'99 YZ250 suddenly stopped running

MXTex

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#1
Yesterday my friends 99 YZ250 suddenly stopped running. I'm looking for tips regarding a troubleshooting process. Here's what I know.
- The bike ran well during the first ride of the day. It sounded good quite frankly.
- On the sencond ride of the day, I noticed the bike breaking up, (mis-firing) when first entering the track and attempting acceleration. It then cleared it's throat and sounded ok.
- About 1/2 lap latter, the motor just stopped. No irregular sounds leading up to this; just stopped and wouldn't start.
- When attempting to re-start, the kick starter was very easy to push. I could easily move it through the stroke with my hand.
- The bike was bought from a local racer about 6 months ago. The previous owner allegedly had the entire bottom end gone through just prior to sale. Additionally, the top end supposedly had only two rides on it. Vertex piston.
- In tearing the bike down to prepare for cylinder removal; I've found the following so far.
- When removing the pipe, there was no copper washer betwwen the head pipe and cylinder. Maybe Yamahas don't have these. My Hondas and Kawis do.
- Viewing exhaust side of piston shows significant blow by but rings are still in place.
- Reed valves look fine.
- Spark plug is heavily fouled; black.
- Moving motor through stroke via kick starter was easier with spark plug removed indicating that some compression was being generated; but not much.

Any help you guys could provide regaring my troubleshooting process and repair would be greatly appreciated.

Jim
 
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#2
I looked at Parts Fish (www.powersportspro.com) and there doesn't appear to be a metal washer that fits between the pipe and cylinder. That's not your problem though. I am clueless as to why it did what it did. Maybe something happened to the rings even though they look OK through the exhaust port. I would remove the cylinder and inspect everything. If there's blow-by after only two rides, something is up here. And when you say the plug was rich, did the bike run rich when it as working?
 

bigred455

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#3
You need to take the cylinder off,i am guessing loss of compression due to the rings ,if part of a ring broke off it would be a good idea to flush the crank out just in case the ring didn't get spit out the exhaust or stuck on the piston.
 

MXTex

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#4
I removed the cylinder this morning. If I can get hold of a digital camera, I'll post pictures. Here's what I found.

- Two gouges running down the intake side of the piston. They are in line with one of the intake transfer ports. I can feel two small ridges on this port.
- The top of the piston has small metal particals of unknown origin stuck to the top.
- Both rings appear to be complete although they are both jammed into their grooves very hard. (can't get them out)
- The wrist pin was very difficult to get out. I fabricated a pusher to get it out; and when it did come out, it was in two pieces. I don't see how this would be the source of the piston gouges.

Questions:
- Potential source of metal particals. Presumably they are what also caused the piston guoges.
- Cylinder verdict. The bore looks fine although that intact port does have two slight ridges.
- How do you flush the bottom end. Mineral Sprits?

I'd appreciate any feedback regarding source of these problems and corrective action. I'll definitely be giving Eric a call.

Thanks,

Jim
 

marcusgunby

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#5
The particles on the piston top could be detonation or parts of the main/big end bearing breaking up.The piston pin being so hard to remove normally means something has got real hot.Has the head got lots of small pin holes in it?
 

EricGorr

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#6
Hi guys,
Jim and I talked on the phone yesterday and the wrist pin was shattered. We think that the problem originated in the big end bearing, with the needle cage fracturing and scattering up the cylinder wall.
The previous owner said that the bottom end was recently rebuilt. If the crank became wedged upon installation with the impact method, (meaning the flyweights are not parallel) then the resultant crank run-out could of caused excess vibration and the bearing damage.
Suzuki makes a special tool that draws the crankshaft into the left side crankcase main bearing. This makes it real easy to assemble the engine. I think the tendency for most home mechanics is to reach for the dead-shot plastic mallet.
Jim Neupert and I will disassemble the engine and keep you updated on exactly what happened to this engine.
 

MXTex

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#7
For all those interested, please find included an update. Eric, please clarify as you see fit.

We shipped the entire motor to Eric. Upon inspection from Eric, it appeared that the big end rod bearing had failed due to improper assembly of the cases. As I understand it, the crank halves were improperly stressed during installation which through things out of kilter leading to premature bearing failure. Additionally, the cylinder bore was found to be damaged requiring an overbore and re-plating. We look forward to receiving the motor next week.

Worthy of noting, I can't say enough good things about the way Eric handles his business. I lost count of how many times I called Eric asking about progress on this project. Each time we spoke, he answered all my questions thoroughly, patiently and with a smile; and this isn't even my motor. Each time you hang up the phone with Eric, you're left with a great feeling of knowing that he cares about you and your project. His level of technical competence is simply incredible.

While riding at the track last weekend, a friend asked me how this project was going. I informed him of progress to date and he asked, "Why did you send the motor so far away? Who is Eric Gorr." I tried to explain, then just asked for his email adress so that I could forward on Eric's website address.

It's going to be fun watching my friend roost with this motor now. I feel confident that it'll be the best 99 YZ250 on the track.

Thanks Eric.
 
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#8
hey eric have you ever dealt with Canadians?? My cousin has a 2000 CR125 and he was thinking of sending his motor to you for an overbore!
 
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#9
MXTex, when i read this topic i suddenly reliezed there was a date gap of a month. Great that you took the time updating us with yr findings.
I wished more people would take the effort to do so.
thnks george