Holley accelerator pump
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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. | 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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| + | ==Primary or secondary== | ||
| + | On a double pumper, the side of the carb (primary or secondary) having the problem can sometimes be found by disconnecting the secondary throttle linkage to allow the carb to run on just the primaries. If the problem associated with the accelerator pump goes away, it can be surmised the problem lies with the secondary side. | ||
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| + | This is useful because while a problem with the secondary pump delivery can sometimes be "cured" by going larger on the primary side, going larger than necessary on the primary side hurts the economy and can cause the carb to not be as 'crisp' as it could be, even though the hesitation is gone. | ||
==Pump cams and shooters== | ==Pump cams and shooters== | ||
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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. Generally, vehicles which normally run at lower idle speeds (600-800 RPM) find | + | 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. |
| − | 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. | ||
===Pump shooters=== | ===Pump shooters=== | ||
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