I have been riding and racing dirt bikes for the past 10 years, and I've never seen this problem before.
I own a 2006 Yz125. For the past 3 years I've been riding this bike, but only recently my problem occurred.
The Problem: It all started in April of 2013,
I blew out a transmission bearing, which I had to split the cases to replace. I ended up replacing the bearing, sealed the case halves and headed out to a local track. After about 10 minutes of riding, the bike seized up.
I brought the bike back to the shop, took the jug off and discovered that the liquid gasket maker had melted around the whole entire crankshaft. I eventually flushed out all of the gasket maker, but nylon on the crankshaft melted which made the crank hard to turn..I ended up having to replace the crankshaft. (This is what appeared to be the problem) This resulted in me ordering a Hot Rods crankshaft, new piston, etc.
I learned from my mistake and the bike was running good (engine wise) for around 4 months, up until I accidentally took it through too much mud and water which got sucked in by the piston, and crankshaft. I took off the jug, cleaned all of the muddy water out of the crankshaft and everything was spotless. After re-assembling everything, it was running as if nothing had happened. The next day I went to a local track and after a good 30 minutes, the bike seizes up.
Again I took the engine apart, un-seized the crankshaft, and found out that the lower crankshaft bearing was shot, meaning there was too much play in the bearing. This must have been the result of the bearing being too dry from the mud and water. (Keep in mind when I flushed everything out, it was as dry as a bone, and then I put 2-stroke oil by the crank)
I ordered up a new Hot Rods Crankshaft and piston, split the cases, put it all together, and broke in the new parts. After the break in process, I was riding for a good 5 hours, and then cleaned up the piston from any carbon deposits, put a new plug in, changed the oil, cleaned the air filter, and she was ready to go!
I went to another local track, and had a good 2 hours of riding on the bike before it seizes up.
I took the jug off, and the lower crank bearing won't budge at all.
When this occurred I was riding at a sand track at very high rpm's.
Any suggestions? Could it be a Hot Rod piston manufacturer error for the last time the crank failed? Is there a greater problem that's occurring? I do run an oil mixture of 32:1
Any help would be greatly appreciated, thanks!