checking intake vacuum

[vtwinsport]

Thought just occurred to me, I always check manifold vacuum off the non ported vacuum stub at the base of the carb. So, today I checked the vacuum off the ported stub (opening to distributor vac advance).
Non ported reads steady 22. Ported reads steady 13.

My question is; should one be checking intake vacuum at idle off the non ported or ported stub? I shouldn't be second guessing this but I've never asked anyone which is right.
thx for your replies

[enaberif]

Originally Posted by vtwinsport

Thought just occurred to me, I always check manifold vacuum off the non ported vacuum stub at the base of the carb. So, today I checked the vacuum off the ported stub (opening to distributor vac advance).
Non ported reads steady 22. Ported reads steady 13.

My question is; should one be checking intake vacuum at idle off the non ported or ported stub? I shouldn't be second guessing this but I've never asked anyone which is right.
thx for your replies

Manifold vacuum and carb vacuum are different.

Carb vacuum will generally fluctuate depending on where your foot is on the pedal where manifold pressure is always a constant number.

If your trying to get an overall idea of vacuum in your motor then you need to find the manifold port to hook up the vac gauge and is usually behind the carb and you screw a fitting into it if one don't exist.

What exactly are you trying to get a reading of vacuum for?

[vtwinsport]

Hey thx for the reply. Well, I've been tinkering with the timing. Originally st at 8 btdc, moved it to 12 after reading some threads online and reading the plugs. Trying to find the best advance with the most vacuum using the non-timed (right port on 1406 edelbrock) vacuum I presume. I've been doing this in conjunction with tuning the carb, trying to get the stock 350 crate to rev easier. Seems constricted, wheezy.

Found this related bit on advance in a search here;
http://67-72chevytrucks.com/vboard/s...d.php?t=274806

[ocbaud]

ported should normally read zero at idle.
manifold vacuum sounds good at 22, i can't get above 16 on mine

[vtwinsport]

ocbaud, you have a higher lift cam in there? longer duration? mine is stock and only 1500 miles on it. Might explain the higher vac.

[ocbaud]

Originally Posted by vtwinsport

ocbaud, you have a higher lift cam in there? longer duration? mine is stock and only 1500 miles on it. Might explain the higher vac.

yeah, mine is modded slightly. comp 270H magnum cam