The two Zenith-Stromberg carburetors that are used on your Volvo may be worn out. You will need to service them at one point in time. You can also replace them with new or remanufactured carburetors. Just keep in mind you want the manual choke, not the automatic ones. You can find replacement links and rods at Pierce Manifolds web site.
This is an exploded view of the carburetor.
The following picture were taken when I disassembled the carburetors.
Here is a diagram for adjusting the needle and the floater.
