This thread reminds me of the change made in 1964, when the end carb throttle plates had the .028" holes added. Rochester engineers had a reason for doing this. Since those people are not available to answer questions today, we can only guess the reason for the questions we have about Tri-Power carbs.
With no holes in the throttle plates, under normal driving conditions, the end carb throttle plates are closed, resulting in zero gasoline flow out of the fuel bowl. Every time you shut down the engine, some of that gasoline evaporates through the air horn vent--about a 3/16" hole. If the end carbs are not opened, and as gas evaporates, the non-volatile ingredients (varnish) become more concentrated and eventually form a gummy residue inside the carburetor.
With the .028" holes, there is some gasoline drawn by vacuum through the holes into the engine. The holes are small enough so that this flow is insignificant and has little effect on operation of the engine. That flow does, however, provide a minute amount of gasoline flow into and out of the float bow., ensuring that little or no varnish collects in the end carbs.
I think the above explains why, beginning in 1957 Pontiac Tri-Powers, Olds' J2, '58 and up Chevrolet triple carbs, and '58-'60 Cadillac three carb setups got a bad reputation. Most of the owners of three carb GM cars were older people, especially with Olds' and Cadillac. After a few months of daily use, where these drivers seldom, if ever opened the end carbs, they became badly gummed up, eventually causing a visit to the dealer because the car didn't run well--or at all. After a few of these experiences, many of these owners said "Take that thing off my car and put a four barrel on it!" I also believe that's why three carb setups were so plentiful in the late '50
s and '60's.
Today, however, most of us regularly "exercise" our end carbs, so the above "gumming up" is not an issue.
That's just my theory. Tell me what you think. I've been adding those /028" holes to all the Tri-Power end carbs I restore.
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BONESTOCK GOATS
'64 GTO Tripower Hardtop (Wife's Car)
'64 GTO Tripower Post Coupe (My Car)
'99 Bonneville SE Sedan
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