I'd been riding my magnum all summer without incident, until yesterday. Yesterday when I compressed the start lever and stomped down on my pedal there was less resistance than normal, and it took a few more kicks to get the engine to turn over. This morning was worse and I was only able to start after biking down a hill. And now the Puch is completely dead. It feels like the pedal isn't engaging the engine at all- when I stomp down, the back wheel makes a whirring sound and spins until it stops of it's own accord. Can anyone tell me how to fix this?