The vacuum port on the base of the carburetor does not provide manifold or ported vacuum. What source of vacuum do you all use for the distributor vacuum advance?
Thanks, Douglas
Thanks, Douglas
Weber 32/36 Vacuum
- Thread starter DougE
- Start date
The 32/36 on my NK has ported vacuum
tdgray
Well-Known Member
The vacuum ports are on the primary side of the carbs. You tie the two together and then route to the distributor via a T
Mike Goble
Well-Known Member
I use full manifold vacuum for my vacuum advance. Better idle, cooler running and less dieseling.
yup. I use same port on my 32/36 set up.
Thanks for your responses regarding the vacuum advance port. I have been using the port in the carb base, which is the manifold vacuum. There is an argument by some that I should be using port vacuum (above the throttle plates) rather than manifold vacuum. Port vacuum is zero at idle, and increases during acceleration. While manifold vacuum is highest during idle then disappears once the throttle plates are opened. Thoughts?
tdgray
Well-Known Member
That’s exactly why you use it the way it’s supposed to… advance at idle is essentially zero. I think I have my distributor set at 5 degrees at 1000. I’d have to look again.
Port vacuum does not increase during acceleration. Manifold vacuum does not disappear once the throttle is opened.
AI says it better than I did....
The core difference between ported and manifold vacuum lies in where the vacuum signal is measured inside your carburetor: [1]
- Manifold Vacuum: Sourced below the throttle blades, delivering constant, high vacuum at idle.
- Ported Vacuum: Sourced above the throttle blades, delivering zero vacuum at a closed throttle. [1, 2]
Both operate identically once the vehicle is cruising or accelerating; the key variance is how they affect your ignition timing while idling. [1]
Manifold Vacuum
Connecting your distributor’s vacuum advance to manifold vacuum pulls in maximum timing advance at idle. [1, 2]
- Pros: Enhances idle quality and smoothness, lowers engine temperatures (which helps prevent overheating), and increases off-idle responsiveness.
- Cons: Can mask mechanical timing issues or cause hard starting on high-compression engines if initial timing isn't properly adjusted. [1, 2, 4, 5]
Ported Vacuum
Connecting to ported vacuum means the vacuum advance is effectively inactive while your foot is off the gas. Timing remains at the base setting until you open the throttle. [1, 2]
- Pros: Prevents idle instability on radical, heavily modified camshafts and lowers idle emissions (which was its primary purpose on mid-to-late 1970s vehicles).
- Cons: Removes advance at the exact time it’s needed for efficient engine cooling and low-RPM efficiency. [1, 2, 3, 4]
Which One Should You Use?
For most classic and street-driven vehicles, manifold vacuum is generally preferred because it provides a cleaner, cooler-running idle and better low-end efficiency. Ported vacuum is often utilized as a "band-aid" for radical camshafts with erratic idle vacuum, or to pass specific emissions tests. [1, 2, 3]