The engine of a Harley-Davidson motorcycle can sputter and waver when a crankshaft position (CKP) sensor begins to fail. The CKP sends data to the motorcycle’s computer data center about the position and speed of the crankshaft, ignition, and fuel injection. A faulty CKP can cause the engine to backfire due to unburned fuel and falter when accelerating. Symptoms of a faulty CKP get worse when the motorcycle is accelerated. Contaminated fuel or faulty spark plugs can also cause similar problems, so you may need to test the crankshaft position sensor.