Post Mortem...

After a complete tear down of the YZ125 engine following the Lake George debacle, a few things have become clear to me. Before I had a chance to split the cases I had speculated that the cause of the engine failure was either 1.) improper jetting (too lean) 2.) old premix that had somehow degraded 3.) a combination of those 2 causes exacerbated by the introduction of ethanol in the fuel that we buy these days.
Now that I have the cases split and the crankshaft in my hand, another kernel of evidence has been discovered. One of the crank seal springs had failed, and had been caught between the crank web and the case. The damage to the case was obvious, but not consequential (I hope). But the seal failure should have been obvious to me much earlier, I just wasn't paying attention to the symptoms.
When a crankshaft seal on a 2 stroke engine fails, the most obvious symptom is a high idle. This is because air is getting past the seal and being consumed as part of the fuel mixture, leaning the mixture because it's downstream from the carb. Add the aforementioned jetting and premix problems with a dash of cold riding conditions, and you have one confused and misguided racer.
I assumed the high idle was a result of having the big bore kit and jetting changes done all at once, so I adjusted it down with the idle air screw. That seemed to work and the bike started and ran fine. Till it blew up.


The crankcase, scored by the spring caught between the case and the crankshaft web.


The remains of the crank seal spring, trying to hide in the web.

As it turns out (no pun intended), it's probably just as well that the engine did what it did when it did. While I was taking the ice wheels off and preparing to put the roller away in the barn I discovered that the 19" front wheel/tire combo had been a tighter fit than I had thought. Tuns out that when the forks were compressed, the screws on the left side of the tire were trying to eat through the left fork. They got as far as the seal and the first layer of ridges on the upper leg. (It's an inverted fork) If I had taken another half dozen laps the fork would have been worn right through. Kinda wish I could have seen the sparks flying though...