Carburetor - Aisan 2-bbl Chevrolet Nova V

APPLICATION

GENERAL MOTORS (AISAN) CARBURETOR NO.

IDENTIFICATION

The Aisan carburetor numbers are attached to the upper left corner of the air horn. If float bowl is replaced, be sure to replace identification tag when reassembling.

DESCRIPTION

Carburetor is a 2-barrel, downdraft design, equipped with an automatic choke consisting of a bi-metallic coil and an electrically-operated ceramic heater. A piston type accelerator pump is incorporated into the primary barrel and an auxiliary accelerator pump system (AAP) aids in cold engine acceleration.

Other features include diaphragms which open secondaries at high speed and full choke operation, throttle positioner, choke opener, choke breaker, deceleration fuel cut and hot idle compensation devices.

THROTTLE POSITION SWITCH

1. Connect a tachometer to engine. Start engine and warm up to normal operating temperature. Disconnect throttle position switch connector. Connect an ohmmeter between throttle position switch connector and ground.
2. Slowly raise engine speed. Check throttle position switch setting speed when ohmmeter shows continuity. Setting speed is approximately 1800 rpm. Adjust setting speed by throttle position switch adjusting screw.

AUTOMATIC CHOKE SYSTEM

Allow choke valve to close. Start engine. Check that choke valve begins to open and choke housing is heated. Unplug wiring connector. Measure resistance with ohmmeter. Resistance should be approximately 19-24 ohms at 68°F (20°C).

Choke Breaker System

1. Check TVSV with cold engine. Start engine. With coolant temperature below 45°F (7°C), disconnect vacuum hose from inner choke breaker diaphragm and check that choke linkage does not move.
2. Reconnect vacuum hose to outer choke breaker diaphragm. Disconnect vacuum hose from inner choke breaker diaphragm and check that choke linkage moves. Reconnect vacuum hose to inner diaphragm and check that choke linkage moves within 1-5 seconds after reconnecting hose.
3. Check TVSV and outer diaphragm with warm engine. Disconnect vacuum hose from outer diaphragm and check that choke linkage returns. Reconnect vacuum hose to diaphragm. If no problem is found with this inspection, system is okay.

Choke Opener Operation

1. Check TVSV with cold engine. Disconnect vacuum hose from choke opener diaphragm. With coolant temperature below 122°F (50°C), step down on accelerator pedal and release it.
2. Start engine. Reconnect vacuum hose and check that choke linkage does not move. Check TVSV, diaphragm and linkage with warm engine. Warm up engine to normal operating temperature and stop engine.
3. Disconnect vacuum hose from choke opener diaphragm. Set fast idle cam. While holding throttle slightly open and push choke valve closed, hold it closed as you release throttle valve.
4. Start engine, but DO NOT touch accelerator pedal. Reconnect vacuum hose and check that choke linkage moves and that fast idle cam is released to 3rd step. If no problem is found with this inspection, system is okay.

AUXILIARY ACCELERATION PUMP SYSTEM

1. Check system with cold engine. Check that coolant temperature is below 122°F (50°C). Remove air cleaner cover. Start engine. Pinch AAP hose and stop engine. Release hose. Check that gasoline spurts out from acceleration nozzle.
2. Repeat step 1) after engine warms up. Check that gasoline does not spurt out from the acceleration nozzle. Reinstall air cleaner cover. If no problem is found with this inspection, the system is okay.

Auxiliary Acceleration

Pump Diaphragm

Check diaphragm operation at idle. Start engine. Disconnect hose from AAP diaphragm. Apply and release vacuum directly to AAP diaphragm at idle. Check that engine rpm changes by releasing vacuum. Reconnect AAP hose. If no problem is found, replace diaphragm.

FLOAT LEVEL

Note. Remove needle valve wire clip and install new inlet seat gasket before adjusting float.

1. Remove air horn gasket, invert air horn and allow float to hang by its own weight. Measure clearance between top of float and air horn. (Scheme 1)
2. Clearance should be.275" (7.0 mm). Adjust clearance by bending float arm at tab "A". (Scheme 2)

Float Level Measurement. Scheme 1

Float Level Adjustment Adjust gap by bending float arm at tab "A". Scheme 2

FLOAT DROP

1. Invert air horn, gently lift up float and measure clearance between needle valve and float arm. (Scheme 3) Clearance should be.066-.078" (1.67-1.99 mm).
2. Adjust clearance by bending outer float tab "B". (Scheme 4) Install needle valve clip after completing float adjustments.

Float Drop Measurement. Scheme 3

Float Drop Adjustment Bend outer tab "B" to adjust. Scheme 4

PRIMARY & SECONDARY THROTTLE VALVES

1. Using a "T" scale, check full opening angle of primary throttle valve. Standard angle is 90° from horizontal plane. Adjust by bending first throttle lever stopper.
2. Check full opening clearance between secondary throttle valve and body. Clearance should be .500" (12.7 mm). Adjust by bending secondary throttle lever stopper.

Bench Adjustment

1. Position throttle shaft lever to high (first) step of fast idle cam. Close choke valve and measure primary throttle valve clearance.
2. Clearance should be .045" (1.1 mm). Adjust by turning fast idle adjustment screw.

CHOKE UNLOADER

Fully open primary throttle valve, check choke valve clearance. Clearance should be.120" (3 mm). Bend fast idle lever to adjust. (Scheme 5)

Choke Unloader Adjustment Bend fast idle lever to adjust. Scheme 5

CHOKE OPENER

1. Set fast idle cam, open throttle slightly and gently push choke valve closed. Keeping choke valve closed, release throttle valve.
2. Apply vacuum to choke opener diaphragm and measure choke valve angle. Angle should be 77°. Adjust by bending tang on relief lever connecting choke opener link to choke valve shaft.

CHOKE BREAKER

1. Fully close valve. Apply vacuum to choke breaker inner diaphragm "A". Measure choke valve clearance. Clearance should be.095" (2.4 mm). (Scheme 6)
2. Adjust by bending tang on relief lever, connecting breaker diaphragm link to choke valve shaft.
3. Apply vacuum to both choke breaker diaphragms "A" and "B". Measure choke valve clearance. Clearance should be.254" (6.2 mm). Adjust by turning diaphragm adjustment screw.

SECONDARY KICK-UP

Fully open primary throttle valve. Measure clearance between secondary throttle valve and body. Clearance should be.0043-.0087" (.11-.22 mm). Adjust by bending secondary throttle lever. (Scheme 7)

SECONDARY TOUCH CLEARANCE

Check primary throttle valve opening clearance as the primary kick lever touches the secondary kick lever. Clearance should be.170" (4.3 mm). Adjust by bending primary kick lever. (Scheme 8)

Choke Breaker Adjustment. Scheme 6

Secondary Kick-Up Adjustment. Scheme 7

Secondary Touch Clearance Adjustment. Scheme 8

ACCELERATOR PUMP STROKE

With choke valve fully opened, check length of stroke. Standard stroke should be.079" (2.0 mm). (Scheme 9)

Air Horn

1. Disconnect choke link. Disconnect pump connecting rod. Remove union and fuel pipe. Remove 8 air horn screws. Disconnect choke link.
2. Lift air horn with gasket from body. Disconnect wires from connector. Remove 1st and 2nd solenoid valves from body.

Float & Needle Valve

Remove float pivot pin, float and needle valve subassembly. Remove air horn gasket. Remove needle valve seat and gasket. (Scheme 9)

Needle Valve Removal. Scheme 9

Power Piston & Pump Plunger

Remove power piston retainer, power piston and spring. Pull out pump plunger and remove boot.

Scheme 10

Scheme 11

1. Disconnect throttle positioner link. Remove 2 bolts. Remove stopper gasket, pump discharge weight, long spring and large discharge ball. Using tweezers, remove plunger retainer and small ball. (Scheme 10) (Scheme 10): Throttle Positioner
2. Remove slow jet. Remove power valve with jet. Disassemble power valve and jet. Remove "TP" levers. Remove primary main passage plug, primary main jet and gasket.
3. Remove secondary main passage plug, secondary main jet and gasket. Remove AAP housing, spring and diaphragm. Remove AAP inlet plug and small ball. Remove outlet plug, short spring and small ball. (Scheme 11) (Scheme 11): Removing AAP Check Ball Plugs
4. Remove primary and secondary small venturies. Remove sight glass retainer, sight glass and "O" ring. Remove throttle lever and fast idle cam subassembly.
5. Remove throttle return spring, throttle back spring, throttle lever and fast idle cam. Remove secondary throttle valve diaphragm assembly and gasket. Remove 3 mounting bolts and vacuum passage bolt.

Cleaning

Clean disassembled parts before inspection. Wash and clean cast parts with a soft brush in carburetor cleaner. Clean off carbon around throttle valve. Wash other parts thoroughly in carburetor cleaner. Blow all dirt and other foreign matter from jets, fuel passages and restrictions in the body.

Inspection

1. Inspect float and needle valve. Inspect pivot pin for scratches and excessive wear. Inspect float for broken lips and wear in pivot pin holes. Inspect spring for breaks and deformation.
2. Inspect needle valve and plunger for wear or damage. Inspect strainer for rust and breaks. Check power piston for freedom of movement. Inspect power valve for faulty opening and closing action.

Exploded View of Air Horn Assembly. Scheme 12

Exploded View of Carburetor Main Body. Scheme 13

REASSEMBLY

Note. Use new gaskets, seals and "O" rings when reassembling carburetor.

1. To reassemble, reverse disassembly procedure. Place gasket and body onto flange. Install vacuum passage bolt. Install 3 bolts. Install secondary throttle valve diaphragm.
2. Assemble secondary throttle valve diaphragm. Position gasket and install diaphragm assembly. Connect link. Install fast idle cam and throttle lever.
3. Install "O" ring sight glass and sight retainer. Install primary and secondary small venturies. Install auxiliary acceleration pump. Install outlet plug, short spring and small ball.
4. Install AAP housing, spring and diaphragm. Install primary main jet and passage plug with new gasket. Install secondary main jet and passage plug with new gasket.
5. Assemble power valve and jet. Install power valve unit. Install discharge large ball, long spring, pump discharge weight and stopper gasket. Using tweezer's, insert plunger small ball and retainer. Install throttle positioner.

1. Install valve seat. Then install needle valve, spring and plunger onto seat. Adjust float level. See FLOAT LEVEL adjustment in this article. Install power piston spring and piston into bore.
2. Install retainer. Install acceleration pump plunger and boot. Install air horn gasket onto air horn. Install needle valve assembly, float and pivot pin. Insert float lip between plunger and clip when installing float.
3. Assemble air horn and body. Install solenoid valve with new gasket into carburetor body. Install eight mounting screws. Install choke, solenoid wire clamp, fuel inlet bracket and number plate. Attach pump connecting link and vacuum hose.

SPECIFICATIONS

NOVA CARBURETOR ADJUSTMENT SPECIFICATIONS