COMPONENT TESTING & ADJUSTMENT
Carbureted engines have incorporated many pollutant control devices, such as a temperature-regulated fuel jet, an air/fuel mixture preheating chamber, and a throttle bypass or over-rev valve. Measures have also been taken to improve the operation and driveability of emission controlled engines such as a cold start device, a constant intake air temperature device and a hot start valve. The temperature-regulated fuel mixture is accomplished differently on the Zenith-Stromberg carburetor than on the SU carburetor.
On the Zenith-Stromberg carburetor, a temperature sensitive bimetal spring (in the temperature compensator) actuates an air valve that varies the air supplied the venturi area, maintaining a constant air/fuel ratio. This ratio is maintained despite changing fuel temperature.
Within the SU carburetor, a temperature sensitive bimetal spring raises or lowers an adjustable jet to maintain the proper air/fuel ratio at changing fuel temperatures.
The air/fuel mixture preheating chamber used on pre-1972 models is located between the primary throttles in the carburetors and the secondary throttles in the intake manifold.Temperature Compensator
See Figures 1 and 2
Please refer to the component legend (following) for the Zenith-Stromberg carburetor. This procedure refers to components by number which makes the legend to which they are keyed necessary.
If the idle speed drops off sharply during extended periods of idling, especially during warm weather, the temperature compensator may be in need of adjustment or replacement. Use the accompanying diagram to help you.
- Remove the screws (7) holding the plastic cover (5) to the compensator and remove the cover.
- With the ambient temperature at or above 85°F (29°C), the valve (3) should be able to be pressed inward with light finger pressure and then return to its position without jamming. If the valve jams and/or is stiff in operation, the temperature compensator should be replaced as a unit. See Temperature Compensator Replacement.
- If properly adjusted, the valve will begin to open at 70-77°F (21-25°C), and will be fully open at 85°F (29°C). The valve may be adjusted while the temperature compensator is still on the carburetor by loosening one of the cross-slotted screws (8) for the bimetal spring (4), and centering the valve so that it opens and closes at the proper temperature. If necessary, the temperature compensator may be removed and isolated at 70-77°F (21-25°C) then adjusted with the nut (9) for the bimetal spring so that the valve is loose in its seat at this temperature.
- Replace the cover and retaining screws on the compensator and check its operation during idling.
If the engine does not return to idle speed soon after the throttle is released, and the throttle control linkage is properly adjusted, the bypass valve may be in need of adjustment or replacement.
- If the engine refuses to lower its rpm to idle speed when the throttle is released, turn the bypass adjusting screw on the front carburetor to the left, and manually lower the idle.
- Run the engine briefly up to approximately 2000 rpm, then release the throttle. If the engine returns to idle speed, turn the screw 1 / 2 turn further to the left. If the engine does NOT return to idle, replace the bypass valve as a unit.
- Remove the air cleaner. While looking into the carburetor bores, observe the air valves. Briefly race the engine and then release the throttle. The air valve of the front carburetor should normally go down to the bridge slower than the air valve of the rear carburetor. Turn the bypass adjusting screw to the right until the normal function is obtained. If the valve cannot be adjusted so that the front air valve goes down to the bridge slower than the rear air valve, the bypass valve must be replaced as a unit. See Bypass Valve Replacement.
ZENITH STROMBERG CARBURETOR
With the throttle at idle, adjust the valve control of the hot start valve so that the valve is against the carburetor lever with the valve piston in the upper position. Coat the contact surfaces on the valve and carburetor with high temperature white grease such as Molykote®. Test the operation of the hot start valve by confirming that the engine returns to idle after several brief periods of racing the engine.SU CARBURETOR
See Figure 3
To adjust the hot start valve, press the control rods down to the bottom position and measure the distance (A) between the rod and the adjusting screw. Adjust the distance to a maximum of 0.04 in. (1mm).
Test the operation of the control rods. Make sure that they do not jam.
See Figure 4
If the flap for the constant air temperature device sticks in one position, engine operation will suffer. Normally, the flap is closed to cold air (intake hose) at an ambient temperature of 70-77°F (21-25°C) and closed to hot air (ducted from the exhaust manifold) at 95°-105°F (35-40°C). Depending on the temperature, the flap can remain in a partially open position, blending the warm and cool air to achieve the proper intake air temperature.
- The operation of the flap may be checked with the housing installed on the car. When the small tab on the flap housing points toward the mark closest to the exhaust heat hose, the flap is open for hot (preheated) air. When the tab points to the mark nearest the cold air intake, the flap is open for cold (outside) air. If the tab indicates that the flap is opening and closing the air sources at the right temperatures, it is operating correctly. If not, check the operation of the flap control thermostat.
- Disconnect the flap housing from the air intake hoses. Immerse the thermostat in lukewarm water. At a water temperature of 70-77°F (21-25°C), the thermostat should be in its upper (toward the flap housing) position. At 95-105°F (35-40°C), the thermostat should be in its lower position (away from the flap housing). If correct operation cannot be obtained, replace the thermostat and flap housing as a unit.
- Replace the flap housing and thermostat assembly, making sure that the thermostat is centered in the middle of the air flow. Secure the hose clamp screw on top of the flap.
See Figure 5
- Remove the carburetor.
- Invert the carburetor and remove the float chamber.
- While referring to the Zenith-Stromberg float adjustment drawing, adjust the float so that the high point (distance B) of the float is 5 / 8 in. (16mm), and the low point (distance A) of the float is 1 / 2 in. (13mm), from the sealing surface of the carburetor housing.
- To adjust the float level, bend the tab at the float chamber inlet valve. Do not bend the arm between the float and the pin.
- When the proper adjustment has been made, install the float chamber to the housing with a new gasket.
- Reinstall the carburetor on the intake manifold.
See Figure 6
- Remove the carburetor.
- Invert the carburetor and remove the float chamber.
- The float is correctly adjusted when the distance A between the float valley and the housing flange is approximately 0.02-0.06 in. (0.50-1.50mm).
- The float is adjusted by bending the metal tab at the float chamber inlet valve.
- When the correct adjustment has been made, install the float chamber to the housing with a new gasket.
- Install the carburetor.
Pull out the choke control one inch from the dash. If the choke is adjusted correctly, the mark on the fast idle cam (see illustration) should be opposite the centerline of the fast idle screw. Adjust the fast idle with the fast idle screw to 1100-1300 rpm. For cars with dual carburetors, make sure you set the fast idle on both carburetors.SU CARBURETOR
Pull out the choke control 0.8 in. (20mm) from the dash. Adjust the fast idle (with the fast idle screw) to 1100-1600 rpm. Remember to set both carburetors if so equipped.Throttle Linkage Adjustment
See Figures 7 and 8
On each carburetor, the link rods should maintain a 0.004 in. (0.1mm)-clearance A-between the lever and the throttle spindle flange when the throttle control is against the intake manifold bracket. To adjust this clearance, remove the ball-and-socket link from the carburetor stud, and turn the socket on the threaded rod until the adjustment is correct. Remember that you are seeking a clearance measured in thousandths of an inch; the link should not require a great amount of turning.
COMPONENT REMOVAL & INSTALLATION
Please refer to the component legend for the Zenith-Stromberg carburetor. This procedure refers to components by number which makes the legend to which they are keyed necessary.
- Remove the retaining screw (6) and lift off the compensator.
- Discard the old rubber seals (1, 2) and replace them with new ones.
- Position the compensator to the side of the carburetor and install the retaining screws.
- Check the operation of the compensator during idle.
See Figure 9
- Remove the three retaining screws (6) and lift off the bypass valve.
- Discard the old bypass valve-to-carburetor housing gasket.
- Position a new gasket and bypass valve to the carburetor, making sure that the orifices and mating surfaces of the valve and gasket align and install the three retaining screws.
- Check the operation of the bypass valve. (See Bypass Valve Testing and Adjustment.)
ZENITH STROMBERG CARBURETOR
The hot start valve on the Zenith-Stromberg carburetor is riveted to the air cleaner. If the valve requires cleaning, it must be done with the assembly in place on the engine.SU CARBURETOR
- Remove the two retaining screws and lift off the valve.
- Discard the old gasket and clean the channels in the carburetor with a low pressure air line.
- Position the valve and new gasket to the carburetor, making sure that the gasket is aligned properly, then install the two retaining screws.
- Adjust the position of the control rod and test the operation of the valve. (See Hot Start Valve Testing and Adjustment.)
- Loosen the hose clamps and disconnect the flap housing and thermostat assembly from the hoses for the intake air, heated air, and intake manifold.
- Install the new flap housing assembly in position and reconnect the three hoses. Make sure that the thermostat is centered in the middle of the intake air flow. Secure the hose clamp screw on top of the flap.
- Check the operation of the new flap housing. (See Constant Air Temperature Device Testing and Adjustment.)
- Pry off the lockwasher for the cold start device and unscrew the channel disc nut.
- Disconnect the return spring and remove the channel disc and spring.
- Remove the two retaining screws and the spring retainer.
- Lift the cold start device away from the carburetor. Press the spindle out of the cold start device housing. Remove the gasket the rubber ring, and rubber seal from the spindle, and discard them. Clean all metal parts with kerosene (not gasoline) and clean the fuel channels with an compressed air.
- Install a new rubber ring and seal on the housing and oil them with light engine oil (use 10W, or if this is not available, 10W-30 will suffice). Install the spindle into the housing.
- Position the housing assembly, with a new gasket, on the carburetor. Fit the spring retainer and install the retaining screws.
- Position the return spring in its retainer so that the spring's short end fits into the retainer slot.
- Hook the channel disc onto the spring's longer end and install the disc on the spindle. Install the channel disc retaining nut and snap on the lockwasher.
See Figure 10
If the engine stumbles upon acceleration, and the damping cylinders are filled to their proper level with oil, the problem may be with the damping pistons themselves. Unscrew the black knobs on top of the carburetors and remove the damping pistons. If the axial clearance A between the bottom of the piston and the retaining clip is not 0.04-0.08 in. (1.0-2.0mm), the piston must be replaced as a unit.
ZENITH-STROMBERG CARBURETOR ONLY
The Stromberg carburetor differs from the SU carburetor in that it has a rubber diaphragm attached to the air valve. Vacuum from the intake manifold acts upon the diaphragm to raise and lower the air valve. Obviously, if the diaphragm rips (which can happen due to age and/or chemical deterioration), engine performance will suffer measurably. This procedure is done with the carburetor on the engine.
- Remove the damper piston from the top of the carburetor.
- Remove the four screws and remove the top of the carburetor.
- Remove the large spring and remove the air valve complete with rubber diaphragm.
- Check the diaphragm for rips and brittleness. If no rips or deterioration are evident, reuse the diaphragm.
- If the diaphragm is ripped or damaged, remove the four screws and remove the diaphragm washer and the diaphragm.
- Fit a new diaphragm. The diaphragm will fit on the air valve only one way: it has a notch in it which must be inserted in the notch on the air valve.
- Fit the air valve and diaphragm on the carburetor. You may have to remove the air cleaner to guide the needle on the air valve into its jet. Place the air valve into the carburetor so that the notch in the outside of the diaphragm fits into the notch in the carburetor body.
- Reinstall the spring and carburetor top. Fill the damper with ATF fluid and install the damper piston.
REMOVAL & INSTALLATION
Two different types of carburetors were used on 1970-72 Volvos. A pair of sidedraft Zenith-Stromberg 175 CD2 SE units were used on 1970 140 series models, and on 1970-72 164 models. A pair of sidedraft SU HIF units were used on 1971-72 140 series models.
All wires and hoses should be labeled at the time of removal. The amount of time saved during reassembly makes the extra effort well worthwhile.
- On the Zenith-Stromberg carburetor, disconnect the hot start valve control. Separate the air cleaner halves and remove the inner half from the carburetors.
- Disconnect the throttle linkage by removing the link rod ball joints from the carburetors. Disconnect the choke cable, taking note of its proper routing and adjustment.
- Disconnect and plug the fuel lines at the float chambers. Remove the vacuum hose for the distributor. On the SU carburetor, disconnect the hot start valve hose.
- Remove the four nuts retaining the carburetor to the intake manifold. Remove the carburetors, gaskets, and protection plate. Keep the carburetor vertical and remove it from the engine are. Safely drain the remaining fuel within the bowls of the carburetor and cap the container. When repairing the carburetor on the workbench, keep the area clean and free from dirt and dust.
- Position the protection plate, new gaskets, and carburetor on the intake manifold studs. Install the carburetor retaining nuts and tighten them evenly until they are snug against the manifold.
- Connect the vacuum hoses, fuel hoses, choke, and throttle linkage. On the SU carburetor, connect the hot start valve hose.
- Install the inner half of the air cleaner to the carburetors. Adjust the idle speed and mixture of the carburetors. (see Engine Electrical .)
- Fit the air cleaner halves together and, on the Stromberg carburetor, connect the hot start valve control.
See Figures 11 and 12
Carburetors are relatively complex. Proper performance depends upon the cleanliness and proper adjustment of all internal and external components. In addition to the usual adjustments performed at the regular tune-up intervals, it eventually becomes necessary to remove, disassemble, clean, and overhaul the entire carburetor in order to restore its original performance.
To overhaul a carburetor, first purchase the proper rebuilding kit. Read the instructions and study the exploded view of the carburetor thoroughly prior to the actual removal and disassembly.
After reading the detailed carburetor rebuilding instructions, the following general procedure may be used. Remove the carburetor and place it on a clean work table. Disassemble the carburetor by removing the screws securing the upper and lower sections together. Remove the damping piston, air valve, spring, metering needle, fuel jet (SU only), and float assembly.
Soak all metal parts in carburetor cleaning solvent. Scrape all old gasket material from the mating surfaces. After the metal parts have been soaked to remove all gum, varnish, and dirt, rinse them off with a clean, uncontaminated, solvent solution. Blow out all passages with compressed air and allow them to air dry. Do not use drills or wire to clean the passages.
Check the throttle shaft and choke disc for excessive wear. Inspect the float hinge pins for distortion. All non-metal parts that are not being replaced should be wiped clean with a lint-free cloth. After all of the parts have been sufficiently cleaned or replaced, assemble the carburetor using new gaskets and seals, and, on Zenith-Stromberg carburetors, a new air valve diaphragm. If any of the replacement seals in an SU carburetor are cork, they must first be soaked in penetrating oil for a minimum of a half hour to avoid splitting during installation.
Assemble the float chamber and adjust the float height. Assemble the air valve, spring, and metering needle into the upper housing. Join the upper and lower housing together, taking care to properly align the metering needle and fuel jet. When the jet and needle are installed correctly, the air valve should drop to the bridge with a distinctive click. Any binding of these two parts will result in poor carburetor performance. Install the damping piston. Install the carburetor on its manifold with a new gasket. Adjust the choke and throttle linkage, and the idle speed. Run the engine through a full cold start, warm up and hot restart cycle to check for proper function of all carburetor systems. Make adjustments as necessary.