The Mysterious Engine Freeze

I bought a 2009 YZ450F from a person who said it seized about ten minutes after riding it one day. At the time I bought it, it was not totally seized as I could move the kick start lever. I removed the head and cylinder and everything looked good and as I rotated the crank it was working freely. I put the bike back together and it started right up and was running well...for about five minutes, then it stalled and the kick start was frozen as if the engine siezed. I put my wrench on the crank and was able to move it backwards about an 1/8 of a turn then forward an 1/8, then back an 1/8, then forward 1/4, then back 1/8th, then forward 1/2, etc. until it was moving freely as usual. I did notice this morning a spot in the rotation of the crank where it (for lack of a better description) kind of clanks a little. I think it is time to split the cases, but I wondered if anyone may know what I should be expecting to find?


Edited by CAMM

Sounds like the big end bearing on the crank is bad. You won't know for sure until you get it apart though.

The big end is a suspect, alright.  The roller cage can break, which allows the rollers to skew out of line, and that can lock things up.  Jiggle the crank around a bit and the re-align and the engine will turn a few more times and lock up again.  Also check for anything floating around near the primary gears.

I'd say a big end failure as well.  I bought a YZ 426F years back that the owner said had a cracked piston because it locked up on him while riding.  I found the same thing as you did and the failure ended up being the lower rod bearing.

I split the cases tonight and everything looks good. Would I even be able to tell if the crank bearings were bad since they are working...somewhat? Is it best just to replace them and assume they are causing the problem?

Yz400f motor froze up

I got what I think is the same issue I pulled the valve cover and can rock the cam chain back and forth about an inch it feels like I could force it in one direction but not the other please help last noise I heard did not sound good when it was running cam chain and valves look good from what I can tell just pulling the valve cover and checking it any ideas?

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