A continuous hole as Glyn describes makes a lot of sense from the manufacturing point of view. The manifold is a very complicated casting.

Anyway, the engine can run on one carb at idle. I'll guess that the only reason for balancing the air flow through the two carbs at idle is to facilitate subsequent adjustment of mixture strength by piston lifting.

 

That mysterious lever is part of the "hold brake" system in automatic cars. It was associated with a solenoid valve fitted to the rear brake lines
Brakes on to a stop and engine at idle the system stopped creep by keeping brake pressure on until the accelerator is pushed again and brake pressure is released courtesy of this switch.
Heath Robinson would have been proud of this idea. Most automatic owners disabled this system after the solenoid valve or carbie switch stuck the rear brakes on.

I find the discussion over carbie tuning at idle a bit strange. Both carbies are involved and if you run one float bowl dry it will only attempt to run on 3 cylinders being fed by the other carbie.

 

Ideal set up! Otherwise other 3 cylinders will idle very lean.

 

Making two into six intake manifolds distribute fuel evenly is very difficult. One into six is even worse. It's likely that operating on one carburetor would result in the cylinder(s) furthest away running weak. Glyn may have been very wise in not going into a porting exercise as improving the air flow can have a negative consequence for fuel distribution.

There were experiments with three twin choke SUs and also with two SUs feeding a plenum that in turn fed long intake runners. Apparently, they all worked very well. Presumably, cost or packaging ended the development of both.

 

My car has this arrangement.

 

Glyn
The same as the dozen or so Mk1 and MK2 HD6 carburettors setups I have handled in my Jaguar experience.

 

I don't believe the British really understood the dynamics of air/fuel flow, this is plain to see in their intake manifolds on the Jag and Roll's Royce's inline 6.
They're all too short with too many hard angles for things to flow properly; yeah, it works, but not very well, it makes the engines difficult to tune and with the manifold being so short they have a tendency to cool the fuel charge off, quenching the combustion chamber temperature.
In the end, I guess the engineers had to make compromises to fit things under the hood.
I'll post a photo of the RR intake manifold later, it's just a disaster.

This manifold from a Dodge slant 6 is so much better, one can see the attempt to make all the paths to the head the same distance _ no sharp corners except where the carb is bolted onto the manifold.

 

But does not have that restriction in the centre of the manifold.

 

Yes, there is a balancing port internal to the manifold that allows the vacuum to equalize inside the manifold. That also allows a greater vacuum flow rate for the power brake booster and reservoir. Uneven flow rates on the carbs at idle would create vibration and torsional vibration in the crankshaft. The port allows the flow to equalize at idle.

Later EFI V12's also have a balancing pipe and the idle air circuit is only on one bank, and the crossover feeds the other bank at idle. Off idle each bank breathes through it's own throttle plate.

 

your explanation seems reasonable... but i wonder how they get around the "...torsional vibration in the crankshaft..." in engines with dedicated carbs and intake manifolds for each cylinder. not every engine is designed with a monolithic intake manifold, i don't think, that even presents an opportunity for balancing. or is it that the engine vibration mentioned is not universal and possibly applicable to a subset of all multi-cylinder engines?

 

 

This I cover in post number 20.

 

You use a flowmeter and balance each carb so it is sucking the same as all the rest. That's how it should be done on a Mark 2 as well, but the hole allows for minor variances and inexactness in the balancing.

 

thanks,

on the multicylinder engines with dedicated carbs and intake manifolds i'm familiar with and even on some without dedicated carbs and intake manifolds, the recommended procedure for balancing is often to ensure, by sight, that the butterflies are all closed when the their throttle shafts are clamped together. ...not as difficult as it might sound. but then again, i don't think that ALL the carbs i'm referring to had dedicated idle circuits like my HD6's that supply fuel and air at idle with the butterflies closed, either.

and i often wonder if when vacuum is measured a leak wouldn't be masked and the observer would never know.

 

Maybe we cast the inlet manifolds in South Africa. We are known to make improvements to Toyota, Mercedes etc. to make them more market appropriate. We have some damn fine engineers.

 

... or the plastic tube, one end in the carb inlet, the other in your ear. Adjust until the turbulence noise is the same intensity from both carbs. It may seem crude, but it works very well.

 

It does indeed. I have balanced the carbs on an E Type with rubber hose method and then used a flowmeter and the results were perfect. You have to be very consistent where you put the hose to listen the sound and do it repeatably on each carb.

 

It's very sensitive with SUs because the air velocity through the closed throat is high. I don't remember ever trying it with fixed throat carburettors and it might not work so well.

 

Works fine on twin sidedraught Weber Carburettors as does plug wire pulling for equal rev drop. I used to pull plug wires on my ALFA's.

Note ~ plug pulling. The front cylinder requires you to pull off the distributor vacuum feed and push a cap over the nipple