My 426 Isn't Running Right

I just bought a 2002 YZ426 this week. I got the opportunity to ride it once the day I brought it home, and it ran great. The power was there down low, up top, and all points in between. Today my friend and I were going to do some trail riding, and the bike fired right up on the first kick, then started to run like crap. At first it was just bogging on the top end a bit, then I went to turn a corner, slowed down, and stalled. I kicked and kicked, and it would not start. We pulled the plug, and it was badly charred, so we put a new plug in, and it still wouldn't kick start. We push started it, and it ran even worse than it did with the old plug. It would sputter and "catch" down low and had absolutely no snap whatsoever, and the top end power was still nowhere to be found. I shut the bike off, and all of a sudden coolant started spewing out the overflow, and I didn't even have it running that long.

I was told these bikes would run fine with racing fuel, and that's what I had in it. I wasn't sure if that was the culprit or not but figured that since I hadn't changed the plug or jetting (I'm pretty sure I'd need to do at least one of those if not both, correct me if I'm wrong) it was a likely suspect, so I put pump gas in it. Still no kick start, and we couldn't even push start it. Now that I think about it, I had racing fuel in it the day that it was running right, too, so now I'm starting to think that something else is to blame.

I'm sorry if this is rather vague, but I don't know what else to say. I'm worried that I just spent $1500 on something that's going to require twice that much to fix. If anyone can figure out some possibilities for what's wrong with the bike, please let me know. I don't worry about much, but I'm damn sure worried about my new YZ.

I'm wondering if the sudden "spewing of coolant" was the cause of a head gasket leak. If the leak is severe enough, cylinder pressure would pressurize the cooling system causing an almost instant overflow.

That could also cause the poor running condition, coolant in the cylinder.

Could the race fuel have caused the bike to run lean? I've read that running with a lean condition long enough can ruin the head gasket.

Alright, I drained the race gas out, put in some pump gas, and it ran much better. It still seems just a little bit more sluggish than it should be, but the difference in how it ran today was night-and-day. I'm much more confident now that it's jetting, or something else that's fairly minor to fix.

I got a Pro Circuit pipe with the bike, and I figure putting it on would be a good excuse to at least check the jetting, and it could give me a chance to check other things as well. Fortunately, I did get a factory manual with the bike, so hopefully it'll be of some assistance in checking everything else.

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