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Holley accelerator pump

3K views 3 replies 4 participants last post by  cobalt327 
#1 ·
Just installed my fresh 383 with 750DP carb and just trying to find-out what tuning issues there are and how to resolve them.

So far, the only obvious thing is a v.slight stumble when suddenly going from part throttle to WOT. For the purposes of 'testing' this, I've been leaving the car on the drive, will raise the RPM to around 2,000rpm, then floor the gas pedal and see how it responds. I adjusted the pump arm nut (on the primary side) 1 full turn clockwise and that seems to have helped as I can't really detect a stumble now. Still got a slight stumble on the secondaries (mechanical), so will probably do the same on there. I haven't measured the pump arm clearance yet.

Does the above indicate I need a higher volume accelerator pump? What am I actually achieving by turning the adjusting nut clockwise? The carb is fresh out the box and totally stock... stock jets worked perfectly on the dyno and so haven't been changed. I checked to see if the pump arm was being actuated as soon as the throttle starts to open and it was (before I even adjusted it).

Thanks.
 
#3 ·
v8hed said:
Just installed my fresh 383 with 750DP carb and just trying to find-out what tuning issues there are and how to resolve them.

So far, the only obvious thing is a v.slight stumble when suddenly going from part throttle to WOT. For the purposes of 'testing' this, I've been leaving the car on the drive, will raise the RPM to around 2,000rpm, then floor the gas pedal and see how it responds. I adjusted the pump arm nut (on the primary side) 1 full turn clockwise and that seems to have helped as I can't really detect a stumble now. Still got a slight stumble on the secondaries (mechanical), so will probably do the same on there. I haven't measured the pump arm clearance yet.

Does the above indicate I need a higher volume accelerator pump? What am I actually achieving by turning the adjusting nut clockwise? The carb is fresh out the box and totally stock... stock jets worked perfectly on the dyno and so haven't been changed. I checked to see if the pump arm was being actuated as soon as the throttle starts to open and it was (before I even adjusted it).

Thanks.
turning the adjusting nut will change the volume of the pump shot, but it's generally better to change the pump cam (the plastic piece on the throttle shaft that the pump arm rides on) to do this, as you can change when in the throttle shaft travel the pump shot squirts as well as total volume. a pack of all common holley pump cams can be bought for 20 dollars on ebay, probably including shipping to england if you search. In theory, all you should do with that nut you are turning is adjust it to "zero lash", so that there is no wiggle room between the lever and the accelerator pump diaphram (the part it pushes in on the underside of the fuel bowl) but it also allows the diaphram to go all the way down when the throttle is released.

Any variation from zero lash will result in either the diaphram not going down full at zero throttle, meaning it doesn't fill up at zero throttle and will not be able to pump a full shot in when the throttle is opened, or by turning the other way beyond zero lash it will leave a gap between the lever and the diaphram that allows the throttle to be opened some distance before any pump shot happens. that may cause a stumble when just barely cracking the throttle, and again may mean that you are again not getting a "full" pump shot when you open the throttle fully. My guess would be that the lever was already pushed by the nut to apply pressure to the diaphram, meaning you weren't getting a full pump shot. This would mean by turning the nut clockwise (I think clockwise?) you get closer to zero lash and a full pump shot.

regardless of which end of the scale you are on, your first step should be adjusting that nut so that the lever is BARELY touching the accelerator pump diaphram with the throttle closed. Except in a few very odd situations, any depression of the diaphram or gap between diaphram and lever with the throttle closed will cause tuning issues.


Hope this helps!


Also, I know nothing beyond carb and engine size, but very few street-driven small blocks need the 50cc (higher of the two available volumes) pump diaphram.
 
#4 ·
I agree w/checking the ignition timing and advance curve. I always get the ignition dialed in before concentrating on fine tuning the carb.

Tuning the carb sitting in the driveway won't tell you very much because the engine isn't under any load. That means there will be a lot more vacuum available to advance the ignition timing by the vacuum advance that might well not be there in the "real world". You really need to take it out to the track to dial it in correctly. As far as the accelerator pump goes, as has been said- no clearance at idle, about 0.015" minimum at WOT so the pump diaphragm isn't damaged. Good advice above for the pump cams as well. The shooter size will also increase the duration of the pump shot if you go to a smaller size, less duration w/a larger size.

In your case if it behaves the same way on the track as it does now, it sounds like advancing the timing from where it's at might help- just don't let the total timing get away from you in the process. You'd prolly be better off using a slightly lighter advance spring instead of altering the initial timing if the curve is set to give you the total timing it needs, and the initial is high enough.
 
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