The amount of fuel that the...
The amount of fuel that the accelerator pump delivers can be adjusted by selecting one of three positions on the pump arm, which alters the pump stroke. The top hole will provide the heaviest shot.
The accelerator pump is a key component of the carburetor in terms of response to sudden throttle application. If the pump's not right, there won't be any burning rubber or snappy response when hammering the "go" pedal. AFB carbs use the familiar displacement-piston accelerator pump, which channels raw fuel to the primary carb barrels. Displacement, as any engine guy can attest, is a combination of bore and stroke. The accelerator pump piston bore is fixed, but the pump has one adjustment, and that is for stroke length. The quantity of fuel available via the accelerator pump can be increased by increasing the pump stroke, and conversely, decreased by lessening the stroke. Changing the position of the linkage rod on the pump arm varies the stroke. The hole closest to the pivot point provides the greatest volume of fuel. It is important to note that the pump-arm position sets the linkage travel, and thereby the stroke. However, if the linkage rod is maladjusted or bent, the piston's actual stroke may not increase at all (if the start height isn't where it needs to be). A quick check can be made by holding the throttle wide open and checking that the piston does not bottom-out before the throttle reaches its open stop. If the piston bottoms prematurely, shorten the pump linkage by bending so full pump travel coincides with full throttle.
We needed a carb to top off...
We needed a carb to top off our Barracuda's near-stock 318, and all we had handy was this 800 cfm Edelbrock carb we've used for engine dyno testing. Although it is ridiculously big for the application, we bench-set the floats and bolted it up for the time being. It didn't perform badly but was rich. A little tuning helped until we could substitute a more appropriately sized carb.
While the pump travel adjusts the volume of fuel available through the accelerator pump circuit, the discharge nozzles affect the pump-shot timing. Large nozzles dump the fuel volume quicker, but since the volume is fixed, a shorter pump duration will result. Conversely, smaller nozzles will deliver the same quantity of fuel but will meter it in more slowly. The required nozzle depends on the combination and is best determined by trial and error. Deep gears and race cars equipped with high-stall-converters generally favor a quick reception of the fuel shot. Discharge nozzles can be drilled, but, for different combo possibilities, assortments of various sized nozzle assemblies (clusters) are available from Edelbrock. The replacement of the pump cluster requires the removal of the air horn to gain access.
We recently installed an 800 cfm Edelbrock carb on our '68 Plymouth Barracuda's tired old 318. We found that the performance seemed good without any tweaking, actually overpowering the tires with a stab of the throttle. Amazingly, with such a preposterously oversized carb, we encountered no drivability problems. The 800 is Edelbrock's largest carb of this style, and our selection criteria certainly deserve some criticism. The 318 is dead stock, except for the duals attached to the factory exhaust manifolds and an antique Edelbrock Streetmaster intake bolted up top. A carb was absent when we purchased this jewel; however, we had the 800 carb handy, and it bolted up and got us down the road. Frankly, we were shocked by how well it worked. A 600 cfm carb would have been a better piece to use. We did find that the oversized carb was grossly rich for our poor little 318. We figured that jetting would allow the carb to work better in our misapplication and also serve to improve economy until we were ready to get serious about a suitable combination of parts.