I went to ride my bike today and it was harder than normal to start, but it started eventually. So I started riding and it didn't seem right, it sounded very low pitched and wouldn't rev past 5k. So I stopped and had a look at it and it just cut out. From then on it was extremely hard to start and would always cut out after about 10 seconds. So I removed the spark plug to have a look at it and it was very oily and the oil was a dark colour. I cleaned the spark plug and then put it back in the bike and it ran like normal for about 2 minutes and then returned to the same problems as before. I didn't have another look at the plug but I am pretty sure that it would be covered with the dark oil again. I am very new to bikes so I am not sure what the problem is. Any advice?