71. The Zenith Carburetor
Bron: http://www.jaimekop.com/CarbManual/Page01/toc.html
Adjustment values for all models can e found on page 31
Index
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 );
4. Throttle Valve Section (with throttle valves, idling speed mixture regulating screw and bypass bores).
The Zenith was used by Mercedes in type 220Sb from 1964 and up until 1972 in the 250/8.
PARTS INDENTIFICATION
B. ADJUSTMENTS
1. VENTING VALVES. (Pre-1972)
Function
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.
Problems
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.
Adjustment
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.
Problem
Examine pump plunger for damage and sticking. A faulty accelerator pump will cause:
1. Hesitation on acceleration
2. Stalling
3. Fuel consumption
Test: Pump lever should have tension‹if not, pump is binding in bore
To check the accelerator pump for proper function the top cover and pre-atomizer in the primary stage must be removed.
1. Start of Injection Immediately on opening the throttle, a powerful jet of fu' should emerge. (See Fig. 1)
2. Injection Amount Using a graduated container‹part no. 111-589-17-21-00, pump for one full stroke. The amount should be 0.7-1.0 cc. (See Fig. 2, 2A.)
Inspect accelerator pump checkballs, inlet and outlet. (See Fig. 2B)
3. Adjust the injection amount by bending the internal lever. (See Fig. 3)
4. Direction of injection. Pre-1972 and 1972 (See Fig 4)
3. FLOAT LEVEL
To establish the fuel level in the carburetor. The Zenith carburetor is highly sensitive to the float level.
1. Incorrect float {fuel) level will cause:
a. Fuel consumption
b. Lean or rich fuel mixture
c. Stalling and hesitation
d. Hard hot starts
2. Float filling with fuel and sinking‹will cause flooding.
3. Fuel needle valve sticking‹will cause flooding.
4. 1972 Models with spring-loaded ball needle valves‹ball can fall out.
1. Remove plate block section. (See Fig. 1)
2. With gasket in place, measure from plate block to bottom of float. (See Fig. 2)
3. Adjust float level by changing thickness of seal ring under the needle valve assembly. (See Fig. 3)
4. Check data and adjustment guide on page 34.
5. Seal ring changes
Seal Ring
Part No.
0.5 mm
000-997-81-40
1.0 mm
000-997-28-40
1.5 mm
000-997-82-40
2.0 mm
000-997-83-40
4. SECOND STAGE DIAPHRAGM
The second stage diaphragm operates the second stage throttle depending upon throttle position and engine load.
If the second stage diaphragm is not operating properly the following may be experienced:
1. Poor acceleration.
2. Inability to reach full speed.
Inspection
1. Locate the two vacuum supply holes for the second stage diaphragm housing. (See Fig. 1 )
2. While the plate block section is removed, lift the second stage diaphragm and place fingers over the vacuum holes in the venturi. (See Fig. 2.)
3. Release the diaphragm. Vacuum will hold the diaphragm if there are no leaks.
4. If the diaphragm is bad, only it needs replacing. (See Fig. 3)
5. Reassemble carburetor using all new gaskets. (See Fig. 4)
5. IDLE FUEL SHUT-OFF SOLENOIDS (used in 1971-72 models)
Idle fuel shut-off solenoids act as check valves. When the ignition switch is turned OFF, the solenoids shut off the fuel at the idle port to prevent "dieseling" or "after-run."
If idle fuel shut-off solenoids are not functioning properly, the driver may experience
1. Dieseling or after-run.
2. Shut-off valve malfunction produces the same characteristics as a "plugged" carburetor idle jet (rough idle).
Testing
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.
IMPORTANT
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. 2 and 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.
7. LINKAGE
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...
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