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did I just break that engine ?


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Hey Christian !

Thank you for your helpful post.

I'm taking note of the shavings cleaning method :)

You're right about the woodruff key, it was dislodged. I put it back, and it was really easy to slip it in : Is that a normal ?

I'm glad you enjoyed the videos :) 

I'll post a new one soon where I put the engine back on, after having done honing on the cylinder and some light sanding on the piston : result : no compression.

I guess it might be a few things : bad piston rings / bad sealing of the flywheel carter ?? dunno will seek online help first then probably pro help.

Will post the video next week... going away for the week-end now.


Hey Lorenzo,

I agree with you, paying a pro might be the sensible thing to do. Unfortunately, I'm not rich, and if I can do it myself and learn through the process, I will try.

I'm a quick learner, and I've always been self-taught on many different fields, I LOVE learning : /

That said, I'm still seeking help from kind nearby mechanics you are willing to share and help. Thanks to internet and forums.

Anyway, I'll keep posting what happen, and hope you'll still give me your helpful feedback.


Cheers !

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The woodruff key should be a nice, snug fit.  The tighter (to the shaft), the better.  If its loose-fitting or visibly damaged, replace it.  It's a cheap part and worth the minimal expense - especially when remounting your flywheel.  When they're so loose that they fall out on their own, it's easy to accidentally push it out of it's slot and behind the flywheel when sliding your flywheel over the shaft during the attempt to line up the flywheel slot to the woodruff key.  Get a good flashlight to confirm it's in position before tightening everything up.

You can totally do most of this work yourself... however, as Lorenzo correctly points out, there are going to be some tasks better left to a professional.  One job for certain - rebuilding your crank if your crank bearing is shot.  Pressing out a crank pin requires a press, fittings and experience... and balancing afterward is something of an art (well, mostly science) that I'd never attempt personally.  Beyond that, motor disassembly and reassembly is definitely within reach.  You'll learn quickly enough... and, if something goes wrong with your rebuild... just tear it down again and rebuild it till you get it right.  

Please don't hesitate to reach out if/when you run into any snags.  Some wonderful, very knowledgeable folks frequenting this site can help steer you right.

Good luck!


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Hi vintagenut,

thank you very much for your post !

I'm going to recheck everything, and specially that woodruff key. I think I'll change it.

If I still get nothing out of the engine, I'm going to try to change the piston and its joints, plus re-bore the cylinder to that new piston size.

Here is a quick rough video (couldn't film everything, like the light sanding of the piston or the inside of the flywheel)


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Doesn't look like there's very much compression on that motor.  Did you check ring end gap after honing your cylinder prior to re-assembly?... or a compression test afterward?  Seeing how easily you're able to push through with your kickstarter by hand makes me wonder if there's enough compression for her to fire.  Even if she does, I think you'd be so much happier with a new top end anyway.  125cc motors have to work so much harder than those of larger displacements and typically need to be refreshed/rebored with greater frequency as a result - at least that's been my experience, anyway.  Keep that OEM piston (in case you stumble across another cylinder to keep as an extra!) and move on to the next overbore.  She'll be so much peppier with a new top end!

Again - great work on the videos.  Very entertaining!

Cheers -



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hey Christian,

the only test I made was to insert a piston ring inside the cylinder, and I could see the light one a section of the ring, not good, but the mechanic I was with (the one who hone the cylinder for me) told me to try anyway.

I'm going to change the top end as you said, you really got me with "so much peppier" ! :D

thanks again for the feed back mate !


Edited by 6ril
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Hi everyone,

I got a tip on a french forum, a forum about old tractors :)

"pour a little oil by the spark plug hole, kick a little, put the sparkplug back and try again"

I did just that. It gave some sealing I guess, the compression got better and the engine just started !



I'm still considering changing the top end, but I'll try it a little as such, if it's not too risky. What do you all think ?


I'll post another video to try to get you to hear the engine without all the rattling.


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Well it was nice for a short time...

I tried to go for a little ride, and 2min in, the engine went "boooooh booooohbbooooo".. stop.

I pushed it back toward home, then it was downhill so I tried to start it going down the hill and it did start, but worked for less then 20 seconds. Same booohboooo stop.

I got the feeling that it might seal while it's cold, but then when the engine start to heat up, dilatation appear and It loose its seal, leak and loose compression.

Could it be ? What do you reckon ?

I'm going the stop messing around and get a re-bore and new piston.  (i keep saying it, got to do it...)

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So, I'm still not making the new top-end step !

After a little more investigation I found out that my new spark-plug it toasted.

Seems that's what stopped my engine, and it's now the second spark-plug that gets toasted. Plus, I got the oil leaking out the engine (by the exhaust).

It makes me feel that I'm in this situation (something I read online) :

the problem :
I found myself with this issue: spark-plugs are getting toasted ( 125 type185 bul)
I've change the "ruptor" and got a brand new condensator, sparkplugs are still getting toasted
I also have oil leaking out the exhaust making a mess all around !
could there be a link between those two problems ?

an answer :
For your toasted spark-plug problem, it's the seal between the 2 halves of the transmission carter that needs to be redone. I have the exact same troubles with my 185. Don't look at the ignition any longer, it's the transmission oil that goes up the cylinder: As a result, you run with an overdosed mix, and spark-plugs do not like that, and the oil oozes out the exhaust.
I've resealed with a brand new seal and loctite AJ45, ans since, no more traces of oil nor toasted sparkplugs.

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If I was you I would change the primary side crank seal and probably the ignition side as well. Especially if this engine has been sitting around for a while. The crank seals do go hard. They are pretty easy to change. You don`t have to split the engine in halves just pull the clutch and the primary drive off and put the seal carrier off. If you do this be sure to put a new O-ring in behind the spacer behind the primary drive  Looking at your videos I would still give it a rebore and a new piston kit. Graham.

Edited by bullylover
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1 small detail I've noticed & would change is to put the wiring along the back of the downtube instead of along the side to protect it from rocks etc, fairly certain the loop just below the tank is for the clutch cable 

I've had several bikes that leaked oil at the front exhaust joint, a small bead of high temperature silicon left for 30 minutes or so before assembly seems to work well

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