20 replies to this topic

[Dusky]

Hi!

 

Where do you take the vacuum signal on twin carb setups? Connect both carbs to the vacuum pipe? Or just take it from one carb?

Would taking it from 1 carb cause inbalance?

 

 

thanks!

Diego

[nicklouse]

Makes very very little to zero difference.

SUs tend to have it from the balance pipe. Webers from one barrel only.

It is not a flow of air.

[KernowCooper]

Standard setups use one, not a issue really and long as its not taken from the engine side of the butterfly as engine vacuum is not nrmally used, but BL/Rover did try it on the MG Metro I think to try and clean up emmisions if I remember correctly.

[nicklouse]

Standard setups use one, not a issue really and long as its not taken from the engine side of the butterfly as engine vacuum is not nrmally used, but BL/Rover did try it on the MG Metro I think to try and clean up emmisions if I remember correctly.

Err most are taken off the manifold so are engine side. And again makes little or no difference.

[nicklouse]

In fact all are taken engine side. If they were not there would not be a partial vacuum as it is the butterfly that allows it to happen.

[timmy850]

Are we talking vacuum gauge or vacuum advance? 

[Guess-Works.com]

Does not matter, as the vacuum only exists between the piston (in the engine) and the butterfly in the carb or throttle body, anywhere else there is no vacuum.

Edited by Guess-Works.com, 19 March 2015 - 09:24 AM.

[timmy850]

On my HS2 the vacuum advance take off was on the piston side of the throttle plate. This meant I didn't get a proper vacuum gauge reading at idle..

There should be manifold vacuum at all times other than full throttle at full load

Edited by timmy850, 19 March 2015 - 08:53 AM.

[Dusky]

Forgot to add: its indeed for the vacuum advance of the dizzy :)

[nicklouse]

Forgot to add: its indeed for the vacuum advance of the dizzy :)

makes Zero difference what it is for. if you want vacuum then between butterfly and engine.

[Mini Manannán]

Standard setups use one, not a issue really and long as its not taken from the engine side of the butterfly as engine vacuum is not nrmally used, but BL/Rover did try it on the MG Metro I think to try and clean up emmisions if I remember correctly.

 

Kerching!  That would explain things to me, we've been thinking my engine is a MG Metro engine and that is another pointer in our theory's favour.  My HIF has the vac advance engine side of the throttle disc and gives advance on tickover.  Since we disconnected the vac advance the economy has gone up hugely and the engine is quieter at the expense of a bit of bottom-end driveability.  I'm gonna reposition the vac advance upstream of the throttle disc this weekend.

[ucchockey]

The take off on my twin su carbs is on the manifold. Only on one side.

[KernowCooper]

You chaps need to read up on manifold vacuum and after the butterfly vacuum, it does indeed make a difference,as most distributors are not designed to read full manifold vacuum.

 

The reason being is that a distributor designed to read vacuum after the butterfly, reading full manifold vacuum as soon as the throttle is open the vacuum drops off enough to only allow the distributor on a slightly open throttle to drop the vacuum advance and go back to the mechanical advance thus costing torque and mpg on a light throttle. Just watch your vacuum gauge which is fitted and subject to full manifold vacuum drop off at the slightest bit of throttle, this does not happen on the otherside of the butterfly. Full manifold vacuum can also advance the timing at idle enough to cause a very rough idle or a misfire

 

All covered in detail by Des Hammill in his book "Ignition Systems"

[Dusky]

I can vouch for des hammil, he has an excellent book about webers too by the way.

So I know vacuum is more of a signal and not an airflow, but connecting 2 carbs, would that make the signal twice as strong?

Or would there be a difference in strenght between ( for example) a HS4 and a hs6 carb? Or are the vaccum takeoff holes in the carb adjusted to be equal?

[Dan]

The dizzy should read throttle edge depression. As above Rover did try to use full manifold depression on various Metros and it caused a hunting, pulsing idle. There was a name for the effect but I can't remember what it was.