Unidentified Port on 40DCOE

[Galwaylotus]

Folks, can anyone identify the port shown in the attached photos? It's the one with the light shining through it. The other photo shows the screw that goes into the port. It's the one between and to the left of the Idle Mixture screw and the Pump Jet Cover plug. It looks as though it has a hole bored through it bit, in fact, it's a blind hole going nowhere. I can't find the screw on my parts list and I wonder what its function is. Could this port be used (with the proper adapter) to balance the carbs? There are two of these ports per carb. Thanks.

[bill308]

Is the hole in question the large diameter one?

I believe this is the discharge end of the choke or cold running circuit.

Bill

[Galwaylotus]

No. It's the one with the light shining through it (just below the large diameter hole in the photo) and from the other side it's the one with the screw head that has a hole in the centre!

[bitsobrits]

It's a vacuum port, I believe. Some series of DCOEs have these to facilitate installation of vacumn advance distributors.

[bill308]

bitsobrits,

I think you are correct in that it is a vacuum port. This may also be a balancing aid where manifold vacuum can measured and compared cylinders-to-cylinder with the aid of a gage or manometer, hose, and a hose barb. Some later Dellorto DHLA's have this feature.

Bill

[Esprit2]

Most vintage DCOEs do not have vacuum ports to connect a manometer for tuning; but I've read that a very few did. I've never seen one before, but it appears that you have one. What is the model number?

Now what you need are the screw-in hose spigots for making the connection to the manometer. The Carbtune II comes with M5 and M6 thread spigots, and they can be purchased separately. What size is the thread on the plug screw? I've made spigots by starting with thick-wall K&S brass tubing with the same OD as the needed thread. Cut off about a 1" length, thread one end for about 3/8", screw on a nut to form a stop shoulder and Loctite it in place, then install a thin washer and small O-ring below the nut. Screw it into the port.

The more modern DCOE -151 and -152 both have manometer vacuum ports, as well as air bleed screws.

Even if the port in question is there for some other purpose, it is in the right place for a manometer vacuum connection. Use it.

Most Dellorto DHLAs have both manometer vacuum ports and Air Bleed Screws (very few don't). That's one of the DHLA's several advantages over the typical DCOE.

Regards,
Tim Engel

[Galwaylotus]

My car came with twin 40DCOE 33 carburettors. I'll probably use my Unisyn to balance the carbs. I just wanted to know what is the purpose of those ports as the screw seems to blank them off. I also own, from my bike days, a mercury 4-tube manometer so I have options.

[Quart Meg Miles]

I fitted 151s some years ago and recently purchased a quad-vacuum gauge from Busters for less than ?55 delivered. The mechanical zeros were all over the place but once corrected (after unscrewing the clear covers and tweaking a screw) all four read exactly the same at 14 psi vacuum on a test rig. The tubes have M5 threads which screw straight into the Webers' monitoring points where Galway shows his mystery ports.

Since using the gauge the car idles on four cylinders, something I can't remember hearing for decades, and it's the best ?55 I've ever spent.

Here are two pictures showing the synchronism between gauges and the best balance obtained (which is better than shown as the needles are jumping a little. Note that zero is the wide white bar 90? clockwise from the needles). Initially the gauge showed up 2 psi imbalance between the halves of each Weber.

Gauge synchronisation

Best Weber balance

I am just a customer of Busters who are primarily motorcycle people.

[Esprit2]

Galway,
Do you have any idea which car the 40DCOE-33 was originally intended for? Anyone? Just curious.

Meg,
I've used a 4-tube mercury manometer and currently have a Carbtune II. I've not used a 4-gauge unit like yours, but I think the key point is connecting to the vacuum ports on the carb.

An airflow meter like a Synchrometer or Uni-Syn affects the very airflow it's trying to measure, while the ports tap the throat vacuum without intruding into the stream. It's so much easier to get an accurate reading with a 4-tube/ 4-gauge set-up via the ports. I encourage anyone who is wondering what to do with their tool budget to invest in a 4-tube set-up.

Spot-on balance makes a huge difference in how well the engine runs, especially at idle.

Regards,
Tim Engel

[Quart Meg Miles]

...the key point is connecting to the vacuum ports on the carb.

An airflow meter like a Synchrometer or Uni-Syn affects the very airflow it's trying to measure,

On my first rebuild, in 1970, I drilled and tapped holes in my manifold but only had a single gauge and crude fittings so it didn't really help the setting up. Tapping M5 holes into the manifold tubes is a simple job but make sure you have a secure seal with a screw, washer and sealant for normal running.

It's important there aren't any leaks from 'O' rings and the headlamp vacuum take-off etc and compression needs to be good too, all of which are not so much a problem with the flow method, but I couldn't get any flowmeters to work as they stalled the engine.

[Esprit2]

The inlet flow is really complicated, with pressure pulses, flow reversion, boundary layers, and all sorts of stuff that might surprise you. The best location for the manometer vacuum port is as close behind the butterfly as possible, which is why they appear on the carbs.

I've seen guys drill & tap the soft mount spacer (it's close to the carb) as well as the manifold runner. But if you have the choice, a carb port is best. If you drill the manifold runners, be careful to tap each one an equal distance from the carb mounting flange in order to be at similar points in the flow field.

Regards,
Tim Engel

[Galwaylotus]

Galway,
Do you have any idea which car the 40DCOE-33 was originally intended for? Anyone? Just curious.

I don't, Tim, and I have looked.
I'm not sure I can trust some of the information that came with the car, e.g. there is a dyno sheet that says it has 30mm venturis whereas I've measured them at 34mm and the pump jet is a 35 whereas the dyno sheet says 45.

[types26/36]

Do you have any idea which car the 40DCOE-33 was originally intended for? Anyone? Tim Engel

The Weber DCOE 33 was original equipment on the Alfa Giulia 1600 Super and 1600 Junior Z Giulia according to my references, I have not found it listed for any other application.

[Galwaylotus]

Thanks once again, Brian!

[Esprit2]

Brian,
Does your handy little list also indicate any other DCOE tipo numbers that also have manometer vacuum ports?

Thanks,
Tim Engel