Tom,
There is rebuilding Zeniths, and there is rebuilding Zeniths.
I had the same concerns as you many years ago, of trying to get a
74 model through California emissions. I "rebuilt" the carbs
following the advice of books etc, and though it did improve the cars
running, it still did not pass the sniff tests. It turns out that
the where the throttle shaft passes through the carb body can wear
a lot, leading to vacume leaks and subsequent high CO levels.
An easy way to check for this is to spray WD40 around the throttle
shaft with the car running, and listen for an immediate improvement
in the idle. If so, you have a leak, and the only way to fix this
is to rebush the shaft; not a job for the amateur. Its been a long time and
I do not remember the costs, but I think that a complete
rebuild and rebush will run around $150 per carb.
The good news is that you will not believe the difference in how your car
will run!
A parting word; make sure all all vacume lines are solid. This is
really the main reason for motors failing or producing marginal results
in the sniff test. And like electrics, they can also be the most
difficult to diagnose (though dead-easy to fix).
Good luck!
Shane in Maryland
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