Is there a convention for which way the handlebar of a motorbike etc rotates
to control the throttle? I ask because I have an electric wheelbarrow with
rotary throttle and it rotates the opposite way to the way I'd have
anticipated, so I wondered what the standard was.

The throttle is on the right hand side. I would expect to rotate the top of
it away from me to increase speed, and towards me (bending my hand back
towards my arm) to reduce speed. But it rotates the opposite way. Looking
end-on to the handlebar, from the end towards the central handlebar axle, it
rotates clockwise.

One of the implications of this is that if the speed control is turned
slightly too far in relation to how fast you are walking, the barrow shoots
ahead, turning the control further *on* - until the barrow leaves your hand
when obviously the throttle closes due to its spring. It's also
uncomfortable to bend your hand back at the wrist to hold the power on; it
would be more comfortable to bend the hand forwards (palm towards wrist).