Primer lock and how not to get an engine fire
The primer is quite simple in that the piston has a lug on it, this lug locks into a detent that is just beside the slot that allows you to pull the primer piston out to prime the engine, once you have primed the engine and the piston is fully in rotate it slightly and pull it out while lining up the lug with the small hole drilled in the retaining ring, the piston will come out wards a little and engage in a detent, you can tell it is locked into the detent as the piston will no longer rotate.
It is vital that you use the check list as a check list and not a "do list" when priming the engine, you should have the engine ready to start with all the controls in the start position ( except for the mags..... But the key should be in the mag switch) before priming, once the engine is primed the engine should be started instantly if you don't start the engine quickly you are setting in place all the ingredients for an intake fire.
The reason for this is that the priming fuel will run back down the intake track and collect in the intake box below the carburetor, should the engine backfire you will then set fire to all the fuel in the intake system, I have seen two PA28's badly damaged and a Robin destroyed by intake fires due to bad practice when priming. ( the Robin has a different way of priming but the principles are the same).