Looks like you have easy access to AVgas. My carb issues (not a Riva) were totally solved by buying non ethanol gas.
Before switching over to non ethanol, if my bike sat more than 3 days it had carb issues.
Tried Seafoam but that did not seem to help. After cleaning the carb for the third time I drained the tank and filled with non ethanol, no more carb issues. Bike has sat for 2 weeks and still cranks at push of button.
Non ethanol is about 10cents a gallon more expensive here, but well worth it.