It sounds like you've got a pretty good engine there- - - -even #5 is within limits for pressure leakage, and that one should improve with enough run time for the rings to seat to the cylinder walls better. Do a good tuneup, and "drive it like you stole it!' These old engines are tough as a two dollar steak, and the more you work them the better they like it.

Otto- - - -once the porcelain is removed, the plug shell is threaded with a 1/4" NPT tap. Install the quick connect with some pipe dope or teflon tape to prevent leakage.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!