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Hello ThumperTalk community! A few years back I bought a 2005 YZ 125 with a then fresh top end. It was broken in correctly and running fine for a few rides before and it bogged out and shut off on me at a high RPM and has not started since then. The bike sat outside for a good portion of the past few years and although there is not much rain or snow where I live, the bike has gathered some rust and still won't start. Now, I am attempting to resurrect the machine as I have the free time. Although, I am new to mechanics, and new to two strokes, so I need some assistance. I have been doing my own research and been working on the bike here and there but just need another half brain... this seems like the right place. So far I have:

 - drained the gas and put a fresh mix in 

 - checked the plug (wasn't fouled and there is spark when I have kick tested it and I have not altered the gap at all) 

 - cleaned the entire carb (bottom of the bowl was covered in residue probably from sitting)

 - pulled out the magneto as it has surface rust on both the inside and outside ( I pulled it off to check the state of the coils and stator : I have attached an image for reference : I know it ain't pretty )

 - Tested the resistance of the coils and the measurements came back as the following: 'Pick-up coil'=10.3 Ohms / 'Source coil 1'=18.5 Ohms / 'Source coil 2'= 04.7 Ohms ... the manual calls for 248-372ohms on the 'Pick-up coil' / 720-1,080 ohms on the 'Source coil 1' / and / 44-66 ohms on the 'Source coil 2' ... my readings all seem far to low and this makes me think that I need a new Stator/CDI set up...  What I found strange when testing the resistance though is that when measuring resistance in kilohms (before I realized my multimeter had an Ohms reader) the numbers came up very different. I understand that 1 kilohm is the equivalent to 1000 ohm BUT when measuring kilohms the 'Pick-up coil' read 7.95 kilohms which would equal 7950 Ohms which is far higher a number than what the manual calls for. The 'Source coil 1' measured out to 8.45 kilohms = 8450 ohms and the 'Source coil 2' measured out to 7.62 kilohms = 7620 ohms. So, not sure what was going wrong here as I was aware to place my tester leads in the right spot. Any ideas? Which reading should I take as correct?

I currently have the magneto pulled off and I was thinking my problem might be that the kill switch is stuck ? I wanted to hear some opinions before I put the magneto back on and try to start the bike again, this time with the kill switch unplugged. I have good compression too. 

Let me know what is up! Dying to get back on the bike! Thanks everyone! As for now, I think that is all I have done to the bike... HELP ME PLEASE ! 

Lucas 

ps - will my rusted magneto be fine to run if I can get it cleaned up a bit ? I have heard yes and no. 

Thanks again! 

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Start with the combustion triangle. Air, fuel, spark. If you have spark out of the plug then you should be fine on the left side of the bike. If you took the carb out my quickest and easiest suggestion is to see if the float hight is right. I've put my carb back together before and had where the needle was stuck and low and behold, no fuel was making it to the bowl. 

Fixed the float height, bike fired. 

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Going back a few years, if it shut off at high rpm (seized ?) and has not run since afterwards,

I'd pull the cylinder head and inspect things before trying to re-start it again.

You simply could have a really messed up piston/cylinder which doesn't create enough compression to start.

 

Easiest visual inspection, pull the exhaust system off and look up thru the exhaust port.

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Thanks mlatour ! That was it... The piston is definitely scored, and the walls of the cylinder don't look pretty... I got the cylinder head off, part of the power valve push rod out, and the cylinder bolts off, should the cylinder be able to pop right off with the push rod in place? Not entirely sure if it could come off now or if I need to pull anything else off? Let me know what you think... Thanks again for the help. 

Lucas

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Also, I tried pulling it off and it wont budge, any tips? Would it be safe to spray a little WD-40 around the bottom gasket and the bolts?

 

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On 7/11/2017 at 7:15 AM, 505 YZ125 said:

Start with the combustion triangle. Air, fuel, spark. If you have spark out of the plug then you should be fine on the left side of the bike. If you took the carb out my quickest and easiest suggestion is to see if the float hight is right. I've put my carb back together before and had where the needle was stuck and low and behold, no fuel was making it to the bowl. 

Fixed the float height, bike fired. 

Thanks for the tips, I will be cautious of that once I get this top end replaced... Took mlatour's advice and pulled the exhaust off and upon looking up the exhaust port sat my scored piston and cylinder walls... :/ Anyone know the best solution for the cylinder? Minimal research seems like the answer is to get it resleeved and then buy a piston kit... What do y'all think about that? 

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That is a Nikasil plated cylinder, ideally you should have it re-plate and size matched to a new piston.

(sleeves are not the preferred way of repairing this)

 

And yes, according to your pics the cylinder should slide off at this point,

try some penetrating oil around the cylinder studs, gently tap it along it's side with soft mallet,

Don't be tempted to pry with a screwdriver around the base gasket!

 

Once the cylinder removed, the up/down play in the connecting rod and

perhaps excessive surface rust may also determine the high likeliness the crank must also be serviced.

(since the bike was left out in the elements without pre-storage preparations)

Edited by mlatour
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Cool. Thanks a lot. I am trying penetrating oil around the studs, gentle taps with a soft mallet, and starting to look for someone to service these parts. Any recommendations for re-plating and size matching for the piston/cylinder? I'll keep you posted about the crank and post pics when I get the cylinder off...

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I'm located in Canada but the names:  Millennium, Langcourt and Powerseal USA often come up in similar forum topics.

Others will chime in as to which is preferred etc.

Also, search online, there are free downloads of the Yamaha service manual in .pdf format.

 

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I just got my cylinder back from USChrome. Cylinder looks great. They also sell top end kits from about every manufacturer at close to competitive prices. It saved waiting on the piston kit to come in before sending them the cylinder. Hardest part of the deal was stripping the cylinder of all hardware including the head studs. If I wasn't so cheap, they'd have done that too, but for a price.

 

Cliff

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On 7/13/2017 at 1:53 PM, Ndrorder said:

I just got my cylinder back from USChrome. Cylinder looks great. They also sell top end kits from about every manufacturer at close to competitive prices. It saved waiting on the piston kit to come in before sending them the cylinder. Hardest part of the deal was stripping the cylinder of all hardware including the head studs. If I wasn't so cheap, they'd have done that too, but for a price.

 

Cliff

Sounds like a good deal regarding the top end kit, you happen to know how much it was to have them pull the studs and put em back in before they send it back to me? 

 

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Also! Here are pics of my piston and cylinder... you still think replating will do the trick based on these pics?

 

My service manual says I need a 'Piston pin puller' to get that puppy out. Anybody know of a cheat code to getting the pin out? 

mlatour? Any takers?

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Last thing... the crank looks good but there is some play where the piston and crank are attached... I am able to slide the piston back and forth a centimeter or so. Is that normal?

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I've always been able to slide the wristpin out without a puller,

perhaps try with a small thread rod / washers of the appropriate diameter before buying a specific tool.

 

Post some pics of your connecting rod and crankshaft, may reveal other things (discoloration due to overheating etc.)

Some side-to-side play of the rod is normal, it's any up/down play on the crank that's bad.

 

I'm sure the replating companies have seen cylinders in worst shape than yours.

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