Thread: Manifold vacuum

I finally found the carby I've been looking for, for a 350 for one of the HK GTS's in the shed. It is a 7029207 which is the carb for a 1969 L46 – the legendary Corvette 350hp 350.

These were manual only.

Just had a question that I could probably find if I lookhard enough. Is the full vacuum port on these early carbs (for dizzy advance) juststraight manifold vacuum? Manuals are different, but can I just use anymanifold vacuum port for full vacuum for the dizzy on an auto?

As this is a manual pre-canister carb, the vacuum port is ported vacuum so no good for an auto. What I meant was can I use a port off the manifold for an auto engine’s dizzy vacuum advance? Or is it best off the carb?
This carby needs a rebuild so I could rebuild it using the throttle base off an auto carby but with a rarer carb like this best to keep it all together.

Full manifold vacuum...

Ported vs. Manifold Vacuum

Originally Posted by EH179

Full manifold vacuum...

Ported vs. Manifold Vacuum

Good read, thanks heaps for the link.

But it doesn't answer my question. I know I need full manifold vacuum. But the carby I have is a manual carby and only has ported vacuum on the vacuum advance port on the front of the Quadrajet.

Can I use as a vacuum source a fitting attached to the manifold? Or is there a science to why the automatic car's vacuum advance is obtained from the carb (which in turn just gets it from below the throttle plates)? What I mean is the vacuum obtained from a manifold runner compromised as it is too close to a particular valve, and is this why it is taken from under the carby?

I can't see why you couldn't run it off the vacuum tree in the manifold runner.

The other option is a spacer with a vac' port.

From memory all vacuum advances I've had took their vacuum from just under the carb.
But in saying that I'm not sure whether "full vacuum" would be under the carb or further down an intake runner?

Originally Posted by guy incognito

Just spit balling here but it is possible that the higher velocity flow directly under the throttle plate past the vac port allows the vac advance to react quicker to changes in throttle opening. The plenum also helps dampen pulses by virtue of volume.

Also spit balling but you might find EFI tuners have had issues depending on where the Map sensor is located in the manifold, could be something in that.

Vac location is not terribly important for brake booster or modulator but "might" matter for the advance operation of the distributor.

Again, just spit balling, me, I would use a spacer or drill the carburettor (or plenum) because I'm not too bright and would rather copy someone who is

Thanks, this is sort of what I suspected too.
Can’t use a spacer, has to fit under a HK bonnet. It is a near concours resto on the oldest known surviving HK V8 GTS so it has to stay body untouched. This is why i’m sticking with straight single carb SBC, TH350 and 9” as no body mods are required on a 5litre, Powerglide and banjo body.

Unless you are using a high rise intake (which I doubt on this car) you will get a small spacer in there easily I would have thought.
Mate has a Victor Junior intake under his HK hood, just.

Cool, will have to wait and see. Air cleaner will be a dual intake Corvette one on a Quadrajet.

Got the 7029207 Quadrajet today, and had a big DOH moment. The 1969 350/350hp Corvette had the latest you-beaut electronics dizzy, and no vac advance! The carby has the vac advance location at the front but it isn’t drilled. So will need a thin spacer under it to create an advance point.

Why not just drill the casting?