[blwncobra]

sounds like carbon build up combined with oil working past the valve seals, when an engine sits for a long period of time oil in the heads tends to leake past the valve seals and loosen the carbon build up on the top of pistons and in the combustion chamber. It is most likely getting the plug dirty, which is why the hard start and the pop (late firing of the plug). just run it at idle and check that the timing is still set and it should take care of itself