Dellorto DHLA Overhaul

[Roland]

I am just doing an overhaul of my Dellorto DHLA's on my sprint and wanted to check the fit of the butterflies in the carburettor bodies.

Everything looks fine with the butterflies and when closed I can't get a 1 thou feeler between the discs and carburettor barrel. I then decided to light test with a strong light in a darkened room. The barrel for No.2 cylinder was showing a perfect seal no light, the other three I have light spilling on approx. 40% of the perimeter.

I did a disc re-centre on the front carburettor and made No. 2 cylinder worse, so both butterflies on this carburettor now spill light on 40% of the perimeter. At least I may now have a better balance across the barrels on this carburettor (I have no air bleed screws).

The question is regarding the light test, is this test too onerous considering the manufacturing tolerances? I don't want to start chasing a 'perfect seal' if it is not achievable.

Tks

Roland

[Craven]

As fully closed butterflies is not an operational position and is an ideal, however the gap is most important as they approach the first of the progression hole in the barrel. Air flow at this throttle position will is a more critical factor.
FWIW

[Roland]

Yes I am all OK with the closed position of all butterflies and the 'first' progression hole, the hole is covered in the same way on all barrels. I've also checked for bent spindles and that seems fine.

Eurocarb have just come back to me and actually indicated that I do need so light coming through on the light test. I therefore suspect that the cylinder No.2 barrel butterfly fit was perfect compared to the other barrels and I therefore pulled more vacuum on that barrel.

I can't be sure but my disc re-centre may have fixed matters.

Roland

[alaric]

Hi. I hope you don't mind me not starting a new thread - the title seemed pretty close to what I was going to use.

I'm about to take my Dellortos off as I've exhausted the tune up route. I have just set the mixtures using a colortune, and have set the carb balance with a monometer. So all should be good, but I think some of the air holes must be blocked, or a pump jet is not working properly. I have a hesitation moving from overrun or idle once the engine is fully warmed up. It doesn't seem to happen until I'm out on a drive. Frustrating, but I wondered if anyone had seen this before, and whether they could comment what they think the issue is likely to be.

Regards.

Sean.

[alaric]

Hi again. In my hunt for an answer I found this post from Rohan back in 2013:

Trying to solve this problem starts with understanding why carbs such as DCOE Webers and DHLA Dellortos have accelerator pump jets.

When the throttles are opened rapidly the pressure in the inlet passages rise rapidly and the mixture goes lean as vaporised fuel deposits on the inlet passage walls or forms large drops of fuel that don't burn in the combustion chambers. The acceleration pump jets compensate for this effect. It is a very crude compensation and not all that critical as to how accurate an individual pump stroke delivery is.

When you open the throttles slowly the acceleration pump and jets make no significant contribution. When you open them rapidly the acceleration pump jet is the major contributor for a second or so of the needed fuel supply. For a mid range opening the mixture provision is a combination of the normal jets and the acceleration jet.

When the engine is hot less acceleration pump volume is needed as the fuel vaporises more easily when the pressure rises in the inlet tract and thus less deposits on the walls or forms large drops.

Based on the above I suspect you accelerator pump in combination with the normal jetting is too rich during midrange acceleration when the engine is hot but ok when the engine is cold as more fuel is needed then and ok when doing rapid acceleration and relying mainly on the acceleration jets or accelerating slowly and relying mainly on the normal jetting

What's the solution ? - a few suggestions

1. Change to a smaller accelerator jet and or pump stroke
2. Change the other carb jetting
3. some combination of the above

The above is easier to do and get right if you have a dyno to do the work on or a exhaust gas analyser to see what's going on with the mixture during real life driving. Neither of these are cheap

Another alternative is to replace the Dellortos with a set of Webers and go to the yahoo group sidedraft_central for setting up and jetting. Not a cheap option either unless you have a spare set of Webers ( which I luckily did have for my Plus2S 130/5)

cheers
Rohan

This came from viewtopic.php?f=40&t=27691

Thanks Rohan for your clear explanation.

I was thinking it is too rich in the transition once hot. The symptoms seem to match the description in the thread.

So I think a rebuild of the carbs is needed, during which I will note what jets are fitted - I know they are not standard - and what the float levels are. The engine is a 711M block with standard bore and ~77mm stroke giving 1650cc, with 1.625" inlets, 1.4" exhaust and CPL 2 cams.

Regards.

Sean.

[Roland]

Sean,

In the last couple of days since I wrote that post I decided I wasn't 100% happy with my carburettors. The PO had done a rebuild on them in 2009 but it amounted to a gasket refresh.

So I have totally stripped the carbs including removing the spindles and bearings. They will be ultrasonic cleaned and vapour blasted. I'll fit new spindle bearings and gaskets and replace anything that I not 100% happy with. I have previously set up a rig to measure the pump jet volume that I will use.

I figured that unless I do this I will never be really confident with these carburettors. I have the older style pump jets which spray vertically and I will change these to the type that spray horizontally.

Hope your carb rebuild goes OK. I was surprised how easily they stripped down.

Regards

Roland

[alaric]

Thanks for the reply Roland.

I haven't removed the carbs yet. I've just been out taking the progression hole covers off, and removing the pump jets to take a look. I think at least one of the progression holes is slightly blocked, but otherwise all looks clean. I'm a bit concerned about the pump jet size though, which is 33. This is the size for the DHLA 40E version, but I have the non E version, which should have a 45 according to the workshop manual. The manual suggests going to a 35 to 'improve driving'. Given my engine is 1650, with larger valves and sprint cam, the 33 should be too small. Des Hammill's book suggests that if the jet is too small then the engine will die momentarily when the throttle starts to open, so maybe there's a clue in this. It's not cheap to try new jets though is it, not at £35 for a set from Eurocarb. I wonder if they have a try and return service - it's not a completely bonkers idea is it?

I won't be buying anything until I have the carbs off and stripped. Any ideas on sources of jets to try? I know I should take it to a rolling road, but I want to explore it a bit myself first.

Sean.

[Roland]

Sean,

My car is a sprint and everything on the carbs is as it came from lotus. So my pump jets are 45 and spray down vertically. I will replace them with the same size that spray into the engine and then can pump the accelerator when starting. I am sure someone with expertise on here will advise on your pump jets but I would also be thinking they are small for your engine.

Eurocarb are expensive and I have been looking if anyone is selling an assortment giving a range of jetting and emulsion tubes for tuning purposes. I haven’t found anything and you probably want original dellorto parts. Try Eurocarb on sale or return, you never know, you can only ask.

I am certainly no expert on these carbs but the bodies are not easy to clean internally, probably an ultrasonic clean is best. If you have a blocked emulsion tube hole you could easily have a blockage elsewhere.

If you do a full rebuild and replace the spindle bearings don’t get them from Eurocarb, they are a silly price. They are a 607rs bearing which can be supplied by a bearing supplier at half the price.

Good luck with yours. Just for reference I reckon a full rebuild on my carbs doing it myself will still be £200 to cover clearing, vapour blasting and parts.

Roland

[alaric]

Thanks again Roland. I'll be taking the carbs off and going through the same sort of cleanup and parts replacement as yourself. I measure an imbalance between barrels within the same carb on both carbs, so need to check the spindles etc too. On the jetting I'm very tempted to get a set of 45 pump jets and try them first...

But just now I have a job that will keep me out of trouble for the rest of the day.

All the best.

Sean.

[alaric]

Hi again.

I've been chatting to Eurocarbs, who have been very helpful but can't send me sets of jets to try. No surprise in that really haha.

Anyway I asked if the guy knew of a rolling road near me, and he sent a link to https://www.nemotors.co.uk. This looks ideal, and is only about 20 miles from me. I'm sure I took my 1400 mini there back in the early 90s. That was terrifying - i thought the engine was going to explode. They quote £120 for a re-jetting service as an indication.

Does anyone have experience with nemotors? I'll not want a power curve to 7k rpm, just jetting to improve the performance and overlap of the three fuel delivery bands.

Regards.

Sean.

[bast0n]

Neil - the owner is a very nice chap indeed and has been brilliant at sorting out various problems for me. I cannot recommend him highly enough. D

PS - he used to be in Ditcheat where perhaps you took your Mini?

[alaric]

Great. Thanks for letting me know. The lady on the phone seemed very nice, and I've booked the car in for a session. She said they prefer that owners leave their cars.... not sure I want to leave it - I'd rather be there as I'm interested - but that's probably why they want owners to go away haha.

Regards.

Sean.

[bast0n]

I always stay and watch the fun!! D

[alaric]

I've been thinking about it and don't want to leave it with them. I'm only down the road so will ask them to let me know what time they'll work on it, and will stay with it. I want the video of the car on the rolling road with all the gory details.

Sean.

[TBG]

Ring them and ask to speak to Neil. He is very accomodating and a good egg. Mention GOU100D a white S3 and stand back!! D