I had my '03 WR450 out for it's first ride since shimming the valves. I had it up at elevation (Brushy Mountain, NC) and it ran like a beast...except it would randomly stall and wouldn't re-fire. It was perplexing. When it wouldn't re-fire, I switched the petcock from On to Reserve and it would light. It ran great the rest of the day on Reserve.
After that day's riding, I shut Off the petcock. The next morning, I just (habitually) set the petcock to On. After about 10 minutes of riding, it stalled again and wouldn't re-light. I put the petcock back to Reserve and it fixed the issue.
So I'm thinking that the petcock may be gummed up or there may be some foreign matter stuck in the valve.
The strange part is that I had given my KDX220 to a buddy to ride that weekend, and the same thing happened - luckily, I was there when he was kicking the crap out of the starter, and I switched it over to Reserve and it ran great. Both bike run 93+ gas (with 15% Ethanol) out of the same pump, and I pre-mix my 2-strokes with Amsoil Dominator.
Can this just be a problem caused by Ethanol? A few of my riding buddies run 100+/No Ethanol gas and they didn't have the same luck I did.
I'll be taking the petcocks off and apart later this week, but I was just wondering if anyone else has experienced something similar.