Engine trouble.

Hey all so last weekend after the baja brawl a buddy and I drove all the way down to kentucky for a 05 yz250f. It looked pretty good (the guy had said it needed a new cam chain and new fork sealz) but wouldn't start. I said screw it be cause 1, I drove all the way down there and 2, I have a complete minus the head donor bike so I wasnt too worried. Anyways we got back and it sat for a week untill last thursday when we were trying to get it started.. I had already changed the oil and filter and put fresh gas in and meticulously cleaned the carb. We checked the chain, it waz brand new, valves werw all in spec save for one intake valve and one exhaust valve. They were a teensy bit tight.

Anyway, we got it runnin and it ran like crap. But we got it tuned in and it started to run like a champ. And then... it just died.. mid stroke... no sputter just died.. and it kept doing that.. we were confused.. very very confused.. we took it all apart again and everything looked awesome.. the spark plug was loose which I found odd. I had read the cam caps expand when it gets warm and that will stop thw cams but I had them torqued to spec.. thw only thing is my buddy was just pushing the bike through its strokes with the kick and it kept getting stuck, like it was seizing or something, but it would be fine and there was no knocking or valve slap or anything.. I'm very confused.. can anybody help?

Make sure that the timing is correct after he got the new cam chain installed. 1-2 teeth off and they don't work so good.

Ya I redid the timing myself. Its in spec. And after it ran for a bit I re checked it... it was fine.. I have no idea whats going on. Could it have been the cam xaps being to tight? Or 2 valves being a lil tight?

Its almost as its hard to kick and won't fire at all.. like I cant get it to to actually turn over..

Unfortunately you're describing the signs of a rod bearing failure. I bought my 03 as a basket case for the same reason. It had locked up on the previous owner and during disassembly it would suddenly turn so he quit with it. I could get the crank to turn 10 revolutions or so then it would lock, back it up and it would free up again. Search around this forum as I have read about others having similar experience with rod bearing failures. If you have the old oil filter you might be able to cut the paper out of it, spread it out, take it outside in the sun and see if you can see any metal chunks or anything sparkly. Clay

Ya it was sparkly... very sparkly.. how hard is a rod bering to change?

  • It requires a complete teardown. I guess you could just split the cases and leave the transmission but persoanlly I'd pull everything for cleaning. There is fine ground up metal everywhere and you don't want to send that through your new crank. As far as what to do with the crank there are several opinions on that. Some replace the rod/bearing with OEM, some replace it with aftermarket. I replaced the entire crank with a kit from Wiseco. Opinions vary on which way to go so you'll have to do your research and decide what you want to do. Clay

if you do tear it down. pay close attention to the oil pump internals. All that metal going thru can scratch the oil pump and greatly reduce oil volume/flow.

take out cam chain tensioner and do the proper procedurre to re install

Sounds like you guys are on the right track, but thought I would throw another idea out before you completely tear it down.  I think it would be worth it to look at the electrical system and make sure you aren't getting a short somewhere occasionally.  You said it felt like it was locking up if you pushed through the kickstarter.  Did it feel like tight like it was hitting top dead center or tight when it shouldn't be getting tight?

Tight upon TDC

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