rochester-b-manual.pdf - mike's carburetor parts

ROCHESTER CARBURETOR SERVICE MANUAL WWW.CARBURETOR-PARTS.COM

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AG

EN

DA

MODEL BC CARBURETOR

DISASSEMBLY PROCEDURE

CLEANING AND INSPECTION

ASSEMBLY PROCEDURE

HEAT SUCTION TUBE INSTALLATION

MODEL BC ADJUSTMENTS

MODEL B CARBURETOR


CLEANING INSPECTION REPAIRS

ASSEMBLY

THROTTLE RETURN CHECK

ADJUSTMENTS


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Model BC ROCHESTER CARBURETOR


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The Rochester Model BC carburetor was introduced in 1951. It is basically a conventional Model B

carburetor with the addition of an automatic choke.

Most of the metering parts are contained in the die-cast air horn assembly and can be serviced by simply

removing the air horn screws and lifting the air horn assembly from the float bowl. This carburetor

incorporates the conventional six systems of carburetion: Idle, Part Throttle, Power, Pump, Float, and

Choke.

This carburetor has been kept basically simple in design without sacrificing the distinctive features of all

Rochester carburetors—e.g., concentric float bowl suspended main metering jets, vacuum-operated power

system, centrally located main discharge nozzle, and a minimum of adjustments and moving parts.

IDLE SYSTEM

The Idle System controls and delivers the proper fuel-air mixture for idling and up to approximately 30 MPH.

As shown in Figure 2-1, the idle fuel first passes from the bowl through the calibrated Main Metering Jet attached to the bottom of the Main Well assembly. This fuel is then drawn up the Main Well by manifold vacuum to the crossbar of air horn. Air joins the fuel through the calibrated air bleeds in the center of the crossbar. This fuel-air mixture is then calibrated as it passes through the idle restriction and is drawn down the passage in the Float Bowl to the Throttle Body.

The idle fuel is then metered to the engine by the idle adjusting needle hole which is below the throttle valve. As the throttle valve is opened to a greater degree, the idle hole which was above the closed throttle valve is exposed to manifold vacuum and delivers additional fuel to meet the increased engine demand.


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Figure 2-1

PART THROTTLE SYSTEM

As the throttle valve is opened further (as shown in Figure 2-2), air at a higher velocity is drawn down the carburetor throat. This creates a pressure drop or suction in the venturi at the main discharge nozzle in the crossbar. As a consequence, fuel and air begin to pass from the main discharge nozzle to meet the increased engine demand. Further throttle opening will result in greater air velocity passing through the carburetor with resultant higher fuel flow from the nozzle and decreased flow from the idle system until it eventually cuts out altogether.


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The calibration of the Main Metering Jet and Air Bleeds in the crossbar maintain economical fuel-air ratios throughout the part throttle or cruising range.


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Figure 2-2

Power System

A vacuum-operated Power System (Figure 2-3) provides additional fuel for sustained high-speed

operation or for increased road load power.

A direct manifold vacuum passage within the carburetor to the engine intake manifold operates this

system. At any manifold vacuum above approximately 5" Hg., the power actuating piston is held in the


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"UP" position. against the compression of the power spring, by manifold vacuum. Thus, no fuel passes

through the ball type power valve.

In accordance with the principle that any sudden acceleration causes a drop in intake manifold vacuum,

the power spring has been calibrated so that at any vacuum below approximately 5" Hg., it forces the

power actuating piston "DOWN."

The end of this piston then unseats the spring-loaded ball in the power valve. The fuel passes readily

around the ball into the base of the Main Well Support. The calibrated power restriction meters the fuel

prior to it joining the fuel from the main metering jet. Conversely, as the manifold vacuum rises above

approximately 5" Hg., the power piston is drawn immediately to the "UP" position, and the spring-loaded

ball of the power valve closes, returning the carburetor to the economical part throttle mixtures.

Figure 2-3

The relief passage which is drilled from the bore of the air horn into the power piston passage serves to

relieve any vacuum built up around the piston diameter. This vacuum, if unrelieved, would draw fuel past


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the piston and down the vacuum passage into the manifold, resulting in an overly rich condition. The

relief passage which is drilled from the bore of the air horn into the power piston passage serves to relieve

any vacuum built up around the piston diameter. This vacuum, if unrelieved, would draw fuel past the

piston and down the vacuum passage into the manifold, resulting in an overly rich condition.

Figure 2-4

FLOAT SYSTEM

The Model "BC" carburetor employs the conventional float needle and seat to control the fuel level in the

float bowl. In accordance with the concentric float bowl design, dual floats are used to maintain the fuel

level (Figure 2-4).

The inlet screen as illustrated can also be a 1” bronze filter.


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This circuit is extremely important in that it controls the level of fuel in the float bowl. If this level

becomes too high or too low as a result of the improper adjustment, dirt, etc., the result can be poor

performance, poor mileage, or both.

On some applications, it is advisable to link the float and float needle with a pull clip, which serves to

retract the needle from its seat if gum deposits cause sticking.

PUMP SYSTEM

To provide fuel for smooth, quick acceleration, a double spring pump plunger is used in the Model "BC"

carburetor (as shown in Figure 2-5). The rate of compression of the top spring versus the bottom spring is

calibrated to ensure a smooth, sustained charge of fuel for acceleration.

To exclude dirt, all fuel for the pump system first passes through the pump screen in the bottom of the

float bowl. It is then drawn past the ball check into the pump well on the intake stroke of the plunger.

Upon acceleration, the force of the pump plunger seats the ball check and forces fuel up the discharge

passage. The pressure of the fuel lifts the pump outlet ball check from its seat. The fuel is then sprayed on

the edge of the venturi by the pump jet and delivered to the engine. The pump jet is targeted at the factory

and no adjustment is required.

CHOKE SYSTEM

To ensure proper starting and driving during cold weather operation, the Model "BC" carburetor employs

a fully automatic choke. (As shown in Figure 2-6.) This choke system is composed of a thermostatic coil,

choke piston, offset choke valve, and fast idle cam and linkage. It is controlled by a combination of intake

manifold vacuum, airflow past the offset choke valve, atmospheric temperature, and exhaust manifold

heat.

The bimetallic thermostatic coil is made up of two metals with different coefficients of expansion. This

coil is mounted firmly on one end to the stat cover and is linked on the other end to the choke valve shaft.

The coil is so calibrated as to hold the choke valve in the closed position when the engine is cold.

As the engine is started, air velocity against the offset choke valve causes the valve to open slightly

against the torque of the thermostatic up the passage in the plunger head and return to the float bowl.


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Without this feature, any vapor pressure built up would evacuate the fuel in the pump system into the

engine manifold, causing poor initial acceleration due to lack of fuel in the pump system as well as

difficult hot weather starting coil.

Figure 2-6

In addition, as the engine starts, intake manifold vacuum is applied to the choke piston, which also tends

to pull the choke valve open. As a consequence, the choke valve assumes a position where the torque of

the thermostatic coil is balanced against the vacuum pull upon the choke piston and air velocity against

the offset choke valve, thereby causing a regulated airflow into the carburetor which provides a proper

mixture during the warm-up period.

During warm-up, the choke piston serves to modify the choking action to compensate for varying engine

loads or acceleration. Any acceleration or increased road load decreases the vacuum exerted on the choke

piston. This allows the thermostatic coil torque to momentarily increase choke valve closure to provide

the engine with a sufficiently richer mixture for acceleration.

As the engine warms up, hot air from the exhaust manifold "stove" is drawn into the thermo-static coil

cover by the vacuum behind the choke piston. This hot air causes a rise in temperature which causes the


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coil to slowly relax its tension. Thus, the choke valve is allowed to move gradually to the fully open

position.

To prevent stalling during the warm-up period, it is necessary to run the engine at an idle speed slightly

higher than that for a warm engine. This is accomplished by the fast-idle cam which is linked to the choke

valve shaft and holds the throttle valve open sufficiently during the warm-up period to give the increased

idle RPM, until such time as the choke valve moves to the fully open position.

While the automatic choke is in operation, the driver may wish to advance the throttle to the full-wide

open position. Since this would decrease vacuum pull on the choke piston, thereby closing the choke

valve, it is necessary to provide increased carburetor airflow by opening the choke valve mechanically.

To accomplish this, a tang on the throttle lever is made to contact the fast-idle cam linkage at wide-open

throttle position so as to partially open the choke valve.

This will also relieve excess choking on starting by allowing more air to enter the carburetor when the

engine is cranked with the accelerator held fully depressed.

DISASSEMBLY PROCEDURE Take plenty of digital pictures as you go along. It will help you determine how parts fit. (assuming they were correct to start with).

CHOKE DISASSEMBLY

1. Loosen the 1/2" brass fitting on choke suction tube.

2. Remove the three attaching stat cover screws and retainers, then the stat cover, cover gasket, and

thermostat coil assembly from the carburetor.

3. Remove the choke baffle plate. These are often missing and not available. While it would be nice

for it to be there, the choke system will still operate without it.

4. Remove the pin spring and rod end clip from their respective ends of the choke rod and remove

the rod.

5. Remove the retaining screw at end of choke shaft and carefully pry off choke trip lever, spacing

washer, and choke counterweight. (Figure 2-7.)

6. Remove the two choke valve screws and then remove the choke valve.


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7. Rotate the choke shaft to free choke piston from housing and then remove the piston and choke

shaft from the carburetor.

8. Remove the choke piston pin and piston from choke shaft.

9. Remove the two-choke housing attaching screws. The choke housing and gasket may now be

removed from the air horn.

Figure 2-7

AIR HORN DISASSEMBLY

1. Remove filter screen retainer nut and gasket with 3/4" wrench and remove filter screen.

2. Remove air horn screws and lift air horns from the bowl.


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3. Place air horn upended on a flat surface. Re-move float hinge pin and lift float assembly from an

air horn.

4. Remove float needle

5. Remove float seat and gasket with 1/2" bit screwdriver.

6. Remove the main metering jet from the main well support.

7. Remove main well support. Air horn gasket may now be removed.

8. Remove power piston and spring.

NOTE: Do not remove the primary idle tube from the air horn.

BOWL DISASSEMBLY

1. Remove the slotted brass pump discharge guide by using a small bit screwdriver to compress top

portions of the guide slightly as shown in Figure 2-8. Spring will cause the guide to pop out.

'Remove pump discharge guide spring and ball. (Note: If the guide is stamped type, remove from

slots in the bowl with a long-nosed pair of pliers.)

2. Remove two-pin springs from the pump link and remove the pump link from the throttle lever

and pump plunger arm.

3. Remove pump plunger from the bowl. Remove pump return spring and inlet ball from the pump

well.

4. Remove pump inlet screen from the bowl.

NOTE: Do not remove choke suction tube from the throttle body.

5. Upend carburetor bowl with suction tube projected over the edge of the flat surface and remove

two throttle body attaching screws. Remove throttle body and gasket.


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Figure 2-8

THROTTLE BODY DISASSEMBLY

1. Remove idle adjusting needle and spring.

2. Remove the idle screw from the throttle lever.

3. Remove fast idle cam.


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NOTE: Due to the close tolerance fit of the throttle valve in the bore of the throttle body, it is not

necessary to remove the throttle valve or shaft from the throttle body.

CLEANING AND INSPECTION

1. Thoroughly clean carburetor castings and metal parts in carburetor cleaning solvent.

CAUTION: Choke coil and housing and pump plunger should not be immersed in the solvent.

Blow all passages in castings dry with compressed air and blow off all parts until they are dry.

CAUTION: Do not pass drills or wires through calibrated jets or passages as this may enlarge

orifice and seriously affect carburetor calibration.

2. Check all parts for wear. Defective parts must be replaced. Note the following:

(a) Check float needle and seat for wear. If worn, the assembly must be replaced.

(b) Check the floating button for wear and float for dents. Check floats for leaks by shaking.

If the vehicle has been sitting for months then any gas inside the float may have

evaporated. A better test is to heat up a pan of water just prior to boiling and immerse the

float. Any bubbles indicates a leak.

(c) Check throttle and choke shaft bores in the throttle body and air horn castings for wear or

out of round.

(d) Inspect idle adjusting needle for burrs or ridges. If necessary, replace it.

(e) If steps of fast idle cam show wear, the cam should be replaced as it may upset engine

idle speed during the choking period.

(f) Inspect pump plunger leather. Replace the plunger if the leather is damaged.

3. Inspect gaskets to see if they appear hard or brittle, or if the edges are torn or distorted. If any

such condition is noted, replace it.

4. Check to see that the lower end of the suction tube is tight in the seal in the throttle body. If not, a

new seal will have to be installed after the carburetor is assembled or poor choke operation will

result from a vacuum leak.

5. Inspect suction tube hex nut packing. If it appears unduly compressed or out of round, it should

be replaced.

6. Check all filter screens for dirt or lint. Clean, and if distorted or still plugged, replace.


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7. The Rochester 1 barrel is prone to warpage. Check the float bowl for warpage with a flat metal

stock. If warped and you have the means to build a jig, the body can be straightened with heat,

otherwise as a last resort you can try double float bowl gaskets. Do not use gasket

ASSEMBLY PROCEDURE

1. Install idle stop screw in throttle lever.

2. Screw idle adjusting needle and spring into throttle body until it is finger tight. Back out screw 1

1/2 turns as a temporary idle adjustment. Make final adjustments on the engine when warmed up.

3. Install fast idle cam with letters RP facing outward.

4. With bowl upended and suction tube projected over the edge of the flat surface, place new throttle

body gasket into position and attach throttle body. Tighten screws evenly and securely.

Note: New choke suction tube seal, if needed, will be installed after carburetor is completely

assembled.

BOWL ASSEMBLY

1. Drop the small aluminum ball into the pump well hole, and replace pump return spring. Press

spring with finger to center it in the pump well. Some B’s have the intake ball located in the

bottom of the float bowl next to the pump well. There should be a screen covering the check ball.

2. Install pump plunger assembly and attach pump link to plunger arm and throttle lever. Attach pin

springs to each end of the pump link.

3. Install the larger steel ball in the pump discharge cavity. (Figure 2-9) Place spring and spring

guide atop the ball. (Figure 2-10) Spread spring guide slot slightly with a screwdriver blade to

keep in place. (Figure 2-11.)

NOTE If a stamped type guide is used, install a new guide into bowl slots until it is a press fit.

4. Press pump filter screen into position.


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Figure 2-9

Figure 2-9


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Figure 2-10

AIR HORN ASSEMBLY

1. Install float needle seat and gasket, using a screwdriver with 1/'2" bit.

2. Place new gasket atop air horns, check to be sure that all air horns and gasket holes are properly

aligned.

3. Place power piston spring and power piston into the vacuum cavity. The piston should ride free in

the cavity. (Figure 2-12.)

Figure 2-12


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4. Attach main well support to air horn securely. (Figure 2-13.)

Figure 2-13

5. Install main metering jet in main well support assembly.

6. Assemble float needle to float and place float carefully into position. Tang at the rear of the float

must face cover. (Figure 2-14.) Install float hinge pin.


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Figure 2-14

NOTE: The float adjustment may be made at this point. For correct settings, see Page 28. DO

NOT put pressure on the needle when adjusting the float.

7. Attach air horn to bowl with attaching screws. Tighten screws evenly and securely.

8. Install filter screen, strainer, nut, and gasket assembly in the air horn.

9. Place new gasket into position and attach choke housing to the air horn. Tighten screws securely.

(Figure 2-15.)


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Figure 2-15

10. Assemble choke piston to the shaft with pin and place into choke housing bore as shown in

Figure 2-16. Rotate choke shaft so that piston rides in the housing cavity.


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Figure 2-16

11. Install choke valve on choke shaft with letters "RP" facing upward. Center choke valve before

tightening screws. Place baffle plate and choke cover gasket into position as shown in Figure 2-1

7 and install choke coil and cover. Rotate cover clockwise until index marks on cover and

housing are aligned.


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Figure 2-17

12. Attach the three retainers and screws to choke housing and tighten securely.

13. Place the choke counterweight on the end of the choke shaft with the tang facing inward toward

the choke housing. Install the spacing washer and trip lever so that the tang of trip lever is on top

of counterweight tang when the choke valve is in the fully open position as shown in Figure 2-18.

14. Install choke rod to counterweight and fast idle cam. The dog-leg of rod must face idle adjusting

needle. Assemble pin spring and rod end clip to respective ends of the rod.

NOTE: Check choke valve for free movement.


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Figure 2-18

HEAT SUCTION TUBE INSTALLATION

1. If the choke suction tube and seal assembly in the Throttle Body requires replacement, proceed as

follows: (Figure 2-19.) This operation must be performed after carburetor is completely

assembled.


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Figure 2-19

(a) Loosen Both Screws Attaching Throttle Body to Bowl.

(b) Place the flared end of the tube with the seal into the throttle body. Using Tool BT-45,

tap lightly to hold the seal in Throttle Body. Rotate tube while tapping seal so that it is

started uniformly in Throttle Body.

(c) Install hex fitting and a bit of Teflon liquid on the upper end of the tube and fasten tube to

choke housing by turning hex nut up finger tight.

(d) Using tool BT-4 5 and hammer, spread seal into throttle body securely.

(e) Completely loosen hex nut and check that tube is tight in throttle body (will not turn).

Then tighten hex nut to choke housing securely.

(f) Tighten throttle body to bowl attaching screws evenly and securely.

(g) After installing carburetor on car, run idling to warm up the engine; check packing joint

with carb cleaner. If there is a leak. the engine will stall or roll.

MODEL BC ADJUSTMENTS

FLOAT ADJUSTMENT


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Two separate float adjustments must be made:

1. With the air horn completely assembled, place the float gauge in position as shown in Figure 2-

20.

2. Adjust the floats by bending the floating button so that the top of the floats just touches the float

gauge between the gauge legs. (Figure 2-20.) Most likely you won’t have this gauge so measure

with a flat metal ruler. Look for specifications below.

Figure 2-20

3. To ensure ample entry of fuel under high-speed operation, hold the air horn right side up and

place the float gauge in position as shown in Figure 2-21.


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Figure 2-21

4. Bend the tang at the rear of the float until the gauge just slides between the air horn gasket and the

floats. (Figure 2-21.) In most instances, a scale dimension is specified on the Adjustment Bulletin

for making this adjustment.

5. Bend the float arms to center the floats between the legs of the gauge. (Figure 2-21.)

NOTE: Re-check the float level to be sure that it has not changed.

DO NOT adjust by bending the pontoons themselves. They can break at the solder point.

CHOKE ROD ADJUSTMENT

1. With the choke cover properly indexed, turn the idle screw in until it contacts the second step of

the fast-idle cam shown in Figure 2-22.


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Figure 2-22

2. Holding the screw tightly against the shoulder of the high step of the cam, bend the choke rod at

the dogleg to obtain the specified clearance between the lower edge of the choke valve and the

bore of the carburetor air horn.

UNLOADER ADJUSTMENT


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Figure 2-23

As shown in Figure 2-23, move the throttle lever to the fully open position. With the choke trip

lever in contact with the choke counterweight, bend the tang on the throttle lever to obtain

specified clearance between the lower edge of the choke valve and bore of the carburetor air horn.

NOTE: No other carburetor adjustments are required. The correct positioning of the accelerator

pump plunger is assured by the proper bending of the pump link at the factory. No change in this

setting should be made. No aiming of the accelerating pump jet is required.


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The Rochester Model B carburetor was introduced in 1950. A detailed description of the operation of its

basic systems has been omitted in this portion of the manual since the basic operating principles are

similar to those discussed under the Model BC carburetor on pages 20-22. This carburetor employs a

manually operated choke system rather than the automatic type, so the portion of the Model BC

discussion on the Automatic Choke will not apply to Model B.

In line with all other Rochester designed carburetors, this model has been kept basically simple and its

circuits are easily traceable and readily understood. The number of moving parts and adjustments have

Model B ROCHESTER CARBURETOR


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been kept to a minimum for ease of service. The following pages have been devoted to a detailed picture

story of the complete disassembly, inspection, and assembly of the Rochester Model B carburetor.


1. Remove the four air horn screws. Then

hold the fast-idle Lever out of. the way

and remove the air horn assembly by lifting straight up.

2. Remove the air horn assembly as

described perilously and remove the float assembly from the air

horn.

3. In most cases., the air horn gasket

should be replaced, particularly if of the

early cork type. Remove the float

hinge pin and lift off the twin float

assembly.

4. Remove the main metering jet and power

valve assembly (consisting of retaining strew, spring and ball)

from the main well support.

5. Remove the main well support. Then lift out the power piston and spring.

6. Lift out the needle valve and unscrew the needle seat from the

air horn.

7. Remove the two retaining screws and

lift out the choke valve. Unhook the

spring from the choke shaft and

remove the shaft.

8. Remove the retainer, fast-idle cam, and

spring.


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9. Remove the retaining screw, flat

washer, fast-idle lever, and spring from the bowl.

10. Rotate the pump discharge guide. with a thin-bladed screwdriver

until it is free and is pushed up by spring

pressure. Remove the guide, spring, and ball.

11. Push the accelerating pump plunger all the way down. Remove the

hairpin retainer, from the pump link and remove the link.

12. Remove the pump plunger

assembly and lower spring.

13. If plunger parts are faulty, compress

the tarp spring, unhook the arm and replace the plunger

(which is serviced as an assembly).

14. Turn the bowl upside down. The

accelerating pump inlet ball check will fail from the bottom of the pump well. If not, apply heat to the outside of where

the ball resides.

15. Remove the pump screen retainer and

screen from the bottom of the bowl,

using a hooked tool or piece of wire as

shown

16. Invert the bowl and remove the two-

throttle body retaining screws.

Remove the. throttle body from the bowl.


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CLEANING INSPECTION REPAIRS

17. Remove the idle mixture adjusting

needle and spring from the throttle body. Also, remove the distributor

vacuum fitting.

18. Wash all parts thoroughly,

preferably in carburetor cleaning

solvent.

19. Blow out all parts and passages and

make sure they are clean and free from carbon deposits, if

not return them to the solvent bath for further cleaning,

20. Inspect all mating surfaces of the

carburetor. Surfaces must be flat to within a few thousandths for the gasket to provide a

tight seal.

21. Inspect the throttle shaft arm for

looseness on the shaft and for wear at the throttle rod hole. Check the throttle

shaft for looseness in the body and the valve

for looseness or binding. 1/32” or less.


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22. If throttle assembly is faulty, the entire throttle

body assembly must be replaced because the idle discharge

and distributor vacuum ports are

drilled in relation to the throttle valve.

24. Inspect the floats for signs of leaks and

the hinge holes for wear. Also, check the hinge pin holes in the

air horn for wear.

25. Inspect the pump plunger leather and springs for damage.

Also, shake the plunger to make sure the by-

pass ball check is free.

26. Air leaks and rough idling may

also result in horn a loose, or improperly

seating power piston'. This

possibility can easily be checked as

follows-

23. Inspect the float needle for wear and Indications improper seating. If damaged, both needle and seat must be replaced as they are serviced as

an assembly. Always replace the needle &

seat.

28. Remove the idle

mixture adjusting screw and check the needle for ridges. A

ridged needle supplies erratic mixtures which

cause rough idling.

27. Check the fit of the power piston in the cover. It must be

free but without excess play, also

inspect the tapered seat on the upper and

of the piston.


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ASSEMBLY

30. Install the distributor vacuum fitting. Install the

idle mixture adjusting screw and spring and position at 1 and 1/2 turns

open. NOTE - Care must be used not to

tighten adjusting screw too tightly or

needle will be grooved.

31. Place the pump screen in its well

and secure with the retainer. Use a

piece of 1/8" drill rod, tapered slightly in the end, to push

the retainer into place.

32. There are three-ball checks — large steel over a smaller

steel one and an aluminum one the same size as the

smaller steel ball. Place the aluminum

bail in the pump well.

33. Roll the bowl back and forth to

make sure the ball is riot sticking in its

seat.

34. Place the lower Plunger spring in the pump well. Then install the

accelerating pump plunger assembly.

Apply 2 drops of oil to the leather cup

(when it is leather) before installing.

29. Place a new gasket on the bowl

and install the throttle body so the

two passage openings line up

with the holes in the gasket. Install the

two retaining screws and tighten them

securely. Some gaskets will cover the hole on the

bottom. That is OK.


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39. Inspect time choice for free operation end normal spring tension

when the valve is pushed open from the

closed position.

40. Install the needle valve seat and gasket and

tighten it securely. Place the needle valve in the seat.

41. Install the spring and power piston in

the air horn.

35. Push the plunger all the way down and install the link and hairpin retainers.

36. Place the larger of the two steel balls

in the pump discharge - opening and install the spring aid discharge guide. Using a close fitting screwdriver, gently spread the head of the discharge guide to lock it in place, or The kit includes a T. Scratch grooves in

the bowl for the T to fit in and use the T.

37. Install the fast-idle lever spring as shown. Then install the lever with the

offset away from the bowl and secure with the flat washer and

screw.

38. To install Ile parts, reverse the order of removal. The letters "RP" or "C" on the

choice valve must be to the top,


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43. NOTE --- There are two power piston springs. That used on 7001374 and 7002050 carburetors are 2 and 3/16" long. That used

on 7002051 and 7003060 carburetors are 2744" . We have

one spring that works for both.

44. Install a new air horn gasket and the main well support.

Tighten the retaining screw securely.

45. NOTE - The gasket for the

carburetor used on late Power glide jobs

is not interchangeable with the gasket used on

the other three carburetors. It is

identified by a notch as shown.

46. Hold the power piston down and

install the power valve assembly, which is

made up of the smaller of the two steel ball checks,

spring. and retaining screws. Make sure the power piston does not

bind.

47. Install the main metering jet. Jets are identified by the last two digits, of their

part numbers. When installing a new jet, be sure to select the correct one for the carburetor being

worked on.

48. Install the float assembly and check

the high and low float positions and

alignment.

49. Install the air horn on the bowl and

tighten the four retaining screws

evenly and securely.

50. Inspect the insulator on the

manifold flange and replace if damaged.

Make sure the insulator is installed so the vacuum ports

are up.


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THROTTLE RETURN CHECK

51. This concludes the list of service

operations. As you have seen, this new carburetor is simple and easy to service. Let's give it quality

service so it will deliver the top-quality performance which has been designed and built

into it.

52. The throttle return check slows down throttle closing and

thus prevents stalling. When the throttle is

opened and the throttle arm moves

away from the piston adjusting screw, the

spring pushes the piston outward.

53. When the throttle is released and the arm

again contacts the piston screw, the piston travel is retarded by the

spring. Thus, throttle dosing is, slowed down and stalling prevented.

54. Installation of Carburetor and Throttle

Return Check on Engine.

55. Throttle Return Check Adjustment

(POWERGLIDE ONLY)

After making the idle mixture and speed

adjustment, stop the engine and place a .090" feeler between the fast

idle! ever and the choke lever cam. Then hold the

return check shaft and turn the adjusting screw

in or out until it just touches the throttle arm.


P a g e 40 | 43

The Throttle Return Check unit is designed to retard throttle closing, gradually slowing the engine down

from a fast idle to the normal idle speed whenever the throttle is closed.

This assembly is a simple dash-pot mechanism, wherein a diaphragm, assembled to a shaft, is moved

between two covers, forcing air to close a valve and pass through a small bleed hole in the shaft. The

volume of air ahead of the diaphragm, being unable to escape quickly, resists the movement of the shaft

and consequently slows the shaft travel. The valve referred to above seats against one end of the shaft.

The closing of this valve prevents the passage of air through the large hole in the center of the shaft.

When pressure is removed from the shaft contact screw, the return spring tends to force the diaphragm in

the opposite direction. Air entering the shaft cross hole and moving up the large center hole in the shaft

lifts the valve from its seat and allows the diaphragm and shaft assembly to rapidly return to its normal

position.

The Throttle Return Check unit has an adjustable contact screw to provide various conditions of throttle

return control.

56. If necessary, bend the throttle return check bracket or throttle contact arm so the adjusting screw is centered with the

arm.


P a g e 41 | 43

ADJUSTMENTS

FLOAT LEVEL ADJUSTMENT

(New Type. Gauge M-250)

With air horn fully assembled, gasket in position, and assembly upended on a flat surface as shown in

Figure 3-1:

1. Place float level gauge in position with gauge tang inserted in discharge nozzle.

2. Bend float arms vertically so that each float just touches the top portion of the gauge.

3. Carefully bend float arms horizontally so that each float is centered in the gauge.

4. Tilt assembly 90% each side (Figure 3-2) and check that floats do not touch gauge. This ensures

that floats will not rub the inner or outer sides of the float bowl.

FLOAT DROP ADJUSTMENT

As shown in Figure 3-3, to ensure sufficient entry of fuel under high-speed operation, it is necessary to

check and adjust the float drop. With cover held right side up and floats suspended freely, carefully bend

the float tang at the rear of the float assembly so that the bottom of the float is the specified dimension

below the gasket surface. (See Adjustment Bulletin.) Install air horn assembly to float bowl and tighten

screws evenly and securely.

Figure 3-1


P a g e 42 | 43

Figure 3-2


P a g e 43 | 43

Figure 3-3

General Instructions

• Take lots of pictures as you take your carburetor apart. This will give you a reference of where

things go.

• Using a cookie sheet with folded up sides will help keep parts from falling on the floor.

• We suggest not removing the throttle shaft, valves, or choke shaft unless they are corroded, or

very dirty. These parts can be easily damaged and are difficult to re-assemble.

• Instruction sheets that come with our carburetor kits are somewhat generic. It may not match

your parts exactly.

• Do NOT use WD-40 around your carburetor. It reacts with ethanol.

• Using Silicon Spray Lubricant on the gaskets will help with sticking in case you need to take the

carburetor apart again.

• Be careful after taking the top of the carburetor off. Turning the carburetor upside down may

cause parts to fall out and you won’t know where they were.

• Screws and jets that are frozen can often be removed after heating outside the screw or jet.

• Stuck check balls can be removed by heating the outside of where the check ball resides and

tapping the carburetor on the work bench.

• Do not discard any parts until complete done. You may have to refer for size, or matching.

Cleaning:

• Clean with carburetor dis-assembled.

• Soak all parts except rubber & electrical in Simple Green for 2 hours. Aluminum parts will get

discolored if left longer.

• Wash parts with hot water if available to remove all chemicals.

• Blow out each passage way taking special notice of the smaller ones. Test each passage that air

goes through the entire passage.

• Blow out the idle mixture hole.

• Check any hole above the idle mixture hole (inside the bore). This is the idle discharge and often

becomes plugged.

• A tooth brush can facilitate cleaning parts.

• Soda blasting, then washing again will make the carburetor look good any will clean any minor

deposits.

• Any corrosion, or deposits that are hard to remove may indicate the passages are also corroded

and the carburetor should be replaced.

• If your engine has been sitting for 6 months or more, the gas has probably turned, and the gas

tank will need to be cleaned as well as the fuel lines. Flushing new gas through the tank will not

be enough.

Assembly:

• Do NOT apply any gasket sealant on any of the gaskets. Gas will break sealant part and the

particles will clog the small passages.

• Test your float.

o Brass floats should be immersed into hot water. As the air inside expands any leak will

be noticeable with air bubbles.

o Plastic, or Nitrophyl floats should be weighed. The weight is in grams. Check our

technical pages for any weight specification that we may have.

• Most gaskets will fit as expected, but you may have to trim some, especially under the venturis.

• Your kit may include multiple gaskets in order to get better coverage out of the kit. Use the one

that fits the best. Look for any opening the gasket may leave allowing air into the carburetor.

Some holes may be casting holes that don’t lead to anything and do not have to be covered.

• Mounting gaskets for multiple bore carburetors do not have to have matching holes. Example a

four-barrel gasket can be open in the middle instead of 4 holes as long as the carburetor has

some kind of passage between bores. The passage is between primary, or secondary, not both.

• When adjusting the float be careful not to put any pressure on the needle. The viton tip is easily

damaged.

• Most idle mixture screws can be cleaned using a soft wire wheel. Inspect for any scoring, which

would indicate over tightening. Screw with scoring should be replaced.

Accelerator Pumps:

• On leather cups run your finger around the inside of the cup to break any manufacturer sealant.

• Apply 2 drops of oil to cups (leather, or rubber) before inserting into carburetor. Do not soak the

cup in oil. The swelling of the cup needs to happen inside the carburetor. Allow the 2 drops of oil

and the gas to do its job naturally.

• Twist the pump as you are inserting to help keep the cup from curling or folding over.

• Test your accelerator pump circuit before putting the top of the carburetor back on. Our

technical pages have instructions on how to do this for most carburetor types.

• Pump wells are usually slight tapered, and the pump will not seal until it gets towards the

bottom.

General Instructions

• Take lots of pictures as you take your carburetor apart. This will give you a reference of where

things go.

• Using a cookie sheet with folded up sides will help keep parts from falling on the floor.

• We suggest not removing the throttle shaft, valves, or choke shaft unless they are corroded, or

very dirty. These parts can be easily damaged and are difficult to re-assemble.

• Instruction sheets that come with our carburetor kits are somewhat generic. It may not match

your parts exactly.

• Do NOT use WD-40 around your carburetor. It reacts with ethanol.

• Using Silicon Spray Lubricant on the gaskets will help with sticking in case you need to take the

carburetor apart again.

• Be careful after taking the top of the carburetor off. Turning the carburetor upside down may

cause parts to fall out and you won’t know where they were.

• Screws and jets that are frozen can often be removed after heating outside the screw or jet.

• Stuck check balls can be removed by heating the outside of where the check ball resides and

tapping the carburetor on the work bench.

• Do not discard any parts until complete done. You may have to refer for size, or matching.

Cleaning:

• Clean with carburetor dis-assembled.

• Soak all parts except rubber & electrical in Simple Green for 2 hours. Aluminum parts will get

discolored if left longer.

• Wash parts with hot water if available to remove all chemicals.

• Blow out each passage way taking special notice of the smaller ones. Test each passage that air

goes through the entire passage.

• Blow out the idle mixture hole.

• Check any hole above the idle mixture hole (inside the bore). This is the idle discharge and often

becomes plugged.

• A tooth brush can facilitate cleaning parts.

• Soda blasting, then washing again will make the carburetor look good any will clean any minor

deposits.

• Any corrosion, or deposits that are hard to remove may indicate the passages are also corroded

and the carburetor should be replaced.

• If your engine has been sitting for 6 months or more, the gas has probably turned, and the gas

tank will need to be cleaned as well as the fuel lines. Flushing new gas through the tank will not

be enough.

Assembly:

• Do NOT apply any gasket sealant on any of the gaskets. Gas will break sealant part and the

particles will clog the small passages.

• Test your float.

o Brass floats should be immersed into hot water. As the air inside expands any leak will

be noticeable with air bubbles.

o Plastic, or Nitrophyl floats should be weighed. The weight is in grams. Check our

technical pages for any weight specification that we may have.

• Most gaskets will fit as expected, but you may have to trim some, especially under the venturis.

• Your kit may include multiple gaskets in order to get better coverage out of the kit. Use the one

that fits the best. Look for any opening the gasket may leave allowing air into the carburetor.

Some holes may be casting holes that don’t lead to anything and do not have to be covered.

• Mounting gaskets for multiple bore carburetors do not have to have matching holes. Example a

four-barrel gasket can be open in the middle instead of 4 holes as long as the carburetor has

some kind of passage between bores. The passage is between primary, or secondary, not both.

• When adjusting the float be careful not to put any pressure on the needle. The viton tip is easily

damaged.

• Most idle mixture screws can be cleaned using a soft wire wheel. Inspect for any scoring, which

would indicate over tightening. Screw with scoring should be replaced.

Accelerator Pumps:

• On leather cups run your finger around the inside of the cup to break any manufacturer sealant.

• Apply 2 drops of oil to cups (leather, or rubber) before inserting into carburetor. Do not soak the

cup in oil. The swelling of the cup needs to happen inside the carburetor. Allow the 2 drops of oil

and the gas to do its job naturally.

• Twist the pump as you are inserting to help keep the cup from curling or folding over.

• Test your accelerator pump circuit before putting the top of the carburetor back on. Our

technical pages have instructions on how to do this for most carburetor types.

• Pump wells are usually slight tapered, and the pump will not seal until it gets towards the

bottom.

rochester-b-manual.pdf - mike's carburetor parts

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