starting to sound like you need to redo the carb job the seller did.
Doesn't take much to clog an idle passage. at least try to (count the turns.) ..remove the idle adj. and blow carb cleaner in the carb orifice (s) followed by air pressure. reinstall with the same turn count and try to adjust the idle again .
check all filters
Seems the 1970 60 J,s have the old style carbs.... easy to clean
the top size dime plug needs to come out to clean 2-3 very small drip holes used for the fuel supply at idling... the needle passage is easy enough to clean . the carb side channel allows fuel to be sucked UP into that chamber via a small channel orifice on the side of the carb . That very small passage needs to be clean so fuel can reach the top side needle and enter the drip chamber.
Everything must be clean. see pict of an 8hp & 18 hp and old carb... same principle see side channel on the 18hp pict. and an old carb. check needle packing to !!
after cleaning install a new dime size plug and tap it in place not to hard with a flat punch and seal with fingernail polish all around