Your Fireblade choke pull that is, the screw cap is designed to push against the clamping tabs and apply friction to the choke plunger shaft, so the the choke will stay on until you push it down manually.
trouble is these plastic parts wear and eventually the screw cap can’t press hard enough to create the required friction.
1.pull up the rubber cap
2.unscrew the screw cap
3.cut a sliver of plastic from a yoghurt top, approx 2mm wide and 10mm long
4. slide the plastic sliver between the plunger shaft and the screw cap (dotted line in photo)
5.tighten the screw cap down
the plastic should sit between the chamfer in the screw cap and press against the clamping tab, you should now have enough friction for the choke to stay in the position you have pulled or pushed it to. You can see the end of the plastic sliver in this photo before I replaced the rubber cap.