Lotus Elan

Dellorto accelerator pump

PostPost by: Donels » Sat Jan 13, 2018 5:48 pm

I have just overhauled the Dellorto's for my +2. I now want to set up the accelerator pump but the Lotus manual contains no advice on how to do this. Before I stripped them down I did note that moving the throttle seemed to give very little movement to the accelerator pump lever and the attaching rod was at the end of its adjustment.

The throttle movement takes the lever over-centre. At this point do I adjust the rod so the accelerator pump at its maximum displacement? Anyone got a setting up procedure or any advice?
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PostPost by: billwill » Sun Jan 14, 2018 1:14 pm

It's more complicated than that. The topic is avoided in the best book on Weber and Dellorto carbs.

{How to Build and Power Tune Weber and Dellorto DCOE and DHLA Carburettors (Speedpro Series) by Des Hammill}

Take great care dismantling the pump, there is a tiny ball bearing and spring acting as a non return valve which can easily get lost.

The first thing to realise is that the pump is not force-driven by your accellerator pedal. instead the relaxing of the pedal causes a fine external spring to stretch and when you press the pedal hard it moves away and allows the stretched spring to push fuel through the pump jet. So it is always a consistent rate.

Second there are two kinds of pump jet, one with the hole in the end squirts downwards, which turned out to not be the best method. The second type has the hole in the side so you arrange it to squirts straight down the inlet tubes towards the engine inlet valves.

No you don't use the max possible squirt as you imply, I read, but I don't recall where, that you adjust the position of one end of the fine external spring so that it squirts about 6 feet. So obviously you have to do this with the carbs off the engine.

But don't take my word for it, do further research before you move ANY of those adjustments.
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PostPost by: john.p.clegg » Sun Jan 14, 2018 2:49 pm

My take on it is that one measures the volume squirted over a number of strokes....

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PostPost by: types26/36 » Sun Jan 14, 2018 2:54 pm

I have collected a little data on the subject from the forum as well as the internet and some books so with acknowledgements to anyone whom I have copied here is some information.
I have never had much success obtaining any consistency with the amount of fuel delivered by the accelerator pump adjustment but hope the info helps.
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PostPost by: Donels » Sun Jan 14, 2018 5:42 pm

Thanks for the info. It kind of makes sense now and it looks like the reason no set up information is given is that none is required, or very little.

I have been reading a book on the theory and tuning for Weber carburettors, not having one for Dellorto's, and the pump volume is set at the design stage and changes required for different vehicles is achieved by the valving. So it looks like the only setting is the 0,5 mm gap to prevent the accelerator circuit operating when idling. If this is correct then you cannot change anything else with the rod length, only by changing the valving. At all other conditions the accelerator pump is either pumping (when accelerating), filling (when decelerating) or static (at constant throttle).

Unfortunately my copy of the manual did not have the 0,5 mm gap info. The volume flow info is only important to check that you have the correct jet size. Too small and it's less than 7.5 cc, too large and it's greater than 7.5 cc.

Does this make sense to you guys?
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PostPost by: john.p.clegg » Sun Jan 14, 2018 6:26 pm

I think you'll find the Dellorto has adjustable pump stroke..

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PostPost by: Esprit2 » Sun Jan 14, 2018 7:46 pm

The stroke length is adjustable, and determines how much fuel is dispensed in a complete stroke.

The spring rate (not typically changeable) and the jet size determine how quickly the total amount of fuel is dispensed. A given amount of fuel can be dispensed almost all at once via a large pump jet, or over a period of time via a smaller stream through a small pump jet. So the variables you can play with and balance are how much, and how fast. Putting your foot to the floor may only take a split second, but the spring loaded plunger remains in action as long as it takes to dispense the full volume through the pump jet... and that can take considerably longer.

If you were an engineer setting up a Dellorto for an new engine application for the first time, you'd go through the full process. However, if you're just cleaning and tuning existing carbs from a well known engine set-up (like a Lotus that came from the factory with the Dellortos installed), then tuning shouldn't require more than a small tweak to the original factory settings. In that case, collecting the discharge of 20 full, slow strokes will confirm that you have the stroke length set correctly. But setting the stroke can be done directly be measuring/ setting the gap in the linkage.

When you're counting the 20 strokes, go slow. Open the throttle and hold it until the flow out of the pump jet ceases. Then close the throttle and pause long enough for the pump to re-fill. Repeat slowly and deliberately 20 times. Pump discharge is typically 7 - 8 cc in 20 strokes.

Dellorto DHLA Accelerator Pump Delivery:
1558 cc Lotus-Ford Twin Cam
7.5cc / 20 strokes ... Lotus Elan - Standard
8.0cc / 20 strokes ... Lotus Elan Sprint and Elan +2 / +2S / +2S-130
8.0cc / 20 strokes ... Lotus Europa Twin Cam & TC Special

Or, written another way:
............................................ Elan Sprint &
......................... Elan Std ... Elan +2/ +2S ... Europa TC
Choke (mm) ........... 30 .............. 33 .............. 30
Idle Jet .................... 50 .............. 50 .............. 50
Idle Air Corrector... 7850-1 ...... 7850-2 ...... 7850-2
Accel Pump Jet ...... 33 .............. 45 .............. 40
Pump Delivery ...... 7.5 cc .…..... 8.0 cc ........ 8.0 cc
… in 20 Strokes
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PostPost by: billwill » Sun Jan 14, 2018 7:57 pm

Image

I don't believe that is quite right, it is not the flexing of the diaphragm that controls the quantity & rate of fuel pumped it is the fact that the pump is driven by the small external spring and not by the throttle movement directly.
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PostPost by: Donels » Mon Jan 15, 2018 6:55 pm

Having looked at all the parts on the bench, Bill is correct. Moving the throttle pedal compresses the long external spring which pushes the lever which in turn pushes the pump diaphragm to squirt the fuel.

When the throttle is released it pulls the rod, which pulls the lever away from the pump and the internal spring pushes the diaphragm back to its resting position, refilling the pump.

The pump stroke can only be adjusted to give the 0,5 mm gap, which is all that can be adjusted and if the jets are correct is the only setting required.
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