See Figures 1, 2 and 3
The YF carburetor is a single barrel downdraft carburetor with a diaphragm type accelerator pump and diaphragm operated metering rods.
ADJUSTMENTS - Float Level
- Invert the air horn assembly and check the clearance from the top of the float to the surface of the air horn with a T-scale. The air horn should be held at eye level when gauging and the float arm should be resting on the needle pin.
- Do not exert pressure on the needle valve when measuring or adjusting the float. Bend the float arm as necessary to adjust the float level.
- Hold the air horn up with the float hanging free.
- Measure the distance between the top of the float at the extreme out end and the air horn under surface.
- Adjust by bending the tab at the rear of the float lever.
- Back out the idle speed adjusting screw until the throttle plate is seated fully in its bore.
- Press down on the upper end of the diaphragm shaft until the diaphragm bottoms in the vacuum chamber.
- The metering rod should contact the bottom of the metering rod well and lifter link at the outer end nearest the springs and at the supporting link. The eyelet of the rod should slide freely on the pin of the metering rod arm.
- On models not equipped with an adjusting screw, adjust by bending the metering rod pin tab.
- On models with an adjusting screw, turn the screw until the metering rod just bottoms in the body casting. For final adjustment, turn the screw one additional turn clockwise.
Fast Idle Cam
- Open the throttle side enough to allow full closing of the choke valve. Be sure that the fast idle screw is not contacting the fast idle cam.
- Close the throttle valve and the fast idle cam should revolve to the fast idle position.
- If adjustment is necessary, bend the choke rod at its upper angle.
- Position the fast idle screw on the second step of the fast idle cam and against the shoulder of the high step. Measure the specified clearance between the lower edge of the choke plate and the air horn wall. Bent the choke rod to adjust.
- Fast idle speed may be checked with the engine warmed up. Speed adjustment is made by bending the choke rod at the lower angle. The speed is given with the adjusting screw on the second step and against the highest step. Adjustment is made with the adjusting screw.
EGR hoses must be blocked off and TCS solenoid vacuum valve wires must be disconnected to do this.
With the throttle valve held wide open and the choke valve held in the closed position, bend the unloader lug on the choke trip lever to obtain the specified clearance between the lower edge of the choke valve and the air horn wall.
- Loosen the choke cover retaining screws.
- Turn the choke over so that the index mark on the cover lines up with the specified mark on the choke housing. Never set it more than two graduations in either direction of the specified setting.
Electrically Assisted Choke
Starting with the 1976 models, some single barrel Carter YF carburetors use an electrically assisted choke to reduce hydrocarbon and carbon monoxide exhaust emissions during warm-up.
Once underhood temperatures reach 95°F (15°F), a bimetallic switch located in the choke cap closes, allowing a ceramic heating element to draw power from a special tap on the alternator.
This causes the choke valve to open faster than normal, thus reducing CO emission during engine warm-up.
After the engine is shut off, the bimetallic switch remains closed until underhood temperature drops below 65°F. Thus, if the engine is turned off for only a short time or if the ambient temperature is above 65°F, the choke will function for only a limited period of time.TESTING
- Detach the electrical lead from the choke cap.
- Use a jumper lead to connect the terminal on the choke cap and the wire terminal, so that the electrical circuit is still completed.
- Start the engine.
- Hook up a test light between the connector on the choke lead and ground.
- The test light should glow. If it does not, current is not being supplied to the electrically assisted choke.
- Connect the test light between the terminal on the alternator and the terminal on the choke cap. If the light now glows, replace the lead, since it is not passing current to the choke assist.
- If the light still does not glow, the fault lies somewhere in the electrical system. Check the system out.
If the electrically assisted choke receives power, but still does not appear to be functioning properly, reconnect the choke lead and proceed with the rest of the test.
- Tape the bulb end of a thermometer to the metallic portion of the choke housing.
- If the electrically assisted choke operates below 55°F (13°C), it is defective and must be replaced.
- Allow the engine to warm-up to between 80°F and 110°F (27-43°C); at these temperatures the choke should operate for about 1 1 / 2 minutes.
- If it does not operate for this length of time, check the bimetallic spring to see if it is connected to the tang on the choke lever.
- If the spring is connected and the choke is not operating properly, replace the cap assembly.
- Unfasten the electrical lead from the choke cover.
- Remove the choke cover retaining screws and clamp.
- Remove the choke cover and gasket from the carburetor.
- Installation is performed in the reverse order of removal. Adjust the choke cover.