The screw fell out of the gas selector...on/off knob...the one way inside that deep center hole. Forgot to tighten it when putting things back together last week. It is listed as (5x25) Is that a 5 mm dia. pitch 1, 25 mm long? I'm sure I can get one at Lowes. HDL gets $2.50 + shipping.
FYI, 5mm metric thread pitches are 0.8mm (coarse) and 0.5mm (fine). Coarse is more likely and more common, but I'm not sure which your fuel valve knob screw is.