Motor Locked UP! Need Advice Please!

I think I may have really messed up my son's 03 YZ450F. I started it took off down the road shifted into second went about fifty feet and the motor completely locked up. Even the kick started won't budge. I wasn't riding it extremely hard and don't think I was over revving it or anything.

I know it would just be guessing with such limited information but I need some advice and direction what to take apart and look at first.

Any advice would be greatly appreciated.


Pull the center timing plug and see if the engine will turn backward with a wrench (or forward, for that matter).

In the end, you're going to need to take it apart to see what's up. You might want to remove the clutch first to see whether the engine or trans is locked, but as you didn't say that it won't roll, it's probably the engine. Inspect the timing and valve clearance as you take it apart.

The most likely causes would be the cams jumping time, the rod bearing freezing, or something on top of the piston.

Thanks for the quick advice.

The bike still rolls fine with the clutch pulled in or when taken out of gear.

At the bike was running great, it never missed or sputtered and gave no warning. It seems like I may have heard a pop or snap at the time it locked up but I'm not sure.

Sounds like I'm going to need to start at the top and work my way down.

I had a kx250f do this and it was a broken valve, a small piece of it was on top of the piston and had stoped it TDC. I wouldnt try to start it even if you get it broke loose.

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