There was one on a stick chopper at the woodyard many years ago. I remember helping the chap I worked for to decoke it a few times. I also remember when it was stopped by throwing the decompression lever it used to take about 5 minutes to come to rest, the flywheel was a heavy lump.
A chap I worked with put diesel in his car before the restrictors were fitted. He only got about a gallon in so decided to top up with petrol and run it out. He came into the car park at work and it was like James Bond laying down the smoke screen in the Aston Martin.
The flap is to stop you using the wrong pump, only the unleaded nozzle fits in it. The nozzle size standard dictates that the delivery nozzles for unleaded fuel shall not exceed 0.85" in diameter while the delivery nozzles for leaded fuel shall not be less than 0.93" in diameter.