Holley accelerator pump
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==First things first== | ==First things first== | ||
| + | ===Pump linkage adjustment=== | ||
Before changing the size of the shooters or the pump cam, be sure the accelerator pump is adjusted correctly- no play and fuel from the shooters the instant the throttle shaft is rotated, and about 0.015" clearance at wide open throttle (WOT). | Before changing the size of the shooters or the pump cam, be sure the accelerator pump is adjusted correctly- no play and fuel from the shooters the instant the throttle shaft is rotated, and about 0.015" clearance at wide open throttle (WOT). | ||
| − | Also, make sure the carb is not idling on the transfer slot | + | ===Transfer slot=== |
| + | Also, make sure the carb is not idling on the transfer slot. | ||
| − | + | The drawing below shows the transition slot as seen with the carb held upside down. The drawing on the left shows an overexposed transition slot. Baseline the throttle blades to give a transfer slot that looks like the image below, right (approximately as long as it is wide, or about 0.020", up to 0.040"): | |
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| + | [[File:Holley transferslot.jpg]] <br style="clear:both"/> | ||
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| + | From this point, if the idle speed is increased to the point where the transfer slot becomes over exposed, a poor idle and off-idle transition can be the result. One sure sign of this is the idle '''''mixture''''' screws do not cause the idle to change when turned in or out. Usually, adding initial timing to increase the idle speed will allow the throttle blades to be closed down some using the curb idle screw. This will return the idle mixture screws to normal and the idle will be smoother and cleaner, as will be the transition from idle to main circuits. | ||
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| + | In addition, the secondary throttle blades may need to be opened slightly to allow more idle air (called idle ''bypass'' air) to be introduced into the engine. | ||
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| + | ===Ignition advance=== | ||
| + | As mentioned above, correct initial advance plays an important part in a clean transition from idle into the main circuits. So the timing has to be sorted out before carb tuning is attempted, else a problem with the timing can seem as though it is a carb-related issue in general, and an accelerator pump issue in particular. | ||
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| + | Also, if there is a vacuum advance being used and it is hooked to manifold vacuum, the engine can have a slight to a major hesitation if the engine is overly dependent on the increased timing from the vacuum advance. In some cases the drop in vacuum and subsequent loss of ignition advance at the hit of the throttle can cause the hesitation. This ''shouldn't'' be the case if the initial timing and the torque converter stall is sufficient, and the vacuum advance amount isn't excessive (limited to about 10 degrees of vacuum advance max), but stranger things have happened. | ||
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| + | More on correctly timing the engine can be seen at '''[http://www.crankshaftcoalition.com/wiki/Hot_rodding_the_HEI_distributor#Ignition_advance Ignition advance]''' | ||
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| + | ===Accelerator pump check valves=== | ||
| + | Another thing that can delay the accelerator pump action is a leaky check valve. | ||
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| + | There are two check valves: | ||
| + | *Under the shooter- usually a pin and weight or sometimes just a pin or just a ball. | ||
| + | *Under the pump diaphragm- this will be a red/orange "umbrella"-type valve (newer carbs) or a "ball and bail" affair used on the older carbs. | ||
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| + | ====Repairing the check valve seat==== | ||
| + | '''''Both''''' the check valves have to seal well or there won't be an immediate discharge from the shooter. The check valve '''seat''' under the squirter can often be repaired if it has an imperfection that prevents it from sealing properly. If you have a used check ball or 'pin' of the same type your carb uses, you can use it to carefully reform the seat in the carb body casting. | ||
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| + | You place the check ball/pin down into the hole, then using a long, thin, flat-ended punch, ''gently'' tap the check with a small hammer. This will remove any raised edges or whatnot that may be causing the check to not seat tightly. | ||
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| + | Under the pump diaphragm, if you have the rubber umbrella, be sure that it is still very pliable and the sealing surfaces on the carb as well as the umbrella are smooth and defect-free. If there's any doubt at all, simply replace the old check valve with a new one. It's a good idea to do this any time the pump is apart. | ||
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| + | If it has the older "ball and bail" style check valve, use the same procedure as for the under-nozzle check valve to re-form the ball seat. | ||
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| + | Removing and replacing the bail is no fun, and must be done very carefully so as to maintain the slight bend in the bail. It's best to re-peen the metal to secure it back into place. Only do this repair if it's necessary. | ||
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| + | [[File:2acc pumpcheck.jpg|thumb|left|500px|The two types of check valves found under the diaphragm: Ball and bail on left, umbrella on right, from bob2000.com]] <br style="clear:both"/> | ||
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| + | Here is a [http://www.youtube.com/watch?v=dW6YGxVTW5Y Holley accelerator pump video] | ||
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| + | ===Shooter assembly=== | ||
| + | [[File:Assembly.jpg]] | ||
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| + | Be sure the '''two''' gaskets are in place and not split. Any leakage past the gaskets will result in a less-than-normal discharge at the nozzles. | ||
==Primary or secondary== | ==Primary or secondary== | ||
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===Pump cams=== | ===Pump cams=== | ||
| − | 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. | + | [[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. | 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. | ||
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| + | ====Pump cam charts==== | ||
| + | ;The following chart is from Jason Kolk, as posted at vintage-mustang.com: | ||
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| + | [[File:Holley pump Cams4777.jpg]] | ||
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| + | ;This is info supplied by Holley: | ||
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| + | [[File:Holley pump cam info.jpg|800px]] | ||
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| + | ;Cams from smallest to largest | ||
| + | Holley says: "Accelerator pump shot relative to cam color from lightest to heaviest are as follows": | ||
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| + | [[File:Pump Cam Volume by Hole.jpg]] | ||
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| + | ====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. | ||
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| + | [[File:Dom pump cam.gif]] | ||
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| + | ====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 shooters=== | ===Pump shooters=== | ||
| − | + | [[File:Brass and reg shooter views.jpg|thumb|400px|right|Brass tube type shooter on left, regular cast zinc shooter on right]] | |
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| + | 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. | + | 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. | 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. | ||
| − | If a shooter size greater than 0.038”-0.040” is required, a “hollow” discharge nozzle 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. | + | [[File:Hollow shooter.jpg|thumb|400px|left|"Hollow" screw]]If a shooter size greater than 0.038”-0.040” is required, a “hollow” discharge nozzle 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. <br style="clear:both"/> |
Be sure the squirter you are using hasn't been drilled oversize! If it has, the number on it obviously will no longer apply. | Be sure the squirter you are using hasn't been drilled oversize! If it has, the number on it obviously will no longer apply. | ||
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