[burnin oil]

Originally Posted by HO455

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

You are correct that the carb has all the circuits in there like pretty much all others and those circuits all build up off the idle circuit. My point was simplified for a reason. The air flow the primary is very limited and there isn't much adjustment to be made easily. A qjet mix and fuel consumption will stay reasonable due to the small primaries. These carbs are not user friendly as far as people learning so it was a leave it be thing. There is just a bigger chance to booger it up compared to other carbs. The OP is learning timing curves so modding a qjet is further along in his future. We all start somewhere and build on that knowledge. It has been a long time since I have been inside a qjet personally. What makes them great for this application makes them bad for my usage. They are alot harder to mod for performance than a holley and I need all those CFMs on the primary side. I don't tend to keep stock SBC around.