No spark on a cylinder of a 2 cylinder motor will not idle fine, it will be a single chug not smooth oposing cylinders. the really test for spark is you can start the motor on a hose and with insulated (very insulated) pliers pull the spark plug lead from the spark plug while running, and the motor should fall off the same amount, if you pull one and nothing happens, that one is not firing,and then the other one should kill the engine (2 cylinder only) A motor that starts and idles good is most likely firing both cylinders good.