Originally Posted by HO455 (Post 8877817)

This information is wrong.
The air fuel mixture at cruise (and anytime the throttle is open above the off idle transfer slots) is controlled by the primary jets and the primary rods. The farther the primary throttle plates are open the stronger the signal developed by the venturies and the more fuel that pulled through the primary jets. The size of the jet sets the maximum amount of fuel and the rods fine tune that amount . There is also a throttle tip in circuit that uses senses manifold vacuum to richen the primary mixture when the throttle is moved open from a cruise. This circuit has a spring that overcomes the vacuum and lifts the rods to richen the mixture to prevent stumbling.
This is the condensed Cliff Notes explanation.

Poor fuel economy can be caused by the wrong jets or the wrong rod or a combination of the two.
Too strong of a spring on the tip in circuit or low vacuum or both will hurt fuel economy.
Improper vacuum advance operation will kill milage. The whole purpose of the vacuum advance canister is fuel economy.
Leakng plugs on the bottom of the carburetor center section will hurt economy. As will at least a dozen other simple things.

The mixture screws ONLY affect the fuel mixture at idle!
The primary bores are 1 3/8" in diameter. Far larger than a dime.
The secondary rods only control the fuel mixture of the air thar passes through the secondary bores of the carburetor. The secondaries are closed in a cruise situation. (Unless your cruising at 4500 plus rpm.under a heavy load)
There are dozens of good books on carburetors out there and some of them are Quadrajet specific. They are much better at explaining all of this than I am able to.
Get a book that explains carburetor operation in a manner that works for you. Read it and ask specific questions.
Your truck should be getting 12 mpg around town and at least 14 mpg at a steady 60 mph on the highway.

Here is a link to my Burban repair thread and there is tuning information of a similar combination to yours in various places.

http://67-72chevytrucks.com/vboard/s...698377&page=19

Originally Posted by Andy4639 (Post 8877935)

One of the best distributers you can buy is a GM HEI out of a mid 80's vehicle. No vacuum advance. They are easy to set up and will provide you with great service.

I had one in my 71 C-10 the whole life of the 305 TPI motor with 300,000 miles on it and I put 1 module in it. They are the best and cheapest for a stock motor. My truck had the 305/700-R4 in it with 4:10 gears and I got 18 MPG out of it running 70-80 MPH.
For a street truck that is used for just driving it and not hot rodding it you want beat the setup.
All these boxes and stuff are hype to sell you the product but @ the end of the day your paying for nothing but fancy electrical gadgets to fail and leave you on the side of the road.
One thing I always kept in mind for my trucks is drivability. I didn't want anything I couldn't get at a local parts house if I was on the road. That is something to keep in mind.:chevy:

Originally Posted by ChevyRacefan (Post 8877958)

That HEI the advance was controlled by the ecu, That is why it had no vacuum advance can.
It will run like garbage on an engine without the ecu controll.

Originally Posted by Andy4639 (Post 8878150)

Are you sure the timing scale is the right one for the motor? If the motor is a 72 and they used a newer timing cover then the timing marks may be off.
Depending on what balancer you have also.
Just asking because I ran into this on the 68 step I was building. It took a bit to figure out.:chevy:

Originally Posted by 68 P.O.S. (Post 8878138)

Here’s how I usually go about setting my timing.
First I pull off the carb and look underneath at the 2 transfer slots in the primary throttle bores. There’s 1 in each primary throttle bore and they’re probably over exposed and look like a rectangle. Using the idle adjustment screw, close the throttle blades all the way, then slowly open it to expose the transfer slots again. Once exposed, adjust them to look like a square, not a rectangle. This ensures you’re only on the idle circuit when actually idling. Once the slots are squared, reinstall the carb and don’t touch the idle adjustment screw again. Then I open the distributor cap and remove the advance springs so the weights move freely and it takes little rpm to reach full advance. This also keeps you from having to nearly redline your engine with your head under the hood lol. Disconnect and plug the vacuum advance. Start her up, bust out the timing light and rev it to like 2000 rpm so the advance weights are fully extended (fully advanced). Then set the total timing to 36 degrees btdc. Lock the distributor down and reinstall the advance springs. Ideally you want springs that allow full advance around 2500-3000 rpm. Then you can take a reading and see what your timing is at during idle as a mental note (this is your initial timing), but do not adjust it, or you’ll screw up your total timing and have to redo it. Next, reconnect your vacuum advance to manifold vacuum and listen for pinging. Your vac advance should be limited to provide between 10-15 degrees of advance. Anymore and its ping city. Check your idle speed and adjustment if needed. Mine is always right on after setting timing this way and never needs adjustment. Boom, you’re done!

Originally Posted by BigBird05 (Post 8878179)

Does the 72 BBC have Transmission spark control like the 72 SBC? The TSC on the 72 SBC uses a temp sender on the passenger side of the block.