B ron: http://www.jaimekop.com/CarbManual/Page01/toc.html
Adjustment values for all models can e found on page 31
A. INTRODUCTION 3 Parts Identification 4
B. ADJUSTMENTS 5
1. Venting Valves (Pre-1972 Only) 5
2. Accelerator pumps 7
3. Float Level 9
4. Second Stage Diaphragm 11
5. Idle Fuel Shut-off Solenoids 13
6. Idle Speed and Mixture 14
7. Linkage 16
8. Choke Cover Tension 18
9. Choke Tie Rod 20
10. Choke Fast Idle Speed 22
11. Choke Gaps 24
12. Vacuum Throttle Control or Dashpot 26
13. Fuel Return Valve (Pre-1972 only) 28
14. Idle Lifting Switch (If applicable) 29 C. TROUBLESHOOTING GUIDE 30 D. DATA AND ADJUSTING VALUES 31
A. INTRODUCTION The Zenith Carburetor is a two-stage carburetor with throttle valve diameters of 35 mm. for stage I and 40 mm. for stage II.
It consists of four main parts which are bolted together:
1. Carburetor Cover (choke plate, idling speed air hole and transition air hole);
2. Plate block (with all jets, accelerating pump, float and float chamber venting valve);
3. Float Housing (with mixing chamber and venturi for stage I and stage II );
The Zenith was used by Mercedes in type 220Sb from 1964 and up until 1972 in the 250/8.
P ARTS INDENTIFICATION
1. VENTING VALVES. (Pre-1972)
Bowl venting gives a reliable hot start.
Ventilation is achieved through the venting valve controlled by the carburetor linkage. When driving, bowl ventilation is in the air cleaner. At idle and when engine is stopped, ventilation is outside the air cleaner. This eliminates starting difficulties caused by fuel vapors entering intake manifold when engine is hot.
The venting valves were eliminated on 1972 models in order to conform to federal requirements. Only internal venting is provided on these models.
1. High fuel consumption can occur if the valve is stuck in the open position.
2. Hot starting problems can occur if the valve is stuck closed.
1. Remove carburetor connecting linkage. (See Fig. 1)
2. Remove vacuum throttle control. (See Fig. 2)
3. Open chokes. (See Fig 3)
4. On late carburetors the vent valve has a stop screw. This has been set at the factory and should not be changed. If it has been tampered with, readjust it to provide 2.5-2.8 mm. vent pin lift. (If changed other carburetor adjustments are affected.) (See Fig 4)
5. On early carburetors, without stop screw, this adjustment must be performed last. This adjustment is made by bending the arm at the notch to provide 2.5‹ 3.0 mm. vent pin lift. (See Fig. 5)
2. ACCELERATOR PUMP
The function of the accelerator pump is to momentarily provide the additional quantity of fuel needed during acceleration, until the flow delivered from the main metering system increases. This is required whenever the throttle is suddenly opened.
The pump is a piston type pump, operated through the throttle linkage. It is used in stage I only. Located inside the float chamber, it is constantly surrounded by gasoline.
When the pump lever is actuated, the pump piston forces fuel through channels and out through the calibrated injection pipe into the carburetor throat.
Examine pump plunger for damage and sticking. A faulty accelerator pump will cause:
2. Shut-off valve malfunction produces the same characteristics as a "plugged" carburetor idle jet (rough idle).
1. To test the idle shut-off valves, disconnect and reconnect the single pin connectors individually with the ignition switched on. The noise caused by the valve opening must be audible. (See Fig. 1)
6. IDLE SPEED AND MIXTURE
The idle circuit is contained in the first stage of the carburetor. Fuel is drawn up into a cavity within the carburetor cover through the idle fuel jet. Here it is mixed into an emulsion with the air entering through the idle air bore. By way of a channel, this emulsion flows to the fuel mixture outlet at the idle mixture screw and to the by-pass bores. The by-pass bores serve to improve transition from idle jet to main jet system when opening the throttle.
The carburetors must be synchronized in order to prevent the possibility of performance problems.
1. With car at full operating temperature, synchronize the throttle plates of each carburetor using "Syncro-Test" device and adapter piece. Adjust to specified idle RPM. (See Fig. 1)
2. Adjust idle mixture screws to the same position by blocking off the idle air bore in the top cover, one at a time, and matching the RPM drop. This procedure prevents damaging the idle mixture screws. (See Fig. 2and 2A)
3. Observing tachometer and exhaust analyzer, adjust throttle valves, and mixture as necessary to arrive at specified idle RPM and specified carbon monoxide. (See Fig. 3)
NOTE: Continually check balance of throttle valves (using "Synchro-Test") and mixture (using idle air bores) as the adjustments are made.
To control synchronization while opening the throttle and transmission shifting.
If the linkage is not properly adjusted:
1. The carburetors will not be synchronized.
2. Shift pattern problems (too early or too late) may be experienced.
1. Long Rod Adjust the long center connecting rod to fit tension free and install. (It must be tension free so as not to change synchronization.) (See Fig. 1,1A)
2. Short Rod with Free Travel With the free travel fully extended the rod should fit neutral from ball joint to ball joint. (See Fig. 2, 2A)
3. Short Rod‹Solid Adjust the rod slightly short from ball joint to ball joint, so that the bell crank is lifted from its rest position by approximately 2 mm. (See Fig. 3)
8. CHOKE COVER TENSION
The Zenith Carburetor is equipped with an automatic choke which is controlled by an electrically heated bi-metal spring. A connecting rod keeps the choke under the tension of the spring which responds to any change in temperature. When the engine is cold, the spring holds the choke closed. When the engine starts, the gap control unit opens the choke to prevent over enrichment.
If the choke cover tension is not properly adjusted the following could occur:
1. Warm up problems.
2. Stumbling, stalling, hesitation.
1. From the factory, both choke covers are adjusted 5 mm. in the rich direction. (See Fig. 1)
2. Re-adjustment is required at the second vehicle service. (See Fig. 2)
9. CHOKE TIE ROD
The function of the choke tie rod is to tie together the operation of the choke plate, bi-metal spring, high idle cam and choke gap.
Prior to making any adjustments related to the choke, the tie rod must be adjusted to its proper basic length. This insures that when the choke flap shuts the high idle cam is turned fully to the high step.
1. Block throttle linkage partially open to free choke cam. (See Fig. 1)
2. Loosen set screw at top of choke rod. (See Fig. 2)
3. Lift relay lever at bottom of choke rod to its full up position. (See Fig. 3)
4. Close choke flap fully, allowing connecting lever to center itself, and tighten set screw.(See Fig. 4)
Note: Late types have 1.5 mm. allen set screw.
10. CHOKE FAST IDLE SPEED
The fast idle speed is critical in keeping the engine running at sufficient speed to overcome the increased resistance of the engine when cold.
If the choke fast idle speed is not correct, the result will be insufficient chokespeed (stalling) in a cold engine, or excessive speed causing rough engagement of the transmission.
Note: Before proceeding with adjustment, remove and discard fast idle speed adjusting screw and screw tensioning spring in the rear carburetor, if one is installed.
1. If a 100° C. temperature switch is installed in thermostat housing, ground cable end so necessary advanced ignition timing is provided. (See Fig. 1)
2. Crack open throttle.
3. Close choke flap by lifting choke relay lever. Release throttle. (See Fig. 3)
4. Start engine without touching throttle. Observe RPM. (See Fig. 4)
5. Adjust to specified RPM at front carburetor fast idle adjusting screw. See page 34. (See Fig. 5)
11. CHOKE GAPS
The choke gap control opens the choke flap slightly to allow the engine to breathe after starting.
1. Choke flap or linkage binds. (Flap won't close or binds and won't open.) Causes rough running and possible stalling when engine is cold.
1. Set carburetor at fast idle by actuating choke step cam. To do this, crack open throttle (See Fig. 1)
2. Close the choke flap by lifting choke relay lever. (See Fig. 2)
3. Release throttle.
4. Start engine without touching throttle.
5. Lift relay lever at bottom of choke rod to its full up position. (See Fig. 2)
7. Adjust at the adjusting screw on diaphragm housing. (See Fig. 4)
12. VACUUM THROTTLE CONTROL OR VACUUM DASHPOT
The function of the vacuum throttle control or vacuum dashpot is:
1. To prevent stalling, when the throttle is suddenly closed. It insures a gradual or slow closing which permits the continuing air supply to lean out the rich mixture of fuel that occurs on sudden throttle closing.
2. It works with emission control. During coasting it holds the throttle plate open slightly.
3. It acts as an idle speed stabilizer when the engine is under load at idle. The plunger extends as the vacuum falls due to the load. This causes the throttles to be opened slightly.
If the dashpot or vacuum control is incorrectly adjusted the driver will experience:
1. Braking problems, if the throttle is held open too long.
VACUUM THROTTLE CONTROL
1. With engine idling, remove vacuum hose from throttle control. (See Fig. 1)
2. Set adjusting screw to provide an engine speed of 1100 to 1200 RPM. Caution: Late units have jam nut on screw which must be loosened first. (See Fig. 2)
3. Reconnect vacuum hose. (See Fig. 3)
4. Adjust compression spring with adjusting plate, so that play between actuating lever and adjustment screw is sufficient to slide a piece of paper through (0.1 mm). While adjusting nut do not allow screw to turn. (See Fig. 4)
(Early models only)
1. With engine off, hold actuating lever to the point where it just contacts the venting valve. (See Fig. 5)
2. Adjust dashpot plunger so that there is 2-3 mm. clearance between the carburetor lever and dashpot plunger. (See Fig. 6)
13. FUEL RETURN VALVE (Pre-1972 only)
The function of the fuel return valve is to eliminate gasoline vapor locks. It does this by allowing unused fuel to return to the tank.
1972 models have an orifice hole for constant return which is automatic and requires no adjustments.
1. Slowly accelerate while watching return valve pin. At 2,000 RPM the pin should be fully lifted. (See Fig. 1)
2. Adjust at spring tension adjusting screw. (See Fig 2)
14. IDLE LlFTING SWITCH
(applies only to K4A Automatic Transmission)
The idle lifting switch controls the two-way solenoid on top of the automatic transmission.
If the idle lifting switch is not operating or adjusted properly, the results can be:
1. Slipping of the transmission on acceleration.
2. Very rough downshift of the transmission.
3. Damage to the transmission.
1. Attach a test light from outlet terminal of switch to ground. (See Fig. 1)
2. Accelerate slowly while observing light. (See Fig. 2)
3 . Adjust switch as necessary by loosening the mounting screws and pivoting switch until light goes out (switch opens) at between 1400 to 1600 RPM.