Editing Holley accelerator pump (section)
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==Pump cams and shooters== Two factors that affect the accelerator pump’s delivery is the pump cam and the pump shooter (discharge nozzle). The pump cam determines the total volume of fuel and affects delivery rate; the pump shooter diameter affects delivery rate and has a major effect on the ''duration'' of the shot. {{Note1}}The old saying “if a little is good, a lot is better” does not apply to the proper tuning of the accelerator pump. Your car’s performance can be just as bad if it receives “too much fuel too soon” as if it receives “too little fuel too late”. {{Note1}}Pump arm adjustment/clearance should be checked and verified each and every time the pump cam and/or pump cam position is changed. ===Pump cams=== [[File:Holley pump cam assort.jpg|thumb|right|4000px|Holley pump cam assortment]] The cams will have 2 (some 3) holes in them. Besides the change to the fuel delivery curve made by changing holes, position #1 is used when the throttle blades at idle are "normal", position #2 is when the tune of the car requires an idle speed of >1000 RPM or so. Position #2 makes up for the additional throttle shaft rotation that's required for the higher idle speed. Cams w/position #3 are usually used on REO 50cc pumps. <br style="clear:both"/> Generally, vehicles which normally run at lower idle speeds (600-800 RPM) find position #1 more useful because they can have a good pump shot available coming right off this relatively low idle. Positions #2 and #3 delay the pump action, relatively speaking. These two cam positions are good for engines that idle around 1000 RPM and above. Repositioning the cam in this way makes allowance for the extra throttle rotation required to maintain the relatively higher idle setting. ====Pump cam charts==== ;The following chart is from Jason Kolk, as posted at vintage-mustang.com: [[File:Holley pump Cams4777.jpg]] ;This is info supplied by Holley: [[File:Holley pump cam info.jpg|800px]] ;Cams from smallest to largest Holley says: "Accelerator pump shot relative to cam color from lightest to heaviest are as follows": [[File:Pump Cam Volume by Hole.jpg]] ====Dominator pump cams==== There are just two Dominator pump cams that come with the larger throttle shaft profile, as shown below. The other cams can presumably be modified to work on a Dominator. [[File:Dom pump cam.gif]] ====50cc pump cams==== The pump cams having three screw positions can be used on a 50cc pump, in position #3. Some are only for use with a 50cc pump. ===Pump discharge nozzles, or "shooters"=== [[File:Brass and reg shooter views.jpg|thumb|400px|right|Brass tube type shooter on left, regular cast zinc shooter on right]] If you need more pump shot '''''sooner''''', then a larger pump nozzle size is required. During acceleration tests, if you notice that the car first hesitates and then picks up, it’s a sure bet that the pump nozzle size should be increased. A backfire (lean condition) on acceleration also calls for a step up in pump nozzle size. Conversely, if off-idle acceleration does not feel crisp or clean, then the pump nozzle size may already be too large. In this case a smaller size is required." If someone can follow the vehicle in question, a too-rich accelerator pump will often cause a puff (or more) of black smoke, indicating a too-rich condition. The pump shooters have a number stamped on their casting which designates the shooter size in thousandths of an inch, i.e., a #25 shooter has a 0.025” discharge orifice. The smaller diameter nozzles lengthen the pump shot duration and are used with heavier vehicles or with vehicles equipped with lower numerical rear axle ratios.<br style="clear:both"/> Larger diameter nozzles (0.035” - 0.037”) shorten the pump shot duration, but deliver a greater initial volume of fuel. These sizes should be used on applications where engine speed will increase rapidly (vehicles with good power-to-weight ratios). Best acceleration is achieved when the accelerator pump delivers the lean best power air/fuel ratio to the engine; not when the maximum volume of fuel is supplied. [[File:Hollow shooter.jpg|thumb|400px|left|"Hollow" shooter screw]]If a shooter bigger than 0.038”-0.040” is required, a “hollow” shooter screw is advised. Above a 0.037” squirter size, a 50cc "REO" pump may also be required, available under Holley P/N 20-11. Make squirter changes in 3-step increments- i.e. if you have a 28, go to a 31, etc. Smaller sized accelerator pump squirters will cause the duration of the pump shot to be longer. Because the fuel cannot be compressed, the spring in the accelerator pump lever compresses then this spring pressure continues to supply fuel through the squirter until the spring has extended back to its original length. An important point should be kept in mind when tuning a double pumper: The secondary accelerator pump must supply fuel for a sufficient time so that the secondary main nozzles can begin to flow fuel to the engine after the secondary throttles are opened. If the nozzles do not start by the time the pump shot expires, bogging will result. <br style="clear:both"/> Also check all used accelerator pump nozzles closely to see if they've been drilled oversize. If a shooter has been drilled out, the number on it obviously will no longer apply. You may have a hard time telling, if they did a good job so a numbered drill bit set or a wire spark plug gap feeler gauge will help determine the exact size. Only drill shooters oversize if it's absolutely necessary. For the best ''repeatable'' results use factory shooters. If you find you have to drill a shooter, no big deal, but do yourself a favor and etch the new size onto it so you'll know what it is later on.
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