Rochester B, BC, and BV accelerator pump circuit technical information, rebuilding help, and parts guide.
HOW IT WORKS
Extra fuel for smooth, quick acceleration is supplied by a double spring pump plunger. Rapid opening of the throttle valve, as in the case where accelerating from low speed, causes an immediate increase in air velocity in the carburetor venturi. Since fuel is heavier than air, it requires a short period of time to "catch up" with the air flow. To avoid a leanness during this momentary lag, the accelerator pump furnishes a quantity of liquid fuel sprayed intot he air stream to mix with the incoming air and maintain the proper air/fuel mixture.
The pump is operated by the combined action of two springs which are calibrated to move the plunger in such a manner that a sustained charge of fuel is delivered for smooth acceleration. The pump is attached by linkage to the throttle lever. When the throttle valve is closed, the pump plunger moves upward in its cylinder allowing fuel to flow through a slot in the side of the pump well into the pump well, through the vapor check ball passage, bypassing the plunger head, and on into the bottom of the pump well. The pump discharge ball is seated at this time to prevent fuel and air from draining into the pump well from the pump discharge passage.
When the pump plunger is moved downward for acceleration, the force of the stroke seats the vapor check ball int he pump plunger head to prevent fuel flow back into the float bowl. Downward motion of the pump plunger forces fuel up through the discharge passage and lifts the pump discharge check ball from its seat and then passes on through the pump jets into the venturi area where it strikes the side of the boost venturi atomizing the fuel with the air and is delivered to the engine.
2 3/4"
2 13/16"
YEARS
1950-51
1952-1962
1957-up
1963-1968
TYPE OF PUMP
slotted stem, leather
slotted stem, leather
round stem, rubber
round stem, rubber
FLOAT BOWL SCREWS
10-32
12-28
12-28
12-28
TYPES OF PUMPS
The Rochester 1 barrel carburetor used 2 styles of accelerator pumps through the years. One has a flat shaft and the other has a round shaft.The flat shaft is easy. The same accelerator pump fits all of the early 1 barrel carburetors. Cars to 1956. Trucks to 1962. These were originally a leather cup type (which is ethanol ready) and the current accelerator pumps of this type are still made with leather cups. The leather cup is not replaceable nor is it interchangable with a rubber cup.
The discharge ball is 90-19
Illustrates the different in return & delayer springs on the 2 types of Rochester B carburetors.
A - Return spring on all Rochester B carburetors. The return spring goes under the accelerator pump. This helps return the accelerator pump to the top position. 66-15
B - Delayer spring on the round accelerator pump stem (late) type of carburetor. The delayer spring goes over the pump stem. A retainer then goes over the stem. 66-44. The delayer spring helps the accelerator pump move in a smoother manner rather that a quick jerk up, or down.
C - Delayer spring on the flat accelerator pump stem (early) type of carburetor. The delayer spring goes over the pump stem. A retainer then goes over the stem. 67-3
The later model Rochester B, 1 barrel used a round shaft type of accelerator pump. In fact two different sized round shaft accelerator pump. When we refer to the size of this pump we are talking about the overall length. AP327: 2.851 and AP462: 2.776.
Our rebuild kit that cover round shaft accelerator pumps include a replacement rubber cup, garter spring, and application tool.
Some of the Rochester B's with the round shaft had a leather cup. This cup cannot be replaced with the rubber cup. In these rare cases you will need to purchase the complete accelerator pump using one of the part numbers illustrated above.
Do you have a pump that looks like this? If so, then use the K1141 rebuild kitplus the 90-36 retainer. These don't come up very often, but when it does you will know what to do.
TECHNICAL AND REBUILD INFORMATION
CHECK BALLS
Pump inlet checkball, 90-20, goes at the bottom of the accelerato pump well on carbs that require it
SPRINGS
Illustrates the different in return & delayer springs on the 2 types of Rochester B carburetors.
A - Return spring on all Rochester B carburetors. The return spring goes under the accelerator pump. This helps return the accelerator pump to the top position. 66-15
B - Delayer spring on the round accelerator pump stem (late) type of carburetor. The delayer spring goes over the pump stem. A retainer then goes over the stem. 66-44. The delayer spring helps the accelerator pump move in a smoother manner rather that a quick jerk up, or down.
C - Delayer spring on the flat accelerator pump stem (early) type of carburetor. The delayer spring goes over the pump stem. A retainer then goes over the stem. 67-3
INSTALLATION TIPS
Polish the pump well with crocus cloth, or polishing cloth. You want the pump to have a smooth glide. Apply a few drops of oil to the cup before installing. That along with the gas will cause the cup to swell up. This will take a few miles of driving. Once the cup is wet doen't let it dry out as that will ruin the pump.
TESTING THE ROCHESTER B ACCELERATOR PUMP
INLET CHECK BALL
The aluminum ball sits at the bottom of the well, at least in the models that use it. When the pump is pushed down, the pressure of the pump holds the ball down and forces the fuel out the exhaust hole into the main discharge. There cannot be anything holding the ball down, or it wouldn't allow the fuel to fill when the pump is rising. If the fuel is feeding back into the well, then the check ball isn't sealing. It should be the aluminum ball. Tap the ball lightly with a brass punch to seat it. If by chance the ball is coming out of the hole completely, then look at the return spring to see if it is keeping the ball from coming out. You may have to spread the bottom of the spring out to fit better.
Some of the B's have the intake ball on the bottom of the float bowl next to the well. A screen fits over the ball. Again it cannot be held down.