Dee & Red,
All you need do is to tighten up the black plastic nut behind the choke knob. It will squeeze a little tighter on the choke knob shaft and hold the choke on for as long as you want it to. This is a plastic nut, so don't overtighten it - it doesn't take much to make a big difference. Try turning it 1/4 turn or less and check the operation.