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Old 07-16-2010, 12:11 AM
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More Quadrajet Stuff-- Question about polished throttle plate

My donor carb core I bought today has a very nicely polished throttle plate. Is this likely a DIY modification, or did certain applications come from the factory this way? The casting # identifies the carb as a 75 Chevy MT.

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Old 07-16-2010, 09:00 AM
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Do you mean the butterflies are polished? If so this is a mistake. If you look at the butterflies from the factory, they have a criss cross pattern on them, this is to help mix the air fuel mixture. Often the factory butterflies on the primary side are replaced when new throttle shaft bushings are added and they use smooth ones as a replacement. When I`m building a Quadrajet one of the mods I do is rough them up good with a file.
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Old 07-16-2010, 11:33 AM
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The butterflies are still cross hatched, but the mating surface on the top and the inside of all 4 of the throats are nice and shiny smooth. My other carb had a slight texture from the casting.
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Old 07-16-2010, 12:53 PM
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This is a cruddy Q Jet I was sent from a salvage yard in CA.
All the threads in the body were shot so I drilled and rethreaded to the
nearest metric size up.
It worked until I went to a Speed Demon.

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Old 07-16-2010, 02:19 PM
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That one looks pretty smooth too. The throats on mine are a near mirror finish, was that one like that too?

The carb I have that isn't polished is a q-jet by Carter, while the other is a Rochester made. I wonder if there was a difference in the manufacturing processes between the two companies?
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Old 07-16-2010, 02:23 PM
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The base seems to be a different material, aluminum, maybe the bodies are a "pot metal" ie. a metal mix.
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Old 07-17-2010, 05:23 AM
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Quote:
Originally Posted by clicketyclack
That one looks pretty smooth too. The throats on mine are a near mirror finish, was that one like that too?

The carb I have that isn't polished is a q-jet by Carter, while the other is a Rochester made. I wonder if there was a difference in the manufacturing processes between the two companies?
Someone has been at the one you're talking about. They were a dichromate conversion finish inside and out, not shiny or particularly "mirror finished". Same thing for the throttle blades. Dull-ish, as stamped and finished, not polished- at least on any that I've seen. And I've seen a few.
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