66 center carb body to throttle plate gasket .
 

Hi, am replacing throttle base on my center carb.
The rebuild kit has two similar gaskets, one thicker than the other.
Which one do I use .

The thickness is not an issue, The important difference is the "vents" that aid in hot starting. Note that these vents are above the throttle plates, so do not add a vacuum leak.

Some may disagree, but this was an engineering change somewhere before 1963. I use the vented gaskets in all the restorations I do. I have six NOS Pontiac Tri-Power carbs that have never been apart. They all have the vented gaskets.

Definitely, use the vented gaskets.

Top or bottom ?
 

So, which one ?

The bottom gasket.
Tom V.

Thank you.
 

Thank you. 👍

I have a copy of a Rochester bulletin dated 1958 which notes in a Pontiac Tri Power page:

" A vented throttle body to bowl gasket is used on all three carburetors to vent any fuel vapors which may form in the carburetor bores to the outside atmosphere. This feature greatly improves hot idle and hot engine starting."

Nothing is mentioned as to whether a hot idle compensator was also used at that time. But by 1959 Cadillac, Chevrolet and Oldsmobile used them and Pontiac had a hot idle compensator by 1961.

Hot Idle Compensator and the vented carb base gaskets do different jobs.

Hot Idle Compensator adds more rpm if the carb temp gets too hot.

Vented Gaskets help with restarts. But make emissions slightly higher after the car is parked.

Tom V.

Vented gasket
 

Here are a few photos of my 66 Tri-Power carbs showing the center and rear vented gaskets.

Chris.

By using the vented gaskets on all 3 carbs as stated that would allow unfiltered air to enter engine when throttle blades are open & why would the fuel vapors not just vent out the top of carbs as the end carbs have no choke & when hot the center carb choke is open ? I believe the vented gaskets may only be beneficial if carbs begin to flood as it would allow some of the fuel to exit & not all go past the throttle blades.

I'm only quoting what Rochester said. I suppose it is such a small amount of unfiltered air that it doesn't matter. But it was their decision.

1966 center carbs uses the vented gasket #7015106.

Delco Rochester Bulletin 9C-627 Date: May, 1967 (1966 389 Tripower) calls for the non-vented Throttle Body gasket #7015345 in end carbs.
Same gasket used on ALL 1959-66 Pontiac Tripower end carbs.

In older Bulletins dated December 1965 ALL 1959-66 end carbs used the vented #7015106 gasket

In Bulletins dated May 1967 ALL 1957-58 Tripower carbs and ALL 1959-65 center Tripower carbs w/o HIC uses vented gasket #7015148.
1959-65 center carbs w/HIC uses vented gasket #7013680.

In older Bulletins Dated 1957 all 1957-58 Tripower carbs uses non-vented gasket #7011326.

I have found using gaskets in the revised bulletins will work best.

FWIW

It was in a different lifetime (maybe 1975 or so), so I do not remember the exact words; however:

I asked about these to a Rochester engineer (who had heard the question before ;) ) and he told me: (a) the vents relieve excess pressure in the throttle body area under hot idle and slow running conditions thereby alleviating an over-rich condition, and (b) the vents connect to a "dead" area in the throttle body, so virtually zero air would ever enter the carburetor, only exit.

If someone has a new old stock Rochester carburetor kit from the early 1960's (I junked all of mine); one will find, in addition to the adjustment sheet, a small yellow sheet stating something to the effect "the gaskets in this kit are correct, use them". I guess Rochester got a few inquiries early on.

Jon.

Here is the reference quoted by pfilean:

https://www.thecarburetorshop.com/Hotidle1.jpg

Jon

From Rochester manual:

"An important design feature used in the Rochester 2-bore carburetor is throttled body venting. Its purpose is to give a quicker hot engine starting after the engine has been shut down for a short period. During extreme hot engine operation, the fuel in the carburetor tends to boil and vaporize due to engine
heat. Some of the fuel vapor tends to reach the carburetor bores and condense on the throttle valves and seep into the engine manifold; by venting the area just above the throttle valves, hot engine starting time can be reduced to a minimum, on applications where the carburetor is exposed to extreme engine heat. There are two methods used in venting the throttle bore area.

1. A special throttle body to bowl gasket is used. See Figure 5A. This gasket has cut-out areas that vent fuel vapors from the carburetor bores just above the throttle valves.

2. The other type of venting is accomplished by drilled holes through the throttle body casting just above the throttle valves. See Figure 5B. They serve the same purpose as the vented gasket.

The location of the vent holes is such that they will not disrupt engine idle or off-idle operation. They are
located above the throttle valves on the side opposite the mixture screws, in an area where the transfer
from idle to the main metering will not be affected."

I use the vented gaskets on all restorations, center and rear carbs. I've had very good results building the carbs this way-fast hot starting, and smooth idle.

I am still trying to figure out why Pontiac added the .026" holes in the end carb throttle plates. This was done in 1964 and continued through 1966. I believe.the purpose is to bleed a very small amount of air and fuel through the end carbs to prevent the formation of varnish in the end carb float bowls if the end carbs are not "exercised." Does anyone have another theory about this?

I also use the vented gaskets in all but 1964-66 Pontiac end carbs.
The latest factory info calls for the closed 7015345 gasket to these.
I can´t imagine the small .030" holes in the throttle valves will pull fuel from the float chamber in any amounts or at all.
My conclusion on the holes is that they are there to pull any excess of fuel vapor residue for combustion to preven varnish build up on throttle valve edges, where other carbs with the vented gasket vents the vapors to the atmosphere.

FWIW

which one is vented, top or bottom

The bottom gasket with the llots is the vented one.

Why do you not use the vented base gaskets in the '64-'66 carbs? Is it because these end carbs have the holes in the throttle plates? Other than that, these carbs are functionally identical to the '59-'63 end carbs.

I drill the small holes in all restorations of the large end carbs, regardless of application, whether the 997 or 683 float bowls.

Dick, it should read:
"I also use the vented gaskets in all but 1959-66 Pontiac end carbs..
The latest factory info calls for the closed 7015345 gasket to these."

The Delco/Rochester bulletins dated may 1967 and later have the #7015345 gasket for 1959-66 Pontiac Tri-Power end carbs, and since they work, why not?