Dizzy vaccuum advance for triples

[parisian62]

Hi everyone,

Should the dizzy vaccuum advance go directly to the centre carb or into the manifold...or does it really matter? Running triple strombergs.

thanks
Stewart

[choppedfan]

Hi everyone,

Should the dizzy vaccuum advance go directly to the centre carb or into the manifold...or does it really matter? Running triple strombergs.

thanks
Stewart

Got mine going into the centre carb

Vacumn through the throttle body of the carby is different to manifold vacumn.

Mick

[Harv]

Stu,

Micks right - the manifold and centre carb are two different things.

The original B-Model carburettors used on early Holdens had a vacuum connection (a steel line) that connected from the carburettor, snaked it’s way around the rocker cover and connected to the distributor vacuum advance. This connection needs to be replicated in multiple carburettor setups. When thinking about this vacuum line, it is important to realize that there are two types of vacuum commonly tapped off a fuel system:
• Manifold vacuum (sometimes referred to as full vacuum), which is used for brake boosters and vacuum wipers. It is tapped off from the inlet manifold, or on the carburettor throttle body at a position below the throttle plates. You get more manifold vacuum when you take your foot off the throttle (this is why pre-EK vacuum wipers work so well when you lift your foot, but run poorly when you have your boot into it driving uphill in the pouring rain!).
• Timed-spark vacuum (sometimes referred to as distributor vacuum) is taken from above the throttle plates. Timed spark vacuum is exactly the same as manifold vacuum – except that it is shut off under zero throttle (i.e. under idle conditions, there is huge manifold vacuum, but zero distributor vacuum). The strategy behind distributor vacuum (generally used in later-model carburettors) is to remove vacuum advance at idle, causing the vehicle to run hotter and combust exhaust emissions (often with the help of air injection systems at the exhaust manifold).

Early Holdens were originally designed to run timed spark vacuum (the vacuum port connection is at the throttle body above the throttle plate). There is no harm in running distributor vacuum with multiple carburettors by tapping into one carburettor and blocking the other two carbs off (plugs are available over the counter from the Carburettor Service Company on Parramatta Road). Tapping into all three carburettors using tee-pieces is absolutely unnecessary.

However, for cars with large cams (high valve overlap and poor vacuum), tapping into manifold vacuum (and blocking off the distributor vacuum ports on all three of the carburettors) can give better vacuum signal at idle, more advance and hence better idling. This can also reduce engine temperature at idle.

Cheers,
Harv

[parisian62]

Many thanks Mick and Andrew for the explanation. I've got mine running into the centre carb. Just making sure I did it right. I must have seen some pics of FB's with vaccuum wipers and early Holdens with power brakes that had me wondering...

Stewart

[Craig Allardyce]

I would definitely go the centre carb connection (ported vacuum). You'd probably have to get the vacuum advance pot on the distributor recalibrated (lighter spring tension) to compensate for some vacuum loss as your now drawing the same CFM of air across 3 carbs and as well as whatever change in camshaft profile.
Under no circumstances would I connect it to the manifold.
I posted on the subject a couple of times regarding the subject of vacuum advance sourced either from the manifold or carby.
Happy reading.

http://www.fbekholden.com/forum/viewtop ... 45#p171945

Cheers

Craig.