The throttle plates (butterflys) on my '68 have a little round, spring-loaded valve near the bottom edge. The new ones in my rebuild kit (genuine S U) are plain, without the valve. What is the purpose of the little valve, and will it be OK to use the new, plain ones without affecting engine performance?

Yes, according to some the little valve was some attempt at emission control. Many remove them and solder up the hole.

Ditto what c1800 wrote. Another attempt at pollution control is the 2nd throttle plate located in the intake manifold which opens when the throttle pedal is pushed far enough. The 2nd throttle directs the fuel/air mixture into a central chamber rather than directly to the cylinders each carb serves. The mixture at low part throttle then finds its way back to the intake runner leading to the cylinders. I remove those 2nd throttles and fill the remaining hole with a 5/16 bolt. The inside end of that throttle plate shaft ends in a blind hole. I grind the head thinner and use a sealing washer with the works installed from the inside. On the outside I double nut the bolt.
Keep in mind that if original the compression ratio of a '68 is a healthy 10:1 and requires premium fuel. If the head has to come off for any reason having hardened seats installed on the exhaust side would be a good move. - Dave

Bill;

C1800 has it right! Spring loaded Valves in the Carb Throttles are the Throttle Overrun Valves, intended to allow some mixture through under closed throttle/maximum manifold vacuum. See: https://www.sw-em.com/SU_Carb_Throttle_Plate_Early_vs_Late.htm#Throttle_Overrun_Valve They are not necessary and a nuisance when they fail.

Dave is thinking of the dual throttle Manifold...

Cheers

Compare the new plates to the old ones as I think they are different in shape and if they are different you should re-use your old ones and if you want you can solder the decel valves shut.

And we also regularly remove the secondary throttle plates and shafts, but we use plugs instead of bolts. Plugs are used for the inner holes for the secondary shafts so we use plugs.

And if needed, I have those plugs in stock.
--
Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com

Thanks to everyone for their replies to my question. I plan on having hardened valve seats installed once I evaluate the overall condition of the engine.