Help! My sons '99 XR 70 worked fine until last weekend when I noticed that it was hesitant to start and kept stalling. While running it was smoking like I had never seen before. The oil level was very low and I filled it to the proper limit, changed the air filter, replaced the plug but still takes a dozen kicks or so to start and it will stall easily until really hot, and smokes terribly from the exhaust:( Any advice would be helpful:) Prior to this it ran effortlessly. Thanks!