The hand throttles are a friction hold at the bottom pivot point. They're kind of a pain to adjust. But do-able.
Bottom pivot of your hand throttle there is the tension nut *** which has a small set screw that MUST be loosened BEFORE turning the nut.. (or you will screw the threads royally)
Just a 1/4 to 1/2 turn tighter usually does it.
Mine used to do the same thing and only in the winter, which i blamed on possible ice/moisture/frost in the assembly.
You could try methyl hydrate over the assembly in sub zero to rid moisture and dry the friction plates without doing the adjustment.