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Bob,
I hope this mechanic friend of yours doesn't also tell you
about the UFO he saw over San Diego recently.
All the carburetor adjustments in the world will not change
the run mixture. The needle valves on the base of the carburetor are air
bleed valves which control air bleed at idle speed. When the engine is
running at operating speed fuel is controlled by the jets installed.
It is possible to lean or richen the mixture by changing jets but except for
race car mechanics who do this continually on race engines, it normally requires
a sophisticated machine to "sniff" the exhaust mixture while running the truck
on a rear wheel dynamometer in order to do much more than an educated
guess.
Beware if the guy also tries to sell you a device that fits
into the neck of the carburetor which will atomize the fuel air mixture thereby
giving you an additional 4 mpg. It is your truck and your money of course,
if it makes you happy having this guy tune your truck, I'm happy
also.
Harvey
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