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2005 YZ250F Problems- hard to kick, locking up

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I'm posting this for my friend. We were out riding a month or so ago and his bike suddenly stalled out and the kickstart locked up. He said it made a "clunk" noise right as it died. He stopped riding that day and i guess he got it to kick over a few days later. Then it was doing alright for a minute, he was still riding it. Then it started doing the same thing more frequently. I told him to put a little oil in the cylinder and try to kick it. He did and said it kicked over way easier. But it's still "seizing" up randomly. One minute it'll be very easy and smooth to kick- like it should be. The next minute it'll be extremely hard to kick.

 

The bike's in my garage now. He brought it over last night and it was kicking over fine for a minute. It actually started up and he rode it down the street but said it's running weird.

 

Am i right in thinking it might need a new piston/top end? He did say putting oil in the cylinder made it turn over easier. Or is it more likely that the bottom end is done? Is there any way to check how healthy the bottom end is without pulling the top end off?

 

Any help is appreciated. I don't have much experience with 4t's. I usually mess with 2 strokes.

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i would bet more along the lines of a siezed intake cam.

 

hey if it siezes, just kick it till it runs again and ride till it seizes again!  dont worry about it

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i would bet more along the lines of a siezed intake cam.

 

hey if it siezes, just kick it till it runs again and ride till it seizes again!  dont worry about it

Thanks for the suggestion. 

 

Not my bike, my friend started it and rode it 50 feet up and down the street to see what it would do since it hadn't been running in a couple weeks. I'm certainly not riding it, or even trying to start it up.

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I need some second opinions here..

 

I ended up taking the top end off the bike to get a better look at what's going on. (it's an '03 btw, not an '05). The piston and cylinder look fine. Also checked valve clearances before i took everything off and they were all in spec. The bottom end, i'm having trouble telling whether or not there's up and down play in the rod. It doesn't have enough play to physically see it moving up and down but it seems like i can feel it a little bit. Hopefully i'm not mistaking it for side-to-side play. If there is play, it's not really noticeable. That wouldn't have made it randomly lock up like it did though, would it? Right now with the top end off, the engine is rotating perfectly.

 

Is there a way i can measure the up and down play in the rod and see if it's in spec?

 

And if it's not the bottom end, it's gotta be the valvetrain like SUnruh suggested. What should i look for when trying to find a problem with the cams/valves?

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When my 03 kickststart locked up randomly it was a broken 4th gear tooth lodging randomly between the flywheel and the stator

Reverse Spin your back wheel in Nuetral and see if tranny sounds suspect. Or remove flywheel and inspect.

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Still trying to figure out what's wrong with this bike. Just saw a couple things on the head that looked kinda iffy but i haven't seen enough 4 strokes to tell.. Hoping someone here can tell me.

 

First, in pic 1, "A", there are these marks where it looks like something was rubbing against the cylinder head wall- but with the cam installed it doesn't look like anything could have been rubbing it, closest thing that could rub it is the valve bucket. Maybe those marks are normal..

 

But in pic 1, "B"- that journal feels weird. It's rough. If i scratch my fingernail on it it kinda scrapes over it, rather than glide across it like it does when i scratch around the other journals on the head. Could that be seizing up the cam? Far as i know, the engine hasn't had any recent work, and the cams definitely haven't been out recently.. If that's the problem, could a machine shop repair it? And would the cam(s) need to be replaced too?

 

Also attached a few other pics just in case there's something i missed.

 

any help is appreciated.

2014-05-02_17-11-43_536.jpg

2014-05-02_17-12-15_741.jpg

2014-05-02_17-12-38_800.jpg

2014-05-02_17-24-26_521.jpg

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