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Blown 2009 YZ250F Project

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Hey all, I'm into buying blown bikes and rebuilding them and just picked up an 09 YZ250F that went boom. Overall condition of the bike is ehhh to okay, the head still looks good, but the crank/piston and possibly cylinder are toast. I'll be doing a full rebuild, possibly SS valves depending on where the current ones are shim wise, and most likely a WR trans swap because I ride trails and fire roads and the MX gearing isn't my preference. The bike came with eBay KKE wheels on it, and a search turned up mixed opinions. Aside from them being heavy, has anyone actually had problems with them?  

 

In yall's opinion, why does it look like this thing seized up? Here's the skinny:

It was reportedly 80-90 out when it seized while riding track hard. Approximately 70hrs on it PO says.

The wrist pin was welded inside the small end of the rod, but no signs of lack of oil (piston had oil in the oil control rings still and up along the rod) 

Head was full of oil, lobes/buckets look fine 

Piston shows signs of severe lean condition/predetonation to the point of pieces of aluminum on the intake valve side of the piston being broken/melted off and smearing up and down the cylinder (obviously why I locked up). Bike was reportedly full of coolant and radiators were clear. Piston is super rough/pitted around top ring land. Head looks fine although has a lot of carbon on it.

 

Bike has an FMF megabomb and has been "rejetted." I haven't opened the carb to see what jets are in it. 

It'll get fully rebuilt, just thought it'd be fun to try to guess the failure mode! Thanks all! Pics kinda suck, but they're all I have until I pull it out of storage (currently snowmobile season!)

 

Thanks everyone!

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Lean jetting could be part of the problem but I would be very suspicious of the oil nozzle and oil supply to it. In those bikes all debris from for instance broken clutch  plates would collect at the oil intake screen and plug it.

 

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Did You tear the bike down to this point ?

If You did, was the clamp tight on the carb at the intake ?  I would look for air coming in on the intake side.

 

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Unfortunately I bought it already torn down, so I have no idea if the carb was in there loose. 

 

I was looking through a parts fiche and I don't think the yz250f got a piston oil squirter till recently. The 450 got one around the 06 timeframe, but I don't think the 250 got one till 12 or newer? I don't see one listed in the fiche for a 2009 at least. Odd, my honda and I think my kawi had one. One less thing to go wrong I guess! 

Edited by charlevoixboy

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I don't think they had a actual bolt on part like from 14 on but i think a passage was incorporated in one of the case halves.

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I'd look at the oil in the filter housing, and cut the filter apart and check the oil pump and screen.  You are going to split the cases, right?  I'd check all the case bearings while I was at it and the clutch as well.  Piston looks oil starved.

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Definitely was detonating to the point of preignition. It might have been lightly detonating and he couldn't hear it, or didn't pay attention to it. It could have been slightly lean, or too much ignition advance (maybe the rotor came loose?), or bad gas. Once something like the ground strap on the spark plug gets hot enough from detonation to cause preignition, all of the damage in the pics will happen in an instant. The molten balls of aluminum on top of the piston is the proof. And oh yeah, there is no such thing as predetonation. 

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Preignition not predetonation, you're correct. My bad! 

 

If definitely looks like the bike was preigniting with the flame front centered around the intake side of the piston. That area is the most damaged and where most the aluminum that's been painted along the piston and cylinder came from. The piston donesnt show the classic four corner seize of an overheat condition due to a malfunctioning cooling system, it instead looks almost oxidized, like it was running lean or had some crappy regular in it instead of premium. Spark plug was a blackish color, not white or abnormally hot looking. But I've always had a hard time getting a good read off the plug in the thumpers. 

The cases will absolutely be split to do the crank and trans swap, and while I'm in there I'll be washing everything down to make sure there's nothing floating around in any oil galleries or passages that may cause new issues once it's rebuilt. 

I also found a jet kit in the pile of parts from fmf. It's one of those power up kits which I'm assuming is just a rebranded JD jet kit kind of deal. It'll be fun to crack it open and see how bad everything looks. 

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Close up of the piston carnage. The wrist pin was also welded into the small end rod bronze bearing, and then that caused it to pivot in the piston until it was all galled out and super sloppy 

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7 hours ago, charlevoixboy said:

I also found a jet kit in the pile of parts from fmf. It's one of those power up kits which I'm assuming is just a rebranded JD jet kit kind of deal. It'll be fun to crack it open and see how bad everything looks. 

FMF power up kits are rebranded Dynojet kits - you'll prob find it has Dynojet on the instructions

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1 hour ago, GuyGraham said:

FMF power up kits are rebranded Dynojet kits - you'll prob find it has Dynojet on the instructions

Good to know! Do they use regular jet and needle numbers or are they a proprietary setup that won't correspond to yamaha jet and needle sizes? 

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Needles are bespoke to DJ.

Main jets, whilst they claim they are different they are just standard off the shelf stuff

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I was told the bike is sprung for an aggressive 140lb class A (ish) rider, and came with what I'm assuming are the stock fork springs in the parts box. Are the spring rates stamped into the forks so I can figure out what I've got in the box and currently in the bike? With gear on I'm probably 200# so I'd assume it's sprung too light. Also, the rear shock spring is solid black (not stock) with a white dot. I don't see any branding on it other than that, do these springs adhere to some color coding or is it just manufacture specific and there's no way to tell other than a spring rate scale? 

 

Thanks!

Edited by charlevoixboy

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Color dot ID's the nominal OEM shock spring rate but, actual rate can vary quite a bit, typically a 5% margin of tolerance.

Only true way is with a spring rate tester but the race sag / residual free sag can determine if it's suited to your weight.

 

Stock fork springs may or may not have any markings, optional OEM ones typically have some lines scribed on one end.

 

Some aftermarket fork/shock springs (Race Tech etc.) may have their manufacturer's part number printed on,

with that number you can usually figure out their nominal rates.

Edited by mlatour

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Does the Head look like there has been some blow-by to a water jacket...possible blown Head Gasket? Water ingestion will give you similar looking detonation scars mate.

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