Say I dump stabilizer in the tank, forgot to run it thru the carb/injector = Clogged jets/spray holes = Hard starting.
Say I haven't cleaned my air cleaner, is kind of dirty, richens the air to fuel ratio = Hard starting = hesitation = no power = dirty AC element.
Say I have electronic ign that does not change timing = Is not it.
Say I have solid lifters and the eng gets hot, opens a valve = Stalls = no start = starts better when cold.