MGB & GT Forum

I am currently installing and tuning a DCOE on my B and I was discussing some installation issues with another forum member when I mentioned that the DCOE I have on my Opel GT is ported for vacuum advance.

Since neither of us had ever heard of a ported DCOE before I thought I would post some pictures for people to see. This carb was originally on a 2ltr MGB I had, but I put it on the Opel GT about 12 years ago when I converted it from a 1900cc to a 2400cc engine. At that time I took the carb and manifold to a Weber specialist to have the manifold modified and the carb jetted however I can’t remember if they installed the vacuum port, or if it was originally there but unused.

The carb is a 45 DCOE 152 model 8D made in Italy. From pictures I have found on the web it looks like the 152 has a blank casting surface between the progression hole cover and the idle adjustment screw that may have been drilled for the vacuum port, however I have found one picture of a DCOE with an identical vacuum port making me think that the port was a factory installed on some models.

I’m posting to ask if anyone has ever seen a carb like this before and to see what people think the performance benefits might be of running a vacuum ported DCOE on the street.

1st Pic is my DCOE with the vacuum port nipple in the lower left near the idle screw and progression hole cover, just above and to the left of the choke actuation lever.
2nd Pic is from the internet with the vacuum nipple in the same location.

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1976 MGBegun
2000cc Engine, 85mm x 88.9mm
Big Valve Dual Spring Head
4 Piston Calipers on Vented Rotors
Koni Front Coil Over Shocks & Rear Tube Shocks
Overdrive Transmission
Advanced Distributors 25D W/ Crane XR-700
Weber 45DCOE9

"Out of the crooked timber of humanity, nothing entirely straight can be built."
Immanuel Kant

A few more pics of my DCOE

Interesting. I don't recall seeing that, but the carbs normally found on an MGB are the DCOE 9 and DCOE 13. I don't know enough about them to know how the other variations differ.

I run mechanical advance only (45D) with my DCOE 13 and get the same mileage I used to get with a vacuum advance 25D....

That's interesting. I figured they would have put the port on the bottom of the carb and there's nowhere down there to mount it. There's potential for better fuel economy, but you'd have to run a manifold type vacuum advance to avoid pinging at freeway cruise speeds if you have a 5-speed or overdrive. With just a 4-speed it wouldn't present a problem 99.9% of the time. Cool!!!

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jeff@advanceddistributors.com

ok, plug up that port and tap two on the manifold to act as a balance tube, use a tee fitting to connect both manifold ports. have a look, enlarge picture for a better look.



Edited 1 time(s). Last edit at 2009-02-25 09:54 AM by Jerry.

I agree with Jerry that the vacum needs to com from both sides. Also need to have the proper distributor for where the vacum is taken from.

I would expect the small tube diameter that matches your vacum source to set up a fair bit fo chatter in the distributor advance plate . The vacum value will "pulse" with no volume in the tube to attenuate the pulse on the way to the distributor.

Peter

Being upstream of the throttle plate, and having such a small diameter hole should eliminate most of the chatter problem. If air was reverting back into the carb like that, your air filters would get soaked with fuel and the car would burn to the ground. Chattering happens in the crossover tube of a stock MGB manifold. I've never seen it happen with ported vacuum.

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jeff@advanceddistributors.com

Jeff:

I was not thinking of any kind of reversion.

I was thinking more along the lines of the air stream entering the carb having a frequency component. Since it is a 4 stroke engine, you are not going to have a continuous pull on the intake. Having separate runners would make this worse no?It might be insignificant, but I have seen small canisters used in the metal vacum line to create a volume in the line going to the distributor. I have this arrangenet on my bugeye and it came from the factoty that way.

Would the crossover tube not serve to eliminate the pulsing effect of the vacum signal? The large volume alone should attenuate any pulses, or am I missing something here?

Pete

No need to add a balance tube because the Opel manifold that the carb is currently running on has a semi circular design (like the Lynx manifold) that incorporates an internal cross over (pic 1), and it's been running smoothly with the vacuum advance distributor for 12 years or so.
I have a 45 DCOE 9 going on the B right now (pic 2) to use with a mechanical advance distributor from Jeff.

I just thought it was interesting to know that a DCOE with ported vacuum does exist and since the DCOE has a bad reputation for giving horrible mileage on the street I thought someone might be interested in this as a possible solution. I'm not sure there is any significant advantage to triggering the advance one way or the other, but one of the first things you Weber detractors say is that the DCOE gives horrible mileage and you need to spend a bunch of money for a new all mechanical advance distributor, and it seems like that may not always be true.

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1976 MGBegun
2000cc Engine, 85mm x 88.9mm
Big Valve Dual Spring Head
4 Piston Calipers on Vented Rotors
Koni Front Coil Over Shocks & Rear Tube Shocks
Overdrive Transmission
Advanced Distributors 25D W/ Crane XR-700
Weber 45DCOE9

"Out of the crooked timber of humanity, nothing entirely straight can be built."
Immanuel Kant



Edited 1 time(s). Last edit at 2009-02-25 03:14 PM by MT-B.

Oops here is Pic 1 of the Opel manifold

Those plugs are for balancing. When you have twin Webers, you can connect up 4 vacuum gauges in a row(same as a motor bike) to balance the carbs. Those webers in the picture have idle bypass screws(One's with the 8mm locknuts) to balance the airflow through both chokes.

The 151(40 DCOE) and 152(45 DCOE) are "Universal performance" type DCOE's... Weber make these now instead of making 200 different specs of DCOE.

The best DCOE for a MGB is a 9 or 13.