I just went and picked up an 03 kx 500 in really good shape today. It's pinging pretty badly though. surges pretty hard. I test rode a junky 86 kx 500 last week and that sucker would scream! Couldn't hardly even give it gas in all 5 gears without the front end coming up super easy. This one is NOWHERE near as strong. It has a new piston in it pretty recently. He showed me the old one which looked pretty good, just a little ring wear. He did say that he mixes the oil pretty heavy in it. It's an older guy, i think he said 52. He gave me his gas jug and I poured the gallon or so out of it in the bike when i got home. After riding it at home there was a good bit of oil on the swing arm from the pipe. It didnt ping when I rode it up the street and back at his house and it didn't get any oil on the swing arm. I'm wondering if that gas that was in the jug had alot more oil in it than what was in the bikes tank at his house. I need to check the plug but it started getting stormy so I put the bike up. Any advice is appreciated. I'm new to the 500. I've been riding a 99 cr 250 since about 03 or so. thanks!