[Cliff R]

The large front vent tube started showing up in 1970 on the CA models like you are looking to purchase. They continued to use the large front vent on Pontiac carbs (with a few exceptions like 301's and the Olds 403 carbs) until 1979. The 1970-74 models used a removable front vent held on by a long front screw and shorter side screw. The 1975-79 model large front vent is pressed into a matching hole in the airhorn. It is NOT exactly in the same location so plan accordingly if you are using Ram Air or Shaker set-ups and mixing/matching Pontiac Q-jets between those years.

The vent tube REQUIRES a matching hole in the air cleaner base and MUST be open. Do not cut off the vent or block it off, it needs to be open. With most air cleaner bases you can cut a matching hole for the vent. It will require a seal to keep unfiltered dirt and debris out of your engine.

The 1970 CA models also used a front 3/8" vent to the charcoal canister. That vent can be blocked off or removed and the hole plugged, it is not needed.

The calibrations used on the manual trans carburetors were slightly leaner than the auto's in almost all cases. That really doesn't matter too often as it is extremely rare to encounter an engine from that period that is still completely "stock" where the factory calibration is going to work well anyhow.

I specialize in custom high performance rebuild kits for those carbs with tuning parts. That's about all I'm doing these days and can barely keep up with it. I require ALL of the engine/drivetrain/vehicle specs and the carburetor part number to put one together.......Cliff