But make sure it is actually locked on the pre take off checks. A pilot in my club once commented that the engine rough ran at low RPM and wouldn't shut down on ICO on one of the club PA28s, so he shut down on the mags. On inspection, it turned out to be the primer which was unlocked and slightly out, by only about half an inch.
It was obviously allowing fuel to be sucked past it into the cylinders making the mixture rich and causing the rough running at low RPM. It was also allowing enough fuel to run the engine at ICO.