Motorcycle Repair: 150cc united motors dirtbike, japanese bike, valve clearance

Question
after you get it started it runs good for awhile the it fouls the plug. put in a new plug runs good awhile and continues to repeat itself!

Answer
This may be one of those learning episodes. Take the spark plug and go one number lower as this is a hotter plug. Example B8ES in bike go to a B7ES run bike like you have when it fouls out do what is called a plug read. The correct color should be a light brown if its black the its gettin to much fuel or the compression is low. Check your valve clearance if its a four stroke. Im not familiar with this bike. Is it a chinese hybrid copying a japanese bike?