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Old 08-05-2003, 08:59 PM
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rochester 4gc carb cfm rating?

Just digging thru looking for the cfm ratings for the old 4g/c rochester carbs. I've got every piece of info I could ever want on that model but have forgotten the cfm rating over the years.. Looking thru the big gm carb manual (bout a foot thick)I see they were on everything from the 283 to the 409. Is there more than one cfm rating? I could just throw a double pumper on my motor but then I wouldnt be different! Thanks for any help! Yes I did go thru the knowlege base... Paul

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Old 08-07-2003, 02:21 AM
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Found this on the net:
Rochester 4G/4GC

Used on a variety of GM automobiles from 1952 to 1967, including the 1961 to 1963 Buick/Oldsmobile/Pontiac aluminum 215 V8's, the 4G was Rochester's first four barrel. Produced in 486, 553, and 692 CFM ratings, the 4G uses a squarebore layout with a conventional fixed jet, power valve and accelerator pump metering scheme. Secondaries are mechanically actuated but use an adjustable spring loaded air valve. Adjustment is achieved by loosening an allen head set screw and inserting a screwdriver. Rotating the screwdriver counterclockwise increases tension. The accelerator pump is also adjustable, using a link with 5 adjustment holes. Four center pivot floats are used. The 4G's used the small diameter 4-7/32" diameter air cleaner, shared by the first generation Carter AFB's, so the usual aftermarket air filter cases will not fit. However there are adapter rings and street rod sources usually carry compatible air filter cases. While the Rochester book mentioned in the reference section briefly covers the 4G, your best bet for information is a GM shop manual from the '50's or '60's.

Model /CFM Venturi Diameter/ Bore Diameter

primary secondary / primary secondary

4 Jet/ 486 1 1/8 1 1/4 / 1 7/16 1 7/16

4 Jet/ 553 1 1/8 1 15/32 / 1 7/16 1 11/16

4 Jet/ 692 1 1/8 1 15/32 / 1 9/16 1 11/16

If you can get an older copy of the Doug Roe book on Rochester Carburetors, it has a pretty good section on the 4G. jf

If you need an air filter adapter for the 4G, the Mr.Gasket pn is 2082, to use a standard 5-1/8" filter base.

Last edited by jimfulco; 08-07-2003 at 02:27 AM.
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Old 08-07-2003, 07:03 PM
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Thanks Jim!
I'll have to go measure it up and see what I've got..I like to tinker so one of these is right up my alley! lol Thanks again! Paul
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