On my 83 watercooled 1.9L digijet FI throttle assembly are two
switches; one for throttle closed (idle) another for full
throttle.

What do these switches actually do to the fuel system when
activated?

The throttle closed idle switch on mine is essentially out of
the circuit. It appears that it is intended to be
activated/closed when the throttle is closed, however at idle
with this switch closed I get surging--back the adjuster screw
out so the switch is no activated and it idles OK. This seems
backwards to me as in order for the engine to idle the switch is
noit activated, nor is it activated when accelerating.

Thanks for your wisdom!