Total advance changes depending on load and engine rpm. Vacuum and Centrifugal work together with Initial to keep the engine running smoothly. To set Initial advance:
- Disconnect the vacuum hose from the carburetor to the vacuum advance at the vacuum advance end and plug (golf tee or small bolt works great) the end of the hose. Plugging the hose prevents a vacuum leak to the carburetor.
- Set the idle speed to spec.
- Now set the Initial advance to spec. for the motor. Usually from TDC to 10 degrees advanced (Before Top Dead Center). If you don’t know pick 5 degrees advanced. Do this with a timing light. It is the only way I know to get an accurate initial advance.
- Once the initial is set to 5 degrees or your spec, connect the vacuum hose and you should see the timing advance another 10 to 15 degrees (reading it with the timing light). If you see this happen you can be comfortable that the vacuum advance is working. The centrifugal advance (if the distributor has one and I guess it does) will not be effecting the total advance if you have the engine set to idle speed. If you want to check it, disconnect the vacuum hose and plug it as before.
- Then advance the throttle to mid-range or higher and you should see timing advance another 10 to 15 degrees above initial setting.
- Now that you know it is working you can connect the vacuum hose again and give the throttle some quick bursts and you will see the combination of initial, vacuum and centrifugal and it will vary depending on the engine rpm and how fast you crack the throttle open. If you crack it fast you will see the total go down first and come back up as the rpm’s catch up with the throttle and centrifugal comes into play.
- Points: Vacuum advance is maximum at idle speed and then drops off with throttle plate opening. Centrifugal is zero at idle speed and goes up with RPM. Initial stays the same regardless of engine speed or throttle plate opening. All this assumes onern important point and that is: The vacuum hose connection at the carburetor is under the throttle plate or put another way between the throttle plate and the intake manifold.
If you don’t get it running right, get a vacuum gage and put on the intake manifold and tell me the reading and I will help further diagnose the problem. You can use a small plastic tee in the vacuum advance line to connect the gauge assuming that connection is under the throttle plate opening. We all might be on the wrong track about what your problem is. A vacuum gauge is seldom wrong. The joy of old stuff is out lasting the problems and solving them. I tell myself this when am pulling out my hair and ready to take a hammer to it.