hi, just bought a 2005 sherco 290, recently had an oil change, i got the bike home put fresh fuel in and it started straight away with choke, if i turn off the choke it stalls after about 20 seconds but would start atraight away again with choke, it then cut out after running for about 20 mins in total and wont go again, there was oil coming out of the breather pipe at the top of the engine and the spark plug had a lot of oil in it, any suggestions to whats causing this?// thanks