Yet another topic guaranteed to spark (pun intended) a lively discussion.
From a wiki source on vacuum advance:
"One source for vacuum advance is a small opening located in the wall of the throttle body or carburetor adjacent to but slightly upstream of the edge of the throttle plate. This is called a ported vacuum. The effect of having the opening here is that there is little or no vacuum at idle. Other vehicles use vacuum directly from the intake manifold. This provides full engine vacuum (and hence, full vacuum advance) at idle.
On some vehicles, a temperature sensing switch will apply manifold vacuum to the vacuum advance system when the engine is hot or cold, and ported vacuum at normal operating temperature. This is a version of emissions control; the ported vacuum allowed carburetor adjustments for a leaner idle mixture. At high engine temperature, the increased advance raised engine speed to allow the cooling system to operate more efficiently. At low temperature the advance allowed the enriched warm-up mixture to burn more completely, providing better cold-engine running."
And I lifted this from a 2009 post from a bsmith123 on another forum:
"When emissions became a priority to vehicle manufacturers, a method had to be found to reduce emissions at idle. The amount of Hydrocarbons emitted out of the tailpipe can be drastically altered by changing the timing: Retarding the timing reduces Hydrocarbon emissions. But retarded timing adversely affects gas mileage at cruise. So a method was needed to retard timing at idle, yet maintain it at normal levels for cruise. The solution was seen to —turn off“ the vacuum advance at idle, yet have it operate normally under all other operating conditions. To do this, a small hole was drilled in the carburetor throttle body just above the position of the throttle plate at idle (NOT in the venturi area), and this hole was connected to a vacuum nipple on the carb. When the throttle plates are closed at idle, they act as an —off“ switch to block the drilled hole from manifold vacuum. As the throttle plates are opened up, the hole becomes fully exposed to manifold vacuum, and normal manifold vacuum is realized at the nipple. Thus, you have a manifold vacuum —on-off“ switch, turning manifold vacuum —off“ at idle, and restoring it to normal operation once
the throttle plate is cracked open. Vacuum advance can be eliminated at idle for good emissions, and instantly restored to normal operation at cruise. At both cruise and Wide Open Throttle (WOT), manifold vacuum and ported vacuum are exactly the same: There is high vacuum at cruise, and virtually no vacuum at WOT. The difference in vacuum occurs only at idle."
So the debate continues!
Hey Guest!
Hey - did you know if you click on the title of a thread it will take you to the first unread post since you last visited that thread?
but were afraid to ask: 
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smilie in place of the real @
Pretty Please - add it to our Events forum(s) and add to the calendar! >> 
