IDLE SPEED AND MIXTURE ADJUSTMENTS

LIMITERS AND LIMITER STOPS

All carburetors manufactured for use in the US since 1968, are equipped with idle fuel mixture adjusting limiters. The limiters control the maximum idle richness and help prevent unauthorized persons from making overly rich idle adjustments.

The plastic idle limiter cap is installed on the head of the idle fuel mixture adjusting screw(s). Under no circumstances are the limiters or limiter stops to be removed or mutilated to render the limiters inoperative. On the Autolite 2100, the power valve cover must be installed with the limiter stops on the cover in position to provide a positive stop for tabs on the idle adjusting limiters.

These days, we cut them off and do what we want.

ACCELERATOR PUMP STROKE ADJUSTMENT

The accelerating pump stroke has been set to help keep the exhaust emission level of the engine within specified limites. The additional holes provided for pump stroke adjustments are for adjusting the stroke to accomodate specific engine applications. The stroke should not be changed from the specified setting.

The primary throttle shaft lever (overtravel lever) has four holes and the accelerating pump link has two holes to control the accelerating pump stroke. The pump operating rod should be in the specified hole in the overtravel lever and in the inboard hole (hole closest to the pump diaphragm) in the accelerating pump link.

The following procedure is suggested:

1. Press the tab end of the retaining clip toward the rod to release the rod from the clip.
2. Position the clip over the specified hole in the over-travel lever.
3. Press the ends of the clip together and insert the operating rod through the clip and overtravel lever.
   1. Release the clip to engage the rod.

CHOKE PLATE PULLDOWN ADJUSTMENT

The vacuum-operated choke modulator is used in place of the vacuum piston. It tailors initial choke opening after engine start, to the existing temperature requirement.

The following adjusting procedure is suggested:

1. With engine at normal operating temperature, loosen thermostatic spring housing retainer screws and set the housing 90* in the rich direction.
2. Disconnect and remove the choke heat tube from the choke housing.
3. Turn the fast idle adjusting screw outward one full turn.
4. Start engine and check for the specified clearance between the lower edge of the choke plate and the air horn wall.
5. If the clearance is not withing specifications, turn the diaphragm stop screw (located on the underside of the choke diaphragm housing):
   1. clockwise to decrease clearance
   2. counter-clockwise to increase clearance
6. Connect choke heat tube.
7. Set choke thermostatic spring housing to specification.
8. Adjust fast idle speed to specification.

FAST IDLE CAM CLEARANCE ADJUSTMENT

The fast idle cam clearance adjustment helps to coordinate the throttle plate opening with the choke plate opening. This allows a smoother, faster running engine during the initial warm-up period.

The following adjusting procedure is suggested:

1. Loosen the thermostatic spring housing retainer screws and set the housing 90* in the rich direction.
2. Position the fast idle speed screw at the kickdown steps of the fast idle cam.
   1. The kickdown step is identified by a “V” stamped on the cam.
   2. The 351C engine makes use of a two piece fast idle lever to provide clearance between the idle lever and the intake manifold.
   3. A tang on the top lever will align with the “V” mark on the cam.
3. Check the specified clearance between the lower edge of the choke plate and the air horn wall.
4. To adjust the clearance, turn the fast idle cam clearance adjusting screw.
   1. Clockwise to increase the clearance.
   2. Counter-clockwise to decrease the clearance.
5. Set the choke thermostatic spring housing to specification.
6. Adjust the dashpot, idle speed and fuel mixture.