[Mgb-v8] Carb to dizzy connection

[Bill Zenkus]

Right
Passenger side port is timed (carb) drivers side port on carb is manifold

Sent from Bill's iPhone!

On Oct 19, 2008, at 8:18 AM, "Paul Hunt" <paul.hunt1 at blueyonder.co.uk> wrote:

Are you sure you have this right?  Normally the two sources are carb and inlet manifold.  Inlet manifold vacuum is near maximum at idle, rises slightly as the throttle starts to open simply because the engine gets more efficient, then gradually tails away as the throttle is opened further.  Carb gives zero vacuum at idle, rapidly rising to a maximum as the throttle starts to open, then is the same as inlet manifold  i.e. gradually reducing to zero as the throttle is opened further.  How much vacuum you get *does* depend on engine speed to some extent, but as engine speed is controlled by throttle opening (except on the overrun) it's the throttle that controls vacuum, and the reducing vacuum with increasing throttle is designed to offset the greater tendency of an engine to pink under wider throttle openings by reducing the amount of advance on wider throttle openings. 
 
It's *centrifugal* advance that increases as engine speed increases, and that takes advantage of the fact that as engine revs increase there is a *reduced* tendency to pink, all other things being equal, so it can take more advance to improve efficiency of combustion and hence performance.
 
Which vacuum source is used simply depends on the emissions requirements - inlet manifold vacuum gives lower idle emissions as the engine runs more efficiently with more advance.  However it has a downside that it makes the engine slightly more difficult to start as some advance is applied even during cranking.  Carb vacuum is usually preferred, if manifold vacuum were better for other than emissions reasons it would have always been used.
 
PaulH.
----- Original Message -----

The passenger side vacuum port is ""timed" meaning vacuum increases with engine speed.

The drivers side port is the "full" vacuum port meaning the vacuum decreases as engine speed increases.