vaccum advace ?
Posting Guru
Joined: Aug 2000
Posts: 1,127
Likes: 3
From: (near) Denver USA
Ideally, there should be little or no vacuum on the carburetor spark port at idle. As soon as the throttle shaft rotates a few degrees (off idle), spark port vacuum should jump to near 10 in/Hg, giving you significant vacuum advance.
You should observe little or no change in total ignition timing at idle when disconnecting vacuum advance, if:
1. The vacuum advance is installed correctly and working (test it by sucking on the advance diaphragm?), and
2. The carburetor is tuned correctly, and
3. The engine is not on its hot idle compensation (HIC) system (or you have HIC disabled).
The HIC system on most '70s engines switched the vacuum advance to manifold vacuum to speed up idle when the coolant temperature reached 210-220F.
You should observe little or no change in total ignition timing at idle when disconnecting vacuum advance, if:
1. The vacuum advance is installed correctly and working (test it by sucking on the advance diaphragm?), and
2. The carburetor is tuned correctly, and
3. The engine is not on its hot idle compensation (HIC) system (or you have HIC disabled).
The HIC system on most '70s engines switched the vacuum advance to manifold vacuum to speed up idle when the coolant temperature reached 210-220F.
