Vacuum Advance on V8S

[bobV8S]

Does anyone know what the valve in the line from the air intake to the distributor is supposed to do? I have tested mine and it seems to be a one way valve that stops the advance working. Yes it is installed the right way as in the bible. Using a vacuum guage I get a good reading on the air intake side and nothing on the distributor side. Any help would be much appreciated.

[GreenV8S]

The vacuum advance is designed to advance the ignition while cruising at part throttle, for economy. But it's cleverly connected to a throttle edge tapping in the throttle housing, so you don't get the advance at tickover. This means that when you floor the throttle from tickover the vac advance unit sees a very brief period of vacuum as the throttle starts to open, then the vacuum is removed again as the throttle opens further. This brief vacuum advance can degrade the throttle response. The one-way valve is designed to prevent this, by almost-but-not-quite stopping the vacuum from reaching the vac advance unit, so the vac advance only comes in when the vacuum remains constant for several seconds. Which is a nice idea, but the valves do tend to get blocked up and even fitted back to front which can mean the vac advance goes all over the shop. My advice if you're in any doubt is replace it with a straight through pipe. One less thing to go wrong, and you won't notice the difference.

[bobV8S]

Thanks for the information. I have done as suggested and everything is fine. I think my old valve must have failed.
Bob