fouling plugs on 2002 CR125?

i have a 2002 CR125.When i bought it (4 days ago)the guy said it would always foul plugs when he rode it with his family because he would have to ride slow, but when he rode it hard, then he wouldnt have the problem.When we were there we also noticed it was running too rich and would spit oil out the sillencer.When i brought it home i changed the spark plug to one that has a bigger spark (BR9ES). I rode it the whole day without a problem it ran perfect. I stopped the bike for about 5 minutes, started it back up , and the plug fouled immediately. my friend thinks i need a new pilot jet. i have a 32.5 in there right now. its about 60 degrees out here. does it need a new pilot jet or just a new spark plug?i am running 32 : 1and what do all the numbers on the jets mean