Vacuum
Thread Starter
Heel & Toe
Joined: Jun 2010
Posts: 15
Likes: 0
Vacuum
I have a 73 w/350 4 SPd with holley 670.I was wondering how much vacuum I need for my distributor. When the car is running and I disconnect the vacuum line there isn't any change in idle.every other car I had had I drop in idle speed.thanks for any and all help
Le Mans Master
Joined: Apr 2004
Posts: 7,106
Likes: 38
From: Newark DE
it's probably connected to a ported vacuum source on the carb. Ported vacuum supplies no vacuum at idle but acts the same as full manifold vacuum once the throttle blades are opened up above idle rpm. On a '73MY I'd be surprised if you weren't hooked to ported vacuum.
Easy enough to check - disconnect vacuum hose off carb, put your finger over the connection opening. at idle there should be no vacuum. Now keeping your finger there manually advance the throttle open from the linkage to just a bit above idle speed and you should feel vacuum sucking on your finger.
If you are already connected to full manifold vacuum (preferred for various reasons since ported vacuum was ONLY used for emission purposes) you should feel vacuum on your finger at idle speed. If this is the case and you get no difference in idle speed when connected to the distributor than either your vacuum cannister is bad or you may even have an incorrect cannister for the level of vacuum your motor is producing.
First determine if you are connected to ported or full manifold vacuum than we can go from there.
Easy enough to check - disconnect vacuum hose off carb, put your finger over the connection opening. at idle there should be no vacuum. Now keeping your finger there manually advance the throttle open from the linkage to just a bit above idle speed and you should feel vacuum sucking on your finger.
If you are already connected to full manifold vacuum (preferred for various reasons since ported vacuum was ONLY used for emission purposes) you should feel vacuum on your finger at idle speed. If this is the case and you get no difference in idle speed when connected to the distributor than either your vacuum cannister is bad or you may even have an incorrect cannister for the level of vacuum your motor is producing.
First determine if you are connected to ported or full manifold vacuum than we can go from there.
Last edited by BarryK; Jul 25, 2010 at 03:57 PM.
