Weber with throttle air by-passes

[Foxie]

I bought a new set of 45DCOE Webers (Spanish manufacture) a few years ago and between myself and my local rolling road man got them running very well.

However, there is a throttle plate air by-pass port on each barrel, with an adjustable needle valve and lock nut, the purpose of which we did not know, and which we always left fully closed.

In the pic it is the nearest item, just below the mounting stud. There is a channel inside the raised section of the castingleading to the inside the throttle plate. It would seem to be some sort of an idle control, as when the throttle plate is open it would be more or less out of operation.

I was never able to find out information on the purpose of these, or how they were intended be used.

Has anybody come across this ?

[mbell]

I believe there purpose is to
- balance the air flow between the two barrels on the carbs. Even with the throttle plates aligned there can be some variation in how much air can pass the plates in each barrel when fully closed. These allows that difference to be cancelled out
- Ensure there is enough airflow past the throttle plates at idle with out having to open the throttle plate to the first progression hole is exposed and providing fuel.

Previously people would drill holes of varying size through the throttle plates to accomplish the same thing.

[richardcox_lotus]

Yes this is standard on Spanish DCOE151s and is to assist with idling. Am away from my books at the moment but Des Hamill defines it as an idle bypass circuit.

They are normally factory pre-set.

[Foxie]

Yes, that makes good sense !

[oldchieft]

you need vacuum gauge set

.

Jon the Chief

[rgh0]

I believe there purpose is to
- balance the air flow between the two barrels on the carbs. Even with the throttle plates aligned there can be some variation in how much air can pass the plates in each barrel when fully closed. These allows that difference to be cancelled out
- Ensure there is enough airflow past the throttle plates at idle with out having to open the throttle plate to the first progression hole is exposed and providing fuel.

Previously people would drill holes of varying size through the throttle plates to accomplish the same thing.

Yes this is their purpose. Delortos have always had them and it was one of their key improvements on the older Webers when Delortos started production.

cheers
Rohan

[Foxie]

I can manage with a flow meter. It was a bit difficult to read your attachment, but I see it confirms my deduction that only one bleed screw on each carb may need adjustment.

[rgh0]

You may need to open both to get the butterfly positioned correctly with respect to the progression holes to avoid an off idle stumble

cheers
Rohan

[billwill]

You may need to open both to get the butterfly positioned correctly with respect to the progression holes to avoid an off idle stumble

cheers
Rohan

Aha... that might be the cure for a problem I sometimes see. I have Dellortos

[Foxie]

You may need to open both to get the butterfly positioned correctly with respect to the progression holes to avoid an off idle stumble

cheers
Rohan

As I understand it, the relative positions of the two butterflies in each carb will not be changed by adjusting the air pass needle

The butterfly/progression holes would first need to be checked. One good way to check the butterfly/ progression holes is by the torchlight method, as mentioned by a previous poster

[mbell]

As I understand it, the relative positions of the two butterflies in each carb will not be changed by adjusting the air pass needle

Correct. Traditionally you adjust idle speed using a screw that opens the butterflies to allow more air past them. However opening the butterflies too far can cause the 1st progressive hole to be active at idle and often result in a of idle stumble. The butterflies need to be in a fairly specific spot to get the best running,

The idle by pass allows additional air to to by pass the butterflies, so it can be used to ensure the engine gets enough air at idle with out having to comprise the butterfly position or drill holes through the butterflies.

I believe the factory sets these to balance out the two barrels of the carbs to a good level of accuracy. So I wouldn't adjust them from factory setting unless I was unable to get the engine to idle with the butterflies in a sensible position (just visible/covering part of the first progression hole).

[richardcox_lotus]

That's interesting because on my back carburettor I cannot get the airflow at idle down to below 3mg on the #4 cylinder according to my meter. Every other choke I can get to 2.5 but this one doesn't change. I can increase the volume of air but not reduce it. I've not checked the butterfly progression hole settings so maybe I should check they are in sync.

[mbell]

That's interesting because on my back carburettor I cannot get the airflow at idle down to below 3mg on the #4 cylinder according to my meter. Every other choke I can get to 2.5 but this one doesn't change. I can increase the volume of air but not reduce it. I've not checked the butterfly progression hole settings so maybe I should check they are in sync.

The butterfly spindle maybe twisted keeping 4 slightly open when 3 is closed. Should be fairly easy to see if this is the case via torch down the barrel and removed progression hole covers.

If it's not this then maybe the throttle plate isn't quite positioned correctly and not sealing as well as on 3. I probably wouldn't mess with it if it runs fine thou.

I am in the middle of a long battle with the webers on my car to get a good balance and running correctly. I probably should have just bought a new set of 40DCOE 151's and had done. On the positive side it's increased my knowledge on weber carbs significantly!