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Space Truckin' · 01-14-2011, 03:35 AM

Hey man this is quite a thread! First I would like to point out that the vacuum valve on your thermostat housing is only open when the engine is warm and is intended to prevent the vacuum advance from being pulled in when the engine is cold. Also your vac source to that valve should be ported (no vac at idle) The EGR system is also a good thing to keep!

Another thing that helps the HEI ignition setup is a good 12 volt lead direct from the battery. Like so:

Example of bosch relay:

Amazon.com: 12 VDC Tyco Relay SPDT 20/30A...
Be sure to use a fuse in the lead from the battery to the relay!!!

Also this thread here on the forum has the vac diagrams for your engine:
http://67-72chevytrucks.com/vboard/s....php?p=3287463

Basically the EGR should see vac at idle with the ngine warmed up but not cold
The distributor should only see vac with the engine warm but not at idle.

There should be 2 temp valves on your engine one is for EGR and the front one is for everything else if I remember correctly.

Several of the ports on the front temp valve are common to each other while one is closed. This is when the engine is cold. The one that is closed should get the vac source (ported) from the carb.

Hope that helps not hinders! I will try to come up with a diagram but I need to know exactly what was removed from your engine and what is left to work with.

01-14-2011, 03:35 AM	#139
Space Truckin' Registered User Join Date: Feb 2006 Location: Texas Posts: 27	Re: Improve mileage 350 with Quadrajet Hey man this is quite a thread! First I would like to point out that the vacuum valve on your thermostat housing is only open when the engine is warm and is intended to prevent the vacuum advance from being pulled in when the engine is cold. Also your vac source to that valve should be ported (no vac at idle) The EGR system is also a good thing to keep! Another thing that helps the HEI ignition setup is a good 12 volt lead direct from the battery. Like so: Example of bosch relay: Amazon.com: 12 VDC Tyco Relay SPDT 20/30A... Be sure to use a fuse in the lead from the battery to the relay!!! Also this thread here on the forum has the vac diagrams for your engine: http://67-72chevytrucks.com/vboard/s....php?p=3287463 Basically the EGR should see vac at idle with the ngine warmed up but not cold The distributor should only see vac with the engine warm but not at idle. There should be 2 temp valves on your engine one is for EGR and the front one is for everything else if I remember correctly. Several of the ports on the front temp valve are common to each other while one is closed. This is when the engine is cold. The one that is closed should get the vac source (ported) from the carb. Hope that helps not hinders! I will try to come up with a diagram but I need to know exactly what was removed from your engine and what is left to work with. __________________ '91 Burb, Fuelly, The Caddyshack '85 3+3 Duelly, Guzzler, Hank the Tank Last edited by Space Truckin'; 01-14-2011 at 06:49 AM.