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TW 125 trouble starting and restarting


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  • 2 weeks later...

Well, I ran compression test with my brand new compression gauge and Its reading about 6 bars. Manual says should be at least 11, ideally 12. So I squirted a bit of oil in and it went up to about 8. So I'm thinking piston. Got it apart now but don't really know how to proceed. Pistons out and the cylinders off and I'm thinking I'll take both to a local bike shop and get them to check for wear so I can get the right piston kit.

Valves look in good nick but is it worth having them out and regrinding you think while I've got it in bits or best left alone? They don't look particularly coked up and the collets look ok.

Anything else worth doing while I've got the top off?

As always any advice is hugely appreciated.

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Thanks folks. A quick update.

A local garage checked the cylinder bore and that's ok However the exhaust valve was quite heavily coked - including the seat.

Inlet is ok, so it looks like the exhaust valve could be the problem. Lapped them both in anyway and de-coked

I'm putting in new standard size piston set. Any tips with regards installing the new piston?

I'm trying to work out which ring should go top and which on the middle. It's supplied with a metal one and a black one which looks like carbon or plastic of some sort. I can't find any info in the manual about this. I'm guessing metal on the top as that will take the most beating.

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Thanks mate. Rocker.

Trying to work out which ring goes on top. Ive put the rings under a magnifying glass and there are no marks. Looking closely neither has a chamfered side - inner or outer.

The only difference seems to be one has a bright outer.

I've been trawling the internet and cannot find any info on this at all.

The order out of the box was bright on top, then black, then the oil rings.

I've put them in the piston in that order.

What do we think?

15808257564_37a39d1715_s.jpgIMG_1829 by tiplerjim, on Flickr

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Thanks airhead. Had to take a bit of a flyer as I couldn't find much info. So I fitted them in what seemed the most likely order according to a site I found re chinese piston rings http://www.dansmc.com/rings.htm. Seems ok. Certainly runs well and is currently starting first push of the button.

So to recap it looks like it was a coked up exhaust valve causing poor compression, enough to cause bad starting but not enough for it to not run. I changed the piston and rings as a Chinese set was about £30. Not sure if yamaha do a genuine replacement any more. So I reground the valves and decoked the head. Hopefully that's it sorted unless I've screwed up in the reassembly somewhere.

Thanks again for all the helpful advice.

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