[turbobill]

On an engine with a factory setting of 7.5 degrees before top dead center, I might venture up to 15 BTDC or so in 2 or 3 degree increments. While you have the timing light on the engine, make sure the centrifugal advance advances the tinming as the RPM's increase.

Set initial timing with the vacuum advance disconnected and the vacuum source pluged. Also check the centrifugal advance with the vacuum advance disconnected and the vacuum source plugged.

One last item. Set the dwell angle to the factory setting and don't forget to lightly lubricate the breaker cam and rubbing block. As the rubbing block wears, the points will open later, thus delaying the spark and retarding the timing from what it was set at when the points were installed and adjusted. Depending on how much you drive, you may want to check the dwell and relube a couple times a year. As the rubbing block wears, the dwell angle will increase.

If the spark advance is over advanced, the main risk is detonation and the resulting engine damage. Detonation is also called pinging, and sounds like a crackling sound, much like diesel clatter. Lesser risk is engine roughness and loss of power. (I tend to think the GMC V6 would fall into the later category as the compression ratio is low)

While it contributes nothing to wide open throttle power, the vacuum advance increases spark timing under light loads. This aids driveability and fuel economy. Make sure it works too.