289 vacuum advance/timing question

[garystocker]

The base timeing on my 1968 289 was set at the suggested 6BTC with the vacuum advance disconnected and plugged. When I revved the engine the centrifugal advance activated at around 2000 RPM: just as it should. Now it is my understanding that when I reconnect the vacuum advance and rev the engine, the timing should advance sooner than before thee centrifugal advance kicks in: if it is working properly.

When I reconnected the vacuum advance line to the distributor, the timing light showed the timing advanced 20 degrees and the RPM climbed by 100 all by itself-I never revved the engine. When i did rev the engine the timing advanced further.

Is is normal for the vacuum advance to have a pronounced effect on timing advance while at idle? I thought it was to be near the base 6 degrees BTC at idle and then advances as the RPMs rise?

What do you all think? I hear no knocking or pinging, it runs smooth, never runs hot...

 

[garystocker]

anyone?

 

[Max Power]

If you are connected to a true ported source, you should not have vacuum through the vacuum advance line at idle. If you attached to a ported source and you do have vacuum, you probably have the idle set to high. You should not get vacuum through the ported line until you crack the throttle.

 

[MitchGT]

Most cars run better with a little more advance than that. the fact that it idles faster with it plugged in kinda shows that the motor wants more advance. Try 10-12 degrees and your motor will probably feel much stronger and more responsive, and still shouldn't ping at all, unless you have really high compression. Since these cars don't need to be smogged, you don't need to worry about the factory settings, so go with what makes the car feel best.