I recently changed my vacuum advance on my points dist. to ported vacuum. (1974 L-82 four speed, headers and edelbrock intake, 10-1 comp.) I have set my mechanical advance to 36 degrees max, found a set of advance springs that allow the full 36 degrees at 2700 PM. After setting my idle up a little(900 rpm) I checked my vacuum at the manifold port. I got a whole 12 -13 inches. I then checked the vacuum at the ported source, got a reading of 0 at idle and 3-6 inches at 2500 rpm. I checked another ported source and got the same reading. On a road test I noticed a little off idle flat spot until I got a few rpm up. I am not to concerened about this as I am my headlight problem. It seems that I am not running enough vacuum to operate my headlights properly(very slow opening). So this morning I changed my vacuum to full and noticed a big difference in idle(smoother and a jump in rpm). I then palced my vacuum guage on and got a full 17 inches(not to bad at a corrected number for 4000 feet in elevation). Now my headlights work great and I have lost the flat spot off idle. I checked my timing with the full vacuum and at idle got 27 degrees.(my base is 11 degrees). At 3000 rpm I have 52 degrees on a cruise speed. The car runs great with no ping. Should I try to run the ported? If so what do I need to do to make it work. Any ideas would be appreciated as I am getting ready to head down south to Redding, Ca. for Kool April Nites next Friday. :cool: :cool: :cool:

 

I think what is happening is the 4000ft is causing your engine loose enough power that things aren't working right. If the manifold vacuum works better use it but remember when you get to the flat lands you may have detonation problems.. If you want to keep using ported then try bringing the advace in even quicker and advacing it when you are in the mountains. When you get to the flatlands set the timing back to where you have it now.

 

It sounds like your vacuum advance diaphram is torn, thus causing a vacuum leak. There should be little or no vacuum at the ported sorce at idle. I think 6in at 2500 is OK, but it should be steady...not 3-6 as you indicated. I don't have my notes with me right now to verify, but it sounds OK.

What vacuum hoses and devices do you have attached to the system? Are you running a hose directly from the Q-Jet to the distributor? Or, do you have one or more thermal vacuum relays (TVRs) in the thermostat housing or intake manifold? If you have some TVRs then they may be cracked, or faulty in some way. I plugged all mine (smog exempt) and routed my hose according to this image https://www.corvetteforum.com/techti...D=86&TopicID=3 that I posted in the tech tips.