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Vented Rochester throttle body gaskets


There is much concern over the entrance of “unfiltered air” through the vents of the “vented” or “slotted” Rochester throttle body to bowl gaskets.

History – during the 1950’s cars became lower in overall height, and engine sizes became larger. Both of these factors contributed to much higher than previous underhood temperatures. These higher underhood temperatures created issues of deteriorated idle, and also increased difficulty in starting hot engines due to vaporization of gasoline in the carburetors. Different approaches were used to combat these issues. One approach used by Rochester was the “vented” or “slotted” throttle body to bowl gasket.

Reference:  1957 Rochester vented gasket service tip

How does this simple “fix” work? When the throttle plates are virtually closed (idle or engine-off conditions), any pressure, which builds inside the venturi area of the carburetor above the throttle plates, may be relieved through the gasket vents; rather than passing by the throttle plates into the intake manifold. This feature will contribute significantly to reducing poor idle/stalling when driving in the city during the summer as well as improve hot start issues ON CARBURETORS WHICH WERE DESIGNED FOR THESE GASKETS. Some carburetors (marine applications, and many others) were designed NOT to use the vented gaskets.

Reference:  Various hot idle compensation devices

Now, the non-issue that many consider an issue – what about the unfiltered air that enters the engine through these vents? And we have also been asked by the novice hobbyist about the vacuum leak through the vents. First, a vacuum leak through the vents is not possible; as the vents are ABOVE the throttle plates. Now, about that unfiltered air. Virtually no air will enter the carburetor through the vents! Consider the workings of the off-idle circuit of the carburetor in order to better understand the previous statement. The off-idle circuit of the carburetor has a vertical slot in the throttle body, which is just covered at the lower extremity by the lower edge of the throttle plate. When the throttle plates are opened a FINITE distance from the throttle body, the velocity of air accelerated by the slot by the position of the throttle plate creates a low pressure area in the slot, thus allowing fuel to be pushed from the off-idle well and flow into the throttle area. ONCE THE THROTTLE PLATES ARE OPENED BEYOND A CERTAIN DISTANCE, THE AIR VELOCITY AROUND THE SLOT IS DECREASED TO WHERE NO FUEL WILL FLOW! Do you doubt the preceding sentence? Consider the various devices for idle air bypass such as holes in the throttle plates, which have been used by the manufacturers on engines requiring lots of idle air to maintain the appropriate distance of the throttle plates to the throttle body to maintain the necessary air velocity. Do you see where we are headed? The vents are in an area of the carburetor (the Rochester engineer with whom I discussed this some 30 years ago called it a “dead air space”) where there is insufficient velocity to cause external air to enter the carburetor. The vent may be considered a one-way valve. Air will flow from the inside of the carburetor to the outside when sufficient internal pressure exists, but will not flow from the outside in.

Not all Rochester G-series carburetors were issued with the vents; many were issued with the “solid” gasket without the vents.

So, to the next question – if I use the vented gaskets on my carburetor, will it improve the hot idle/hot start issues?

Answer – we highly recommend using the original parts unless you have the facilities to do a thorough test using both. In any event, NEVER use the vented gaskets on a marine application in an engine in a closed bilge.

So how do you know which is the correct gasket for your carburetor? I cannot answer that. We know because we have invested in original Rochester documentation, so that we can put the proper gaskets in the kits we produce.

One other historical note that may be of interest. 40 or so years ago, when we started building our reference library, we purchased lots or ORIGINAL Rochester parts kits to have original samples. The Rochester kits for the G series carbs contained, in addition to the kit instructions, a piece of yellow paper with words similar to: “ATTENTION – the gaskets in this kit are correct as supplied”. Evidently, Rochester received lots of questions concerning the gaskets.