Classic/Antique Car Repair/65 Caddy Fleetwood Brougham Rochester 4GC Rough Idle
Jon Hardgrove wrote at 2007-01-04 21:36:03
Dick - thank you for the reference. I would like to add somewhat to your answer.
Rochester made many different 4-G series carburetors. Many of these actually HAVE the cutouts in the throttle body to bowl gasket, and these are FACTORY.
Hopefully, you will allow a link to this website that is a copy of an original Rochester bulletin that explains these cutouts. This link is: http://www.thecarburetorshop.com/Hotidle1.jpg
In a nutshell, these cutouts enter a "dead" (Rochester's term) area of the airstream. According to Rochester, outside air never enters the airstream through these cutouts; rather, excessive pressure created by very warm ambient temperatures and excessive heat due to traffic may exit the airstream at these points; thus reducing stalling or rough-running conditions in summertime city traffic operation.
This was an inexpensive method (Stromberg and Carter often machined bleed holes in the throttle body or bowl, which are not as obvious to many observers). Much less expensive to do the cutouts in the gasket.
Incidentally, you may remember the old (1950's and early 1960's) Delco 'V-series" carb kits. Rochester had so many calls from mechanics concerning the cutouts that Delco inserted a piece of yellow paper with the kit instructions that stated: WARNING - The gaskets in this kit are correct for the application!
I truly doubt that Rochester saved money by use of the cutouts.
The proper gaskets should ALWAYS be used. Using a gasket with the cutouts in the wrong carburetor may cause the engine to run lean in traffic. Using the gaskets without the cutouts may cause stalling problems if used in a carburetor designed for the cutouts. If properly used, the cutouts will not cause an idle issue.
ASE Doc wrote at 2015-04-17 15:29:42
I want to say that I just overhauled a 4GC carb for a 64 Coupe Deville yesterday. We are a NAPA service center and I used the NAPA Echlin kit. The kit had several options for the body to throttle plate gasket, including one that did not have the vapor vent passages and fit correctly.
The carb on disassembly had the vapor vent gasket in it. Whether that was added during a previous overhaul, I don't know. I agree that an unmetered air leak, like that created by the vented gasket, is not a good thing. I've tried to find more information on this carb to get some idea of what Rochester was going for with this gasket design. It seems like there must be some variation of the 4G carb design that was altered in some way so that the vented gasket was used. I can't imagine that the engineers intentionally designed an unfiltered, unmetered air leak into this carb.
I've yet to run the engine since the overhaul. It actually seemed to idle fairly well before hand. I'll see how it runs with the carb redone and make adjustments from there.