kx250 2003 wont run

[yamaha35]

i have a 2003 kx250 just bought from private owner, bike starts up runs really good, they owner had been running the bike at 32;1 and had black ooze all over, they shop told me to change it to 40;1 now the bike runs the same but now the bike wont stay ideal just dies if i let off the throttle what could be wrong or how can i fix it,

[JessKruid]

for starts, try adjusting your idle screw if you feel it could be turned up without being at too high of a rev.