Holley 1920 Help needed

Hello,

I just purchased and installed a Holley 1920 on my '69 A100 Van, slant six and it starts great and runs pretty good, a little rough on acceleration but it idles nicely.
The problem is I was smelling gas fumes and searched for leaks - there are none I could find but as I was searching (with the motor off) I noticed the vacuum valve all of a sudden open up and spurt gas.
Any ideas why this would happen and how to fix it would be appreciated!
Raymond