About three weeks ago I bought a 81 Yamaha maxim 650 and it ran great. About a week ago I was not able to get it started. I noticed the oil was low so I put a quart of four-stroke 20w-50 in it put it. However, it still was not running. When I turn it on it will cough than die. I checked the battery and it seems to be fine. This is my first bike so I am entirely unfamiliar with trouble shooting it. Any advice would be greatly appreciated, thank you.