Lotus Elan

Dellorto DHLA Pump Jets

PostPost by: Roland » Thu Sep 22, 2022 11:11 am

I have done a full rebuild on the Dellorto DHLA's for my Elan Sprint. I have just set up all the pump jets giving 8 cc per jet over 20 strokes of the butterfly/throttle. I have approx 7-8 mm of thread left from the pump jet adjustment nut to the end of the threaded rod.

The thing is I have had various technical discussions with Eurocarb and they indicate the pump lever stroke length (at the end of the lever) should be 5 mm. If I measure the stroke I have based on the settings above I have a 9-10mm stroke length at the end of the lever on both carbs, no where near the 5mm Eurocarb are quoting. I cannot see how I can be this far out, the pump jets seem to work well with a 6ft spray and everything is refurbished and clean. I have new pump diaphragms and springs, I didn't renew the pump non return valve or the ball bearings below the pump jet weights because the seem in good condition.

Appreciate if anyone can advise on the pump jet stroke length they have on a pair of properly performing Dellorto DHLA's.

Tks

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PostPost by: alaric » Thu Sep 22, 2022 10:35 pm

Hi Roland.

I have to set the pump jet volume on my Dellortos this weekend, so should be able to let you know what stroke length I end up with. The carbs are ready to go back on the car, apart from the pump jet adjustment and float level setting.

Sean.
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PostPost by: Roland » Fri Sep 23, 2022 12:04 am

Sean,

Thanks that will be helpful.

I have fully rebuilt my carbs but I didn’t replace the pump jet NRV’s or the pump weight ball bearings. The long stroke may be due to these being worn. This evening I inspected the pump weight ball bearings in one carb. One ball bearing feels slightly worn in my hand but it’s almost impossible to pick up the wear with a micrometer. However I rolled it across a glass plate and it always tracks to the right, the other one rolls straight. So I may just replace the pump weight ball bearings and the pump jet NRV’s.

Roland
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PostPost by: alaric » Fri Sep 23, 2022 6:55 am

Hi again.

I replaced the pump weight ball bearings - I lost one as I took them out - they rolled too well.

I found one of the non return valve ball bearings was stuck with deposits from evaporated fuel, but just cleaned it with carb cleaner to free it up.

I need to do the test but actually you need to compare against well working regularly used carbs - which is what you asked for in the first place. I'll still let you know what stroke I end up with.

Sean.
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PostPost by: Roland » Fri Sep 23, 2022 8:51 am

It will be interesting to see what stroke lengths you get.

I suspect that there are a lot of Dellortos out there that don't meet the 5mm stroke length Eurocarb gave me, there are a lot of variables. Having gone this far I would like to get them performing as new.

Roland
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PostPost by: Roland » Fri Sep 23, 2022 11:58 am

Carb pic 2.jpg and
Carb pic 1.jpg and
Sean,

Here are two photos showing one of my carbs with butterflies in the closed and open position, you can see the stroke length approximately and the position of the adjustment nuts.
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PostPost by: Donels » Fri Sep 23, 2022 6:16 pm

When I rebuilt my carbs I did not have the 5mm info you describe so calculated the piston volume to give 0.4cc per stroke (8cc per 20 strokes). It gave a stroke length of 4.5 mm. Are you sure the Eurocarb figure is the length of the rod and not the stroke length setting?

The stroke length is the important bit, the Rod length is immaterial.
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PostPost by: Roland » Fri Sep 23, 2022 8:12 pm

Donets,

Eurocarb are quoting 5mm stroke length at the end of the lever. The lever magnifies movement 4x so this is a 1.25mm movement at the piston/diaphragm. So they have used a piston diameter of 28.5mm to deliver 0.8cc to two pump jets. This seems quite reasonable, I guess you may have used a larger piston diameter, the diaphragm makes it tricky to determine exactly.

My lever movements are too large and I must have losses at the pump jet NRV and pump jet ball bearings, at least I am beginning to reach that conclusion.

What stroke length did you measure at the end of the lever during physical testing?

Roland
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PostPost by: alaric » Sat Sep 24, 2022 11:44 am

Hi all. I took the carbs back off the car last night and measured the stroke length. I have reassembled the levers using the same pushrod as original, and adjusted the nuts to give me the same lever length as when I took them apart. I get stroke length of about 6mm on one, and about 8mm on the other. This is before any fuel goes in. So I don't think I'm helping much until I measure and set up using the volume of fuel. I should get out there later today. Sorry not to be more helpful.

Sean.
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PostPost by: Roland » Sat Sep 24, 2022 12:08 pm

Sean,

No worries, my carb refurbishment is moving slowly.

The old settings on your carbs are closer to the Eurocarb 5mm than mine. Be interesting to see what you get on the retest.

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PostPost by: alaric » Sun Sep 25, 2022 10:36 am

Hi. I collected 20ml after 50 squirts, so have 0.4ml, or cc, per opening of the butterflies, or 0.2cc per jet. There was some spillage and the fuel that lined the collecting bowl is not included, but that's roughly what I have from my first test, before adjustment.

I'm going to check this again... I seem to have half the volume, but, actually, the rate of fuel delivery is going to be determined by the pump spring force and jet size, and I have squirts that are reasonably long.

I suspect one of us has made a mistake Rupert - since you have twice what i have its not an unreasonable conclusion. It's probably me. I'll check the other carb and repeat.

Hammil's book doesn't mention this volume - at least I couldn't find it a moment ago when I checked. The book just has a pic and says don't touch...

Sean.
Last edited by alaric on Sun Sep 25, 2022 11:23 am, edited 1 time in total.
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PostPost by: alaric » Sun Sep 25, 2022 10:52 am

Hi again. I repeated with the same carb and got the same result.

With the second carb I got 0.22cc per jet per opening.

I think that's close enough for me... OK I can't resist making a tiny adjustment to the second carb to make them the same.

Sean
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PostPost by: alaric » Sun Sep 25, 2022 11:21 am

Hi again. I repeated with the same carb and got the same result.

With the second carb I got 22ml collected after 50 pumps, or 0.22cc per jet per pump.

Looking at this video https://youtu.be/eKEqqimaF3M

He's aiming for 14 to 15ml after 40 pumps, and mine is 20ml after 50, so that's 16ml after 40. So I reckon I'm getting pretty much the same. If I adjust the nuts that'll reduce the stroke a little.

Sean
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PostPost by: alaric » Sun Sep 25, 2022 2:08 pm

Hi again.

I now have both carbs adjusted to deliver 19ml after 50 pumps, giving me the same volume as in the video link that I posted above. The stroke length on each carb is now about 6mm, maybe 6.5 - it's hard to measure accurately with a ruler.

So I'm putting the carbs on this evening, once my head has cleared from the petrol fumes...

Sean.
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PostPost by: Roland » Sun Sep 25, 2022 4:07 pm

Sean,

I looked at that Scuderia Usignolo video before and thought it may be wrong (I actually sent them an e-mail a few days back, no reply yet), or applicable to the Alpha they are working on (was it a 1300 cc engine?). All data I have seen for an Elan is quoting 20 strokes and 7-8cc per barrel. Specifically for a Lotus Elan Sprint 20 strokes at 8 CC per barrel is the number I have (16 cc per carb), Eurocarb also confirmed this figure.

So I think your setting the pump jets at half the rate they should be, this must translate into a performance shortfall since you have half the fuel from the pump jets when opening the throttle fully.

I will check my numbers tomorrow but maybe worth double checking before putting them back on.

Rgds

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