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2007 yz250 problem- motor toast?

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so tonight was riding, came into a corner and all of a sudden no power, felt like the motor was getting bogged down and flooded with fuel. died on its own, tried to kick it, kicker was seized. started pushing it back to the truck to head home, going down a hill accidentally hit the shifter, and it bump started, again, bogged/no power feel. shut it off immediately. tried kicking while holding kill button, kicker was now free. got home and started tearing down, nothing seems wrong. gotten down to having the head off. piston rings look good through exhaust port, no piston scoring noticeable either. looking at the piston dome and head dome, looks like a few dings on both. with some very odd buildup on one area of the cylinder head. no matching buildup on piston though. thoughts? my initial thoughts were crank/bearings? but figured this would have grenade the top end with it.

been burning off about 15 gallons of VP U4.4 and MS109 mixed with amsoil dominator oil 32:1 (yes have it jetted richer for fuel)

bike is completely stock with a fresh top end maybe 10 hours ago, earlier this spring, with a weisco piston kit

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Once you get the cylinder off check the rod for up and down play in the big end bearings. I'm guessing they're on there way out and you caught it before it caused major damage.

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That's what I was thinking. It's all original cept the piston and cylinder (I replaced both this spring). Was going to look at a new crank. OEM still the best? I know theres a big debate between OEM and weisco

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That's what I was thinking. It's all original cept the piston and cylinder (I replaced both this spring). Was going to look at a new crank. OEM still the best? I know theres a big debate between OEM and weisco

oem, pro x, hot rods,

wiseco has a few bad batches it seems like but there are bikes out there with hundreds of hours on wiseco cranks.

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It might just be dirt in the pic, but there's a good bit of damage done to that head, and the new piston. looks like something blew up at one time and left those pits. I'd send the head out for machining, all those high and low spots in the head hold heat and can cause detonation/pre- ignition. They will modify the head at the same time for optimum squish and your fuel preference. Is the spark plug electrode still attached? Thats how they look when the electrode comes off and bounces around the combustion chamber. Excessive dirt from a dirty air filter could cause it too

I'd stay away from that ebay rebuild kit, $260 for top and bottom end can't be good. poor quality and something won't fit properly, just stick with a good known brand and u will be ok.

Edited by NitrousR1

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it is not dirt, they are pits. ill be sure to send it off. but no to the spark plug questions. plug was completely normal in appearance. and i alternate air filters every ride, i have 3 brand new filters i switch out and keep clean with soap and water. there may have been a little dirt in the fuel. i run it through a filter as i pour it into my fuel container but its always a possibility. going to continue the teardown this weekend, but planning on new top end, crank and bearings in the lower end. ive been running ATF for oil as i keep hearing locally and on here that it does just fine in 2smokes. anyone else have other recommendations for tranny oil?

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http://pages.ebay.co...id=300928599997

Haven't done any more teardown but anyone ever seen, used or heard any feedback on this kit?

When they dont list brands, its bad news, garunteed those are all the worst of the worst, wiseco crank, namua piston. and cheap paper gaskets. Dont do it.

Cheapest i would go

hot rods crankshaft kit

pro x topend kit D size

oem, cometic or athena gaskets.

It would be cheaper and better to rebuild the crank, kenoconnorracing.com there methods for rebuilding are the best and they use a quality japanese pro x rod.

ATF type f works great. I used shell rotella triple protection 15w 40. as 4 stroke engine oil, or transmission oil. It meets the jaso ma certification for wet clutches. And you can get 5 gallon pales of it for dirt cheap at walmart.

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check the pistons ring locater pins.let us know if there both still there.

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Piston is actually good except the dome of it. Trying to spin the crank you can feel it went, not much play but it feels like sand is binding it. All the crud in the pix is metal shavings. Got a new OEM crank and top end kit ordered. Cylinder is still good, a very minor grove on the intake side of it that came out with barely any rubbing with green scratch pad. There is a fowl smell, like a mix of burnt oil, seared metal and melting plastic though haha

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Piston is actually good except the dome of it. Trying to spin the crank you can feel it went, not much play but it feels like sand is binding it. All the crud in the pix is metal shavings. Got a new OEM crank and top end kit ordered. Cylinder is still good, a very minor grove on the intake side of it that came out with barely any rubbing with green scratch pad. There is a fowl smell, like a mix of burnt oil, seared metal and melting plastic though haha

To me with all the score marks on that piston skirt that thing is shot. Post a few good pictures of the cylinder walls.

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The marks seen on the cylinder wall you can't feel, nothing catches on them. Scorch marks maybe? Any way sending off my cylinder and head to millenium for whatever they need to do. Already ordered new jug, head, top end kit, crank, bearings and power valves, all new OEM. Want back up parts like this for future tuneups.

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yz cranks are famous for going like yours did. more so in the higher revving hotter running 125s. heres an 08 i did stuffers melted just like yours

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these guys have a fix, watch this video

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How many people have done this? What kind of results?

Shoulda mentioned the mod in the video to the crank

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