Technical Ported vs "common vacuum" advance.

See my last comment

Ben

 

Well actually....

This is for my 261 on a 123Ignition module.

 

Personally I have always had a better lower idle on venturi vacuum. When I tried manifold vacuum I could not get a nice smooth idle in the 500 rpm range.
When I purchased a Tom Langdon ignition for my sons GMC 6 we could not get a repeat idle on manifold which Tom recommended. I moved it the venturi and it smoothed right out. 10-1 compression, 3-2’s idling on all 3, and Howard cam.

 

At idle you are barely filling the cylinders, so you need a lot of advance to ignite at the right time.
If your vacuum can doesn't do it, because it sees no vacuum, than what does it?

I don't know about your motor (cam/carb etc.), but I saw al lot of carbs that are "idling" on the transfer slot cause the idle circuit doesn't cut it anymore with just a few degrees of advance at that rpm.

Also a lot of folks just think they are using ported, cause the port (outside) is above the throttle, but internally that port goes down below and sees full vacuum, like on the old Rochesters. It's easy to find out - just pull that hose while idling, If it is really ported, nothing should happen.