Join Date: Dec 2001
Location: Belleville, Illinois
Thanked 22 Times in 17 Posts
Re: vacuum advance question
When everything is set and working properly, the vacuum advance only gives additional advance at light to moderate throttle opening, which increases the fuel efficiency of the engine. With more throttle opening, the engine can't tolerate as much advance without pinging, but that's where you start to lose vacuum, and so you get less advance.
In the '60s some high-performance engines didn't have a vacuum advance on the theory that they were extra complexity on an engine where fuel economy wasn't an issue.
If you have the basic timing too far advanced, the vacuum coming in could be giving you too much total advance. That will raise the voltage requirement to fire the plugs. If everything in the ignition system isn't up to snuff, that can cause a misfire.
If everything else is in good condition and set properly, the vacuum advance will only increase your gas mileage - mainly the highway mileage.