If your RBS carburetor is not idling properly, there are a few potential causes that you may want to consider:
Idle adjustment: The carburetor's idle speed is typically adjusted using a screw located on the side of the carburetor. If this screw is turned too far in one direction, it can cause the carburetor to either idle too fast or not idle at all.
Dirty carburetor: A dirty carburetor can cause a number of issues, including poor idle quality. If the carburetor has not been cleaned in a while, it may be clogged with dirt, debris, or gummed-up fuel. This can prevent the carburetor from functioning properly, causing it to idle poorly or not idle at all.
Leaking gaskets: If the carburetor has leaking gaskets, it can allow air to leak in or fuel to leak out, which can affect the carburetor's ability to idle properly.
If you recently rebuilt your RBS carburetor check to make sure the gasket fit correctly. It is possible you used the wrong gasket and it is letting air into the carburetor from outside.
Worn or damaged parts: If the carburetor has worn or damaged parts, it can cause it to idle poorly or not idle at all. This could include things like the throttle shaft, needle valve, or accelerator pump.
Use spray carburetor cleaner and spray around each gasket area, like the mounting gasket, the intake manifold and any vacuum hoses coming from the carburetor.
To troubleshoot and fix an RBS carburetor that won't idle, you may need to perform some basic maintenance and inspections, such as cleaning the carburetor, replacing any worn or damaged parts, and adjusting the idle speed. If you are not comfortable with working on the carburetor yourself, it is recommended that you have the work performed by a trained mechanic or technician.