Help me understand these vacuum readings
Thread Starter
Melting Slicks
Joined: Mar 2007
Posts: 2,520
Likes: 11
From: Hinckley OH
Help me understand these vacuum readings
Went to an autocross on Monday and my engine started running rough. It ran rough all the way home and when I shut off the lights they didn't close so I figured I had a vacuum leak. Today I put the vacuum gauge to a manifold port on the carb and it was reading 12" of vacuum. I do have a bigger cam but that's low for it. Now comes the strange part. I started to disconnect vacuum lines to find out where I was losing it and when I disconnected the line that goes to the PCV valve and the charcoal canister, the vacuum jumped to 17" with the line open. I put my thumb over the line and the vacuum fell to 15". I reconnected the line and tightened it and the vacuum is still 15" but now the engine is running smooth and the headlight doors open and close.
I don't understand why the vacuum would rise with a line disconnected. Can anybody explain it?
I don't understand why the vacuum would rise with a line disconnected. Can anybody explain it?
Thread Starter
Melting Slicks
Joined: Mar 2007
Posts: 2,520
Likes: 11
From: Hinckley OH
OK, I have a theory. If the two ports are connected inside the carb, then the large amount of air being sucked into the open line could create a venturi effect on the port with the vacuum gauge connected to it. That would increase the vacuum reading on that port and in turn on the vacuum gauge.
