65 300HP Carb & Intake

[Kenneth G.]

I rebuilt my 3721SB Carter AFB and am puzzled by a couple of things:
(1)There are two holes about 1/4inch dia. in the intake manifold on either side of the four holes for air intake. The carb gasket clearly allows for something (fuel or air?) to flow between them since there is half-moon channel in the gasket to accomm. this. These holes in my intake had been filled with bondo which I removed. Engine seems to run OK, but what is purpose of holes. No help in any of many manuals I have.
(2)My setup has a spacer about 3/8" thick between intake and carb. One manual refers to intake insulator. Is that what this is and is it standard? I live in a small town with nobody to discuss these things with. Thanks

[William C.]

Re: 65 300HP Carb & Intake

Spacer is the insulator. The holes are part of the carb heat system for very cold climates and is triggered by the heat riser valve being closed, redirecting exhaust heat thru the intake manifold, and some of that heat going under the carb via the holes you mentioned. Gien today's galsolie and typical driving habits of modern Corvette owners, these passages are quite often plugged to somewhat mitigate the effects of changes in gasolie formulations over the last 40 years or so. Ther should also be a stainless steel plate between the insulator and tha carb to prevent the exhaust hasses from coming directly in contact with the aluminum of the AFB, such contact will erode (eat) the aluminum of the carb in short order. If it were mine, I'd drill and tap the holes and plug them.

[Duke W.]

Re: 65 300HP Carb & Intake

That is the "heat slot" that allows exhaust gas from the heat riser passage to heat the idle/off idle fuel circuits and throttle plate area to prevent carburetor icing. Unless you regularly drive your car in cold, damp weather it is a good idea to thread the holes for pipe plugs to block this passage or install Welsh plugs. Bondo is certainly not the way to go.

The "spacer" is referred to GM as "insulator", and it should be topped by a stainless or galvanized steel "heat shield" that protects the aluminum body of the AFB from direct exhaust gas impingement. The gasket is positioned between the manifold and insulator.

Details of the assembly are in the AIM. You should also have a 1963 Corvette Shop Manual and the 1965 Supplement.

Duke

[Joe R.]

Re: 65 300HP Carb & Intake

Hi Ken:

Most people recommend plugging the "hot slot" to help keep the carburetor cooler. I threaded and plugged the holes in my manifold as shown in the attached photo. If you decide to do this let me know and I can send you a couple of the plugs (and loan you the tap as well). I had to buy a box of 25 plugs so I have quite a few spares.

I think this job is best done with the manifold off the car, since there is a slight risk of some metal chips getting into an exhaust port and then into a cylinder via an open valve.

Another option that does not involve threading the holes is to use a press-fit plug of the type used for the so-called freeze plugs in the block. Various sizes are available from the Dorman catalog at most auto parts stores. If you check the archives, John Hinckley has posted a nice photo of what that solution looks like (and he may post it again if he reads this post).

There's no real harm in leaving the holes open, but the extra heat under the carb causes more problems than it solves unless you frequently start your car in sub-freezing weather.

[Timothy B.]

Re: 65 300HP Carb & Intake *NM*

[Timothy B.]

Re: 65 300HP Carb & Intake *NM*