Dominic,
You bet a back plate can leak and cause irratic running (lean). I rebuilt a Traxxas Pro .15 and ended up replacing the back plate because the start shaft through the back plate was very worn and caused an air leak. But this was an old engine.
There are occasionally bad engines out of the box, but that is the exception, not the rule. If the back plate is leaking the inside of the recoil and the one-way bearing will be really wet.