting any adjustments.
- Thoroughly warm the engine. If the engine is cold, be sure that it reaches operating temperature.
- Check the torque of all carburetor mounting nuts and assembly screws. Also check the intake manifold-to-cylinder head bolts. If air is leaking at any of these points, any attempts at adjustment will inevitably lead to frustration.
- Check the manifold heat control valve (if used) to be sure that it is free.
- Check and adjust the choke as necessary.
- Adjust the idle speed and mixture. If the mixture screws are capped, don't adjust them unless all other causes of rough idle have been eliminated. If any adjustments are performed that might possible change the idle speed or mixture, adjust the idle and mixture again when you are finished.
Before you make any carburetor adjustments make sure that the engine is in tune. Many problems which are thought to be carburetor-related can be traced to an engine which is simply out-of-tune. Any trouble in these areas will have symptoms like those of carburetor problems.