Which vacuum port for dizzy?

[Chris Pucci]

Standard Holley distributor 345 v8 application circa 1971.

My carb offers me both 'timed' and 'full' vacuum options.

Until about 3 days ago I didn't know there was a difference...


Prior to me monkeying with it - it was hooked to the 3 port vacuum "tree" on the rear drivers side of the manifold. In my infinite wisdom I figured it should be attached to the base of the carb since they gave me the ports there... :d

thanks in advance (pun intended... )

 

[Michael Mayben]

standard Holley distributor 345 v8 application circa 1971.

My carb offers me both 'timed' and 'full' vacuum options.

Until about 3 days ago I didn't know there was a difference...


Prior to me monkeying with it - it was hooked to the 3 port vacuum "tree" on the rear drivers side of the manifold. In my infinite wisdom I figured it should be attached to the base of the carb since they gave me the ports there... :d

thanks in advance (pun intended... )

Connect the distributor vacuum advance directly to the "timed" or "ported" (interchangeable terminology) vacuum connection on that edelbrock. Looking at the front of the carb, that will be the fitting to the left of the center vacuum tap which is for connecting pcv. That vacuum source is above the throttle plate in the passenger side primary venturi, thus it's referred to as a timed or ported vacuum source. At idle speed, with the carb idle mixture adjusted correctly, it should show a vacuum level of between 0"hg and about 4"hg. The vacuum measurement at that point will then increase as the throttle is opened, thus actuating the distributor advance...depending upon which advance canister that distributor has, it will begin to actuate at around 5>7"hg if functional.

That "three port vacuum tree" you describe is the thermal-control vacuum switch which was an emissions system component. That switch was fed from a ported vacuum source on the oem carb.

Older sv engines (some) had an actual "vacuum tree" that was connected directly to a manifold vacuum source.

 

[Chris Pucci]

That "three port vacuum tree" you describe is the thermal-control vacuum switch which was an emissions system component. That switch was fed from a ported vacuum source on the oem carb.

Thanks!

The above makes since. When it arrived it had both carb ports and the dizzy plumbed to the 3 port tree.

I will make the correct connection. It didn't make sense to use the full vacuum port as it was making 'full' vacuum at idle -- which quickly pulled the vacuum advance in and bumped things up to 25-30 degrees at idle...

 

[Chris Pucci]

I tested both ports - and per the carb setup instruction sheet and mayben the passenger side port made 0", while the driverside port made about 15" at idle.

I have the distributor setup on the passenger port and it seems to work fine. No advance at idle, vacuum advance comes in as I rev it up.

Despite my own resistance I am learning something new on this truck everyday....

 

[Michael Mayben]

Agin...major accomplishment!

I'd like to see that manifold vacuum level up around a minimum of 17.5"hg...a "normal" level of 19>20"hg is achievable on a fresh motor with a fresh tune.

Your idle rpm of 700 is just right for that slushbox-equipped motor...if ya do that "power timing" thang once you are able to take it down the highway, then I bet you will end up with a base timing of around 10*btdc on regular grade fuel, then re-adjusting the carb idle mixtures and curb idle peed will get you real close to 19>20!