@Stephen: I guess you are right. I could check by opening the airbox and removing the filter while the engine is running. If my digital tacho doesn't change, that supports your opinion/experience.

@ericandchi: Great idea. I was thinking about soldering a knob to the brass screw (as can be found on the Internet) but reckoned there would not be enough space for my finger on the lhs screw. Adding a few cm of control wire from a bicycle brake between the knob and the screw should help. Thank you.