2t oils

[dwint51]

I have a 50cc Chinese 2t and im using walmart oil .ive been having a hard time since I bought this with fouling plugs.i removed the cap off my air fuel mixture screw and leaned it down a little and had to raise the idle.seemed to be running fine and starting better.went for a ride today and it wouldn't start.took out the plug and it was black.put in a new plug and when I came home the new plug was white on the electrode.it was only maybe 4 miles with new plug.how can one plug be black and no adjustments a new one be white?