Dellorto Mystery Tube?

[William2]

Before I attempt to try and fire up my newly rebuilt twincam, I noticed that on one of the Dellorto carbs there is a small brass tube that is positioned underneath the carb inlet flange on number 2 cylinder as shown in my photo. Not sure what it is there for and whether I should somehow seal it off or not? An educated guess is that it is an outlet for vacuum advance distributor if fitted? Any help would bev much appreciated

[Esprit2]

That's a ported vacuum spigot for the distributor.

If you're distributor doesn't have a vacuum capsule, or you don't plan to use it, then cap that spigot. If you leave it open, it will be an air leak, and will mess up the fuel mixture of that one throat.

Regards,
Tim Engel
Lotus Owners Oftha North (LOON)

[tonycharente]

That's a ported vacuum spigot for the distributor.

If you're distributor doesn't have a vacuum capsule, or you don't plan to use it, then cap that spigot. If you leave it open, it will be an air leak, and will mess up the fuel mixture of that one throat.

Regards,
Tim Engel
Lotus Owners Oftha North (LOON)

Hello Tim,
I'd be pleased to benefit (again...) from your knowledge and experience, please.
A French friend of mine has a European spec Elan Plus 2 S130/5. A local garage is trying to improve its running and has noticed that there is small vacuum tube on one choke (Twin Dellortos) but a non-vacuum-capsule distributor. He is wanting to try reverting to what presumaly was its original European spec set up by fitting a vacuum-capsule distributor. What's your view on the benefits or otherwise of having a vacuum-capsule distributor - I thought that they were only fitted on cars exported to markets with harsher antipolution laws than in the UK to reduce polution and not to improve running?
Many thanks
Tony

[Chancer]

It d?pends if it was fitted with a vacuum advance distributor or a vacuum retard type, other members will confirm what Elans had fitted for the European market, I think the American emission market used a vacuum retard system and I think the UK market used a Dizzy with neither vacuum advance or retard but am not confident.

To answer your question, a vacuum advance system (which I suspect the Elan never had) gives better fuel economy and throttle response on partial throttle openings at the cost of timing scatter (which I think is why it was not used on the Elan) I dont think that there is any benefit to an emissions vacuum retard system other than to get through the emissions testing, others may know more and/or better.

[Esprit2]

Not all Dellorto DHLA carbs have a ported vacuum spigot that you describe. Actually, DHLAs with the spigot are relatively rare. Are the Dellortos original to that TC engine? Or are they replacements off another, non-Lotus engine? I ask because I don't believe the Dellortos used on the later Twin Cams included that vacuum port. Having said that, I've never owned a late (post '72) Plus 2 with Dellortos.

The Federal emissions Twin Cam with Zenith Stromberg carbs did use a distributor with 5 degrees of vacuum retard (41225), but in that case, its purpose was to minimize Oxides of Nitrogen on closed-throttle over-run, and at idle. Note, that was with Zenith Stromberg carbs on an Federal-emissions engine. I'm not aware of any non-emissions, Euro Twin Cam ever using vacuum retard (or advance) with Weber or Dellorto carbs.

How you "Improve its running" depends upon what your goal is. Better performance is usually best achieved with a simple centrifugal advance distributor and an aggressive advance curve. One plug-n-play hop-up trick often used by owners of Federal emissions twinks is to replace the vac-retard distributor with a Euro centrifugal advance distributor. I don't know of anyone who has intentionally gone back the other way.

If the goal is to make the engine more civilized (maybe he should buy a Miata), then a little vacuum retard can result in a more smooth, less nervous idle. The affect goes away as soon as the throttle is cracked open. The downside to that is the engine will be a little less responsive, less crisp off idle (you have to slap it to wake it up). For a silk scarf and wicker basket sort of owner who is more interested in taking his lady on a peaceful picnic drive, a few degrees of vac-retard can make the engine a little more mild mannered at, or just off idle. Personally, I don't think the small benefit is worth the effort. Besides, if he's really interested in taking his lady for a ride, then he should keep in mind that adrenaline is the ultimate aphrodisiac.

If he's not replicating a Lotus TC spec for some other market for the sake of 'originality', and he's just modifying the engine for his own purpose, then IMHO, it would be best to simply add a vacuum retard capsule, but keep the more aggressive advance curve of the centrifugal distributor. The emissions distributor with 5 degrees vacuum retard also has a really lame, neutered advance curve.

Regards,
Tim Engel

[rgh0]

My 73 Plus 2 40DHLA Dellortos do not have a vacuum spigot. The have a casting boss on the top of each throat for a vacuum connection but it is not drilled out.

The Mid 80 45 DHLA Dellortos I bought to convert my S1 Esprit from Strombergs to Dellortos has vacuum tappings fitted to these ports and capped off.

Like Tim said no Dellorto or Weber Twin Cam had a vacuum retard capsiule. These were only used on the emission Stromberg engines for the USA and I would not advise copying that

cheers
Rohan

[RogerFrench]

With respectful nods to Tim and Rohan, I think you could make a case for a vacuum advance distributor with less aggressive centrifugal advance, particularly with the Plus 2.
However, I'd rather not work out how much advance you got from each.

[alan.barker]

On my 1972 Sprint and 1973 +2S130 the Dellortos have little brass screws/plugs for the Vacuum take offs.
This is a pleasure to use my Carbtune and balance Carbs in 5 mins and without needing to remove the Airbox.
Spot on.
Alan

[Esprit2]

The manometer vacuum ports are a significant advantage of the Dellorto carbs over the previous Webers. But just to be clear, those are not the same vacuum ports being discussed in the previous messages.

Regards,
Tim Engel

[rgh0]

With respectful nods to Tim and Rohan, I think you could make a case for a vacuum advance distributor with less aggressive centrifugal advance, particularly with the Plus 2.
However, I'd rather not work out how much advance you got from each.

Hi Roger
Yes a vacuum advance can certainly help cruising economy. I was referring to the vacuum retard capsule arrangement used on Stromberg engines in the USA for emissions purposes that you dont want to copy. Keith on Sidedraft_central has used vacuum advance on his Weber Elan.by rigging up a tapping of each carb throat and connecting them together to get a smoothed average vaccuum.

cheers
Rohan

[alan.barker]

Sorry i went off subject when i spoke about CarbTune and 4 manom?tres.
The Vacuum Spigot in photo does seem strange because surely petrol can drain into it like Stromberge engines with low level inlet manifold. On Stromberge engines this was changed to high level inlet manifold.
Alan

[vxah]

I looked at the photo and thought that the "tube" is in the wrong place for vacuum advance as is normally used?
The drilling would need to be on the top so as to be closed off by the throttle plate at idle then, on the engine side of the throttle when opened in order to monitor the vacuum signal? As said the signal from one throat would be far too rough to use with a vacuum capsule!
On my +2 I drilled and tapped the intake runners out of sight and linked the four ports together for a smooth signal, with the electronic timing box you can turn off the vacuum advance until a given rpm so the vacuum signal does not need to be ported. My problem is what to do with the system now that I have got it? As I understand it the reason for the vacuum advance is to start the burn earlier as the carburettor leans off at cruise loading causing the fuel air to burn slower.. But these carbs don't really seem to lean lean out at cruise do they.. Or do they??

[RogerFrench]

Indeed Rohan, the retard capsule was one of the first things I threw away from my Federal Europa TC!

[/b]

With respectful nods to Tim and Rohan, I think you could make a case for a vacuum advance distributor with less aggressive centrifugal advance, particularly with the Plus 2.
However, I'd rather not work out how much advance you got from each.

Hi Roger
Yes a vacuum advance can certainly help cruising economy. I was referring to the vacuum retard capsule arrangement used on Stromberg engines in the USA for emissions purposes that you dont want to copy. Keith on Sidedraft_central has used vacuum advance on his Weber Elan.by rigging up a tapping of each carb throat and connecting them together to get a smoothed average vaccuum.

cheers
Rohan