lars

I have posted this tech "how-to" on the C1/C2 Forum, but with the number of Holley-equipped early C3s, I thought it should be posted here on C3 for you guys to have access to, also...

One of the common problems I see on many older Q-Jet and Holley carbs is damage to the bottom surface of the carb’s throttle plate caused by people not using the correct gasket “pack” and reflective heat shield required on the “hot slot” intake manifolds.

In order to promote better cold-weather drivability, GM used an exhaust groove crossover under the carb’s mounting flange on many carbs from the 1950’s through the 1960’s. This groove channeled hot exhaust gas under the carb to promote better atomization and vaporization of the fuel entering the manifold during cold weather conditions. The slot was sealed with a gasket and a stainless steel shield to prevent the exhaust gas from directly contacting the carb, and the carb was heated by contact with the stainless steel shield.

Over the years, people did not realize the purpose of the shield, nor the function of the hot slot exhaust crossover. So after rebuilding the carb, they installed the carb in the “normal” fashion: An open-plenum gasket on the manifold, and the carb bolted directly on top. This resulted in hot exhaust gasses entering the carb’s vacuum system, and the hot exhaust very quickly eroded the aluminum bottom surface of the carbs, destroying the carbs.

Here is a typical “hot slot” (aka, “smiley-face”) QuadraJet intake manifold for a big block (aluminum) and a small block (cast iron) application. You can see that the big block manifold has been run without the correct gaskets and heat shield from the exhaust gas ingestion into the intake and under the carb’s throttle plate:




The Holley-equipped engines also used the hot slot feature, and have the same problems:


The correct gaskets for these manifolds are a high-temperature bottom gasket with a stainless steel plate on top to seal the slot. The factory then mounted the carb directly on top of the stainless shield. To reduce the direct heating into the carb, I like to place a standard base gasket on top of the heat shield, making the complete gasket set look like this (Q-Jet gaskets shown – Holley system the same except with Holley bore pattern):


So what happens to a rare GM OEM Holley carb when it is mounted and run on a hot slot manifold without the correct gaskets and shield? The entire bottom surface of the carb, where exposed to the exhaust gas, is completely eroded and wasted away, destroying the carb and the vacuum passages in the throttle plate.

Here is a rare GM OEM Holley List 3419 (1966) that has been run without the shield. You can see the exact shape of the hot slot eroded into the bottom surface of the carb from one side to the other:


The erosion is so severe that the vacuum passages are completely gone:


This is what the throttle plate should look like:


Since these carbs, and their parts, are no longer available, we have to save the parts we have, and here is how to save this one…

Using a similar throttle plate to the one on the 3419, a mold was made of the vacuum passages using a polyester resin. Steel wires, bent into “L” shapes, were inserted into the resin before it cured so the mold could be “lifted” out once cured:


Here is the vacuum passage mold once removed from the “model” throttle plate:


The slow-drying JBWeld (not the 6-minute stuff) is some really strong, heat resistant stuff. It can be successfully used to fill wasted areas, as long as the base material surface is properly prepped:


The throttle plate was given a thorough bead blast after having been degreased. The mold was then placed into the correct position in the damaged plate. Drill bits, sprayed down with WD40 as a mold release, were inserted into the vacuum passages to prevent them from being filled with epoxy:


All the wasted areas and voids were then filled with epoxy resin. The throttle plate was covered with a sheet of wax paper, and a ¼” aluminum plate was firmly clamped down to the surface to create a smooth, flat surface with plenty of “squish” for the epoxy:


The whole thing was placed in an oven at 180 degrees and allowed to cure rock hard. Once removed from the oven, the squish plate was removed along with the wax paper, and the mold (which had been sprayed down with mold release) was removed. A little trimming and lap sanding of the surface, and the throttle plate looks like a new plate:


Check out the “before and after” side by side. Yes, these two really are the same throttle plate:


Throttle shafts re-installed with new bushings:


…and throttle plate assembly re-installed to the carb:

Notice that the Holley throttle plate is retained with only 6 screws - not 8. The screw holes in the center are left empty so the screws cannot drop down into the intake if they come loose. The other 6 screws are all retained and captured by the base gasket.

As long as this carb is mounted to the correct gasket and shield configuration, this carb will perform like a brand new carb, and we’ve put another one back on the road again!



Lars

bazza77

Wow , outstanding job .
It definately would be a feel good moment having that plate back on a classic vette or any car really and hearing it running again .