Hook up the multi meter and just lightly "flick" the throttle open / close .. to see if the volts vary in the closed position (it's in the closed position that is giving you grief)
Other than that .... give the whole throttle system and hoses a good internal clean ... including the "flame trap can" on the drivers side rocker cover (unscrew and wash in Kero)
Manually hold open the throttle butterfly and clean all internal sides & edges
Cheers
Mike

