I'm learning to fly on a PA28 and am suprised that the primer lock is not more positive, you can turn it too far and it pops out again,but I always check it thoroughly just to make sure.
I'm also suprised that the mixture lever does not have a detent (like a gear lever) to prevent accidental operation, as I have heard of low hours pilots inadvertantly operating mixture instead of carb heat on a PA28.
Lister