Vacuum question

[rockyrivermark]

Small block

I have a vacuum reading of 20 on the manifold
I have a vacuum reading of 10 on the carb ports.
Checked two ports to verify.
Should the carb and intake vacuum be the same?
If so I'm guessing carb gasket?
Thanks
Mark

[Wrenchbender Ret]

No they are not always the same. Most outlets on the carb are ported vacuum. They are on the upper side of the throttle valves. They are used often for emission control items. Sometimes the ported vacuum is used for dist advance. That varies with different engines. At part throttle about 2-3 k RPM's they will be near the same.
George

[Wrenchbender Ret]

They will not be the same. Most outlets on the carb are ported vacuum. They are used mostly for emission control devices. Some engines use ported vacuum for dist. advance. They will be near the same at no load part throttle.
George

[rockyrivermark]

Oh. Got it. Thanks!

[68 P.O.S.]

Correct, manifold and carb may not always match. One factor is the condition of your carb. With the vac gauge still connected to the manifold vac inlet on the carb, adjust the idle mixture screws for best vacuum. Hopefully your vac will get a little higher than 10" at the carb.

[davepl]

Any ports BELOW the throttle blades should match the manifold, though. Any ports above, will depend on how they're routed internally (like ported vacuum for the dirt).