Timing Marks on the 273 [66 Barracuda]

[RustyRatRod]

Nope, but that is what I used to think also, the vacuum should be highest at idle.
A lean fuel charge burns slower so you need more spark advance, which is what happens when you have a lot of vacuum.




THere is no vacuum advance at WOT only at idle or cruising.

There is zero vacuum at idle on a ported vacuum inlet. Only when the throttle blades are cracked open or at cruise. Vacuum drops off on sudden acceleration, or WOT.

 

[Mattax]

Nope, but that is what I used to think also, the vacuum should be highest at idle.
A lean fuel charge burns slower so you need more spark advance, which is what happens when you have a lot of vacuum.

Yes vacuum is relatively high at idle on most factory (street) engines. Higher vacuum can occur under low throttle cruise and does occur during deceleration.
Yes a lower density charge burns slower.

However, idle should not be lean unless one is trying to reduce CO in the exhaust gasses.
This is why the late 60s-early 70s CAP and CAS systems targeted idle mixtures as lean as 14.2 AFR.
Here's a snip from from page 3 of the 1967 Chrysler Master Technicians Conference 240:

For CAP they also reduced unburned hydrocarbons in the exhaust by retarding the timing at idle. This completed the burn when the piston was further down in the cylinder.
The slight loss in power and efficiency could be compensated with slightly higher idle speed and rapid timing advance off idle.
For a number of years this allowed Chrysler to meet emmissions standards without resorting to really expensive tricks like catalytic converters.

ref: Heat riser affect wideband?

Idle is a relatively high load situation. When the clutch is let out, or the selector placed in gear at idle, the engine is maxing out. If you've ever stalled a vehicle going into first, you;'ll know this is so.
see more here: racingfuelsystems-Desired Carburetor AFR Characteristics At Different % Load

On Chysler products (and many others) the distributor should generally have no vacuum signal at idle.

from Ignition System Analysis (Session 259): 1969 and (session 136) 1959.

However if the throttles are a little too far open, there may be a signal, so its good practice to plug the line. Also other makes, models, years sometimes used manifold vacuum, or had the timing port so close to the throttles that a little signal bleeds over. So again, its just good practice, even though it is not always neccessary.

And since the OP was asking about timing, below is factory timing of 1967 4bbl 273 with and without CAP to illustrate two points:
1. Why non-CAP and CAP distributor have such different initial timing specs.
2. How one can get false initial timing reading when measured at an rpm above idle spec.