[JUSTINH]

Stupid question of the day:
What role does the vacuum advance play when your distrib has weights on it to advance it?

I found a crack in my vacuum line. Dont know how long its been there. How can I tell if my vacuum advance is even working? Can I use my timing light to watch it advance with and without the vacuum hooked up?

[BOBCRMAN@aol.com]

check in the knowledge base for specifics, but, vac advance makes your engine more responsive and drive able at part throttle operation, also helps gas milage

[Frisco]

Originally posted by JUSTINH
Stupid question of the day:
What role does the vacuum advance play when your distrib has weights on it to advance it?

I found a crack in my vacuum line. Dont know how long its been there. How can I tell if my vacuum advance is even working? Can I use my timing light to watch it advance with and without the vacuum hooked up?

Bob answered what the vacuum advance does. Yes, you can use your timing light to watch the advance with and without the vacuum hooked up.

Just remember that the total advance you see when the vacuum advance is hooked up and the RPM of the engine is up around 2000-3000 will NOT be what you will have under load (driving) conditions. The vacuum will not be there under load and the vacuum advance will not be working at that point. I mention this because when I was just learning I had set my Total advance to 36 degrees (with the vacuum advance disconnected and plugged) and thought I'd check it with the vacuum hooked up. I about freaked. Advance was upwards of almost 50 degrees. I forgot that the vacuum drops off and that figure will not be there under load.

[78SilverShark]

Frisco, then why is there the vacuum advance? How exactly does it help with gas mileage if, when under load and at high RPMs with little vacuum, it gives away? Can you explain this?

Thanks!

78SilverShark

[johnsongrass1]

Advance is there to increase power without detonation problems. When under cruise or really light throttle the air flow through the carb will generate vacuum that pulls on the canister, advancing timing for better fuel efficiency. When you step into the throttle, the vacuum will decrease in the dist. due to the sudden momentary loss of vacuum in the carb, this will retard timing and prevent detonation under loads, saving the engine. You must have a way to increase timing when there isn't a vacuum signal to the canister under load to allow earlier spark timing to compensate for rising engine RPM and increase drivability. That's where the mechanical advance will come into play. Seems alot people seem to get confused as to which timing measure is the primary advance mechanism, the vacuum advance. The mechanical advance is there to help support the vacuum at times where vacuum isn't available. The trick is with Mechanical timing can only be read at idle because there will be no vacuum to tell the vacuum advance what to do. Unless you hook the canister up to full timed vacuum port, but then you would be defeating the purpose of the timing retard feature under higher loads, which would allow less use of mechanical timing. I hope this explains well enough.