It's also possible that an old Cox engine is too gummed up or too tight to turn over easily. The symptom will be just a pop once in awhile instead of at least burning off the prime with a 1 second run.
If the engine is "free" enough to run, the prop should dance back and forth with a gentle flick.
These engines were sold by the millions to beginners and are some of the worst engines you could ask a beginner to run and try to keep running well for the long haul. There are so many little nit-picky things internally that can keep you from getting it to run well or to run at all.

Be sure to run 25% oil with at least some castor oil involved.