Sounds like you're checking ported vacuum and not manifold vacuum. Ported vacuum is above the throttle plates in the carb or throttle body and only used to advance ignition timing. 'Normal' is 18-20-inches but even stock older engines can be less. If you engine is in good condition, 15-inches may indicate retarded timing with a stock, mild cam but it may be 'normal' for a mild street cam. The deciding factor here is how the measured vacuum reacts when the throttle is 'blipped'.
Some carburetors have both ported and non-ported vacuum connections you can tap into. The Edelbrock AFB type 1400 series has both and many Holley's probably do as well. Other places to find manifold vacuum is anywhere on the intake where power brakes attach. Lots of variables here. Here's a good site with some (manifold or non-ported) vacuum readings information - http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm Dale McIntosh <http://www.chevellecd.com> ChevelleCD.com <http://www.chevellestuff.com> ChevelleStuff.com _____ From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jim & Chris Sent: Sunday, June 29, 2008 7:18 PM To: The Chevelle Mailing List Subject: [Chevelle-list] vacuum where is the best place to check the vacuum and what should it be ? 15in.? I have 6in. of vacuum at the timing advance port on the carburetor.the timing is at 8 deg. BTDC .when I unplug the vacuum advance hose nothing changes. idle and timing remain the same and the car is driveable and seems fine. Jimmy.C
