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DHLA45 E Correct Accelerator Pump Time Constant - Engine & Ancilliaries/Gearbox - The Lotus Forums #ForTheOwners Jump to content


DHLA45 E Correct Accelerator Pump Time Constant


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Hi all,

I've recently rebuilt the carbs for my Elite. Spec 5 calls for a pump jet 38V 42H, ie the jet has a vertical and horizontal outlet. Unfortunately as I have discovered these were experimental and have been discontinued. I was in touch with Gary at Eurocarbs, and he recommended a replacement between 50 and 60. Have gone for 60 for the moment to get the car going.

I am busy setting up the carbs now, and I would like to ask whether anybody knows what the time constant should be of the accelerator pump for the Lotus cars with these carbs? This is the amount of time that it takes the volume of the pump "cylinder" to be depleted through the pump jets on engagement of the pump. It is set by adjusting the volume of the pump via the nut on the sprung linkage attached to the throttle axle.

From the available literature, I know what the required amount of fuel output is for 20 depressions, however I'm rather interested in flow duration so that I can expect the correct acceleration profile.

Any help would be appreciated!

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My rough as guts response is about 3 seconds. Had the carbs off recently, and a depression of pump gave a raw fuel squirt lasting 2-3 seconds. Carbs haven't been touched in that area at all. Just had a look at the carb tuning book by Des Hammill and can't see any reference to that, except for jet sizing. good question though.

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Thanks for the tip Clive. It is a good question I thought as nowhere is much thought given to this. I get the 20 pumps thing, but surely the time constant is actually more important.

 Bringsme to think, if your application requires smaller/larger pump jets, surely you should require less/more volume out per pump to maintain the release duration to provide the acceleration profile you're looking for?

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There are also different diaphram tension springs to 'squirt' the fuel with different pressure to achieve different durations, so it will be a corelation of pump jet orifice size, pump jet volume and the diphragm spring - I would imagine that will be a tricky balancing act without a proper rig, although I would have though volume would be the primary concern  (adjusted by the nut)  

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