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?
Elan +2
Elise mk 1
Donels
Fourth Gear
Fourth Gear
 
Posts: 681
Joined: 10 Sep 2016

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.
Bill Williams

36/6725 S3 Coupe OGU108E Yellow over Black.
billwill
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 5060
Joined: 19 Apr 2008

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....

John :wink:
User avatar
john.p.clegg
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 5744
Joined: 21 Sep 2003

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.
Attachments
1.jpg and
2.jpg and
3.jpg and
4.jpg and
5.jpg and
6.jpg and
Brian
64 S2 Roadster
72 Sprint FHC
User avatar
types26/36
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 3872
Joined: 11 Sep 2003

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?
Elan +2
Elise mk 1
Donels
Fourth Gear
Fourth Gear
 
Posts: 681
Joined: 10 Sep 2016

PostPost by: john.p.clegg » Sun Jan 14, 2018 6:26 pm

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

John :wink:
User avatar
john.p.clegg
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 5744
Joined: 21 Sep 2003

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
Esprit2
Fourth Gear
Fourth Gear
 
Posts: 561
Joined: 02 Apr 2008

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.
Bill Williams

36/6725 S3 Coupe OGU108E Yellow over Black.
billwill
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 5060
Joined: 19 Apr 2008

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.
Elan +2
Elise mk 1
Donels
Fourth Gear
Fourth Gear
 
Posts: 681
Joined: 10 Sep 2016

PostPost by: RedBarnMan » Thu Jan 18, 2018 9:57 pm

This is useful information. When I first got my +2S130/5 with Dellorto's it was a real ***tard to start from cold and I could flatten the battery trying. Using the choke made no real difference most of the time. Then I discovered that when fully depressed, the accelerator pedal was not fully opening the carb butterflies and presumably was not engaging the accelerator pump. Manually opening the accelerator fully at the carbs for a few seconds resulted in much improved starting. The odd thing is my car only had 10k miles on the clock when bought and the accelerator cable looked original. The car also started much better when I put in an electronic trigger from Powerspark ignition along with a new coil.
RedBarnMan
First Gear
First Gear
 
Posts: 20
Joined: 23 Aug 2013

PostPost by: JonB » Fri Jan 19, 2018 8:42 am

@RedBarnMan

With the S130 engine and Dellortos, you do not need the accelerator to start the engine. I found on mine that once I'd applied some adjustments to the Dellortos per the manual and balanced the carbs with a hand held flow meter, I just needed about 25% choke and no throttle to start it. Fires up second time, usually (which I attribute to the fuel pump priming the carbs on the first attempt, as I don't start it often these days, it being up on stands). A fill stroke of the throttle prior to start results in flooding.

I blame the poor starting I'd experienced previously on the carb imbalance, which was entirely self inflicted. At that time, my starting regime was the same as you describe, but with a greater than 50% chance of flooding.

I have an electronic ignition like you.

Cheers
JonB
User avatar
JonB
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 2396
Joined: 14 Nov 2017

PostPost by: john.p.clegg » Fri Jan 19, 2018 9:07 am

+1 the above..

John :wink:
User avatar
john.p.clegg
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 5744
Joined: 21 Sep 2003

PostPost by: Donels » Fri Jan 19, 2018 9:19 am

I have rebuilt the carbs with new pump diaphragms and then attempted to set the pump according to the required 0,5 mm clearance. This is impossible as the adjusting nut is at the end of the rod and the pump does nothing when the throttle is operated.

The only way to set the pump up is to measure the flow via the volume over a number of strokes as described by Esprit2. I have no idea how the 0,5 mm setting can be applied.
Elan +2
Elise mk 1
Donels
Fourth Gear
Fourth Gear
 
Posts: 681
Joined: 10 Sep 2016

PostPost by: alan.barker » Fri Jan 19, 2018 9:32 am

I think the trick is to find out what works on your car for starting and your technique :wink:
For me with Dellortos on a 1972 Sprint and Dellortos on a 1973 +2S130 i never ever use the choke and no starting problems. Same as the two +2 Elans with Webers i had before, 2 or 3 squirts with the accelerator no choke :wink:

So if you have found the technique that works for your car stay with it 8)
Alan
Alan.b Brittany 1972 elan sprint fhc Lagoon Blue 0460E
alan.barker
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 3680
Joined: 06 Dec 2008

PostPost by: JonB » Fri Jan 19, 2018 10:23 am

Hmm, yes. I should have added a "YMMV" to my last post.
User avatar
JonB
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 2396
Joined: 14 Nov 2017
Next

Total Online:

Users browsing this forum: No registered users and 16 guests