340 carburetor selection advice

I run a 2.5 to a 4.5 primary power valve depending on Vacuum at idle with a larger Cam. Vacuum at idle divided by two. No power valve in the secondaries. You can play with air bleeds. All 4 corner carburetors come with a secondary idle adjustment screw to use it for idle after you square the transfer ports on the primaries.

There were times with the secondaries closed it was idling high with the primaries square. Then you can drop the idle RPM with the primary idle adjusting screw. If its idling to low you would raise it with the secondary idle screw. You should never go past square on the primary transfer ports.

The carburetors I buy usually have a blank power valve in the secondary metering block. Probably why its there and why they sell them. I would never buy a new a standard double pumper for any car with a large cam any more. If so you usually have to drill holes in the venturi valves. That is the old school way and I see many double pumpers that it is done to.

4 corner is the new way for carburetion. The latest a Holley sniper fuel injection system .