View single post by Esprit2
 Posted: 01-19-2017 04:22 pm
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 311
The accelerator pump's external linkage is stroke-adjustable. A longer/ shorter stroke controls the total amount of fuel dispensed by the pump per stroke. The pump jet controls the rate at which it's dumped into the throat and the duration of the stream. The linkage is spring loaded, so stepping on the throttle applies spring pressure to the diaphragm without necessarily forcing it to instantly move full stroke. Then a small jet will result in a tiny stream that lasts a relatively long time, while a larger jet produces a larger stream that ends more quickly. In the end, the total volume is set by stroke length, then jetting controls how fast & long it's dumped.

To check the current setting, put a slender graduated cylinder (chemistry set) into each throat to catch the pump jet's output. Then fully stroke the throttle 20 times, holding each stroke long enough for the flow to cease before stroking again.

7cc to 8cc in 20 strokes is in the normal range. 7cc is about right for a 2.0 907 with the idle circuit's mixture properly set. If the idle is a little lean, then the engine may need a little more help from the accel pump... but don't exceed 8cc.

Don't use the accelerator pump as a crutch, or a reason not to address the idle circuit. The correct approach is to get the idle mixture right first, with no off-idle hesitation, just sitting in the driveway/ on the dyno!! Then use only as much pump as the engine needs to help the car off the line.. a heavier car needs more help than a light one. The pump is the last tuning priority, not the first.

Tim Engel

Last edited on 01-19-2017 09:49 pm by Esprit2