CB125 smoking right cylinder

Ollie125 · Derestricted Danger Joined: 22 Oct 2015 Karma :

Hi All,

just finished a top end rebuild of my cb125tdc (re-bored to .5mm with new pistons and rings to match). Started to run it in (only done about 15miles) keeping it under 5k, but the right hand cylinder is smoking quite badly. Cant think will that would be- especially as it has new rings etc? Right plug has oil on it but it certainly isn't coming through the rings. Any advice would be welcome. Many thanks, (please see attached video)
https://www.youtube.com/watch?v=CLqcnrpmmfU
Ollie

(repost as not sure what happened to the first post)

WD Forte · World Chat Champion Joined: 17 Jun 2010 Karma :

What does it smell like?
Not sure if the smoke is oil or condensation/water in that vid
Water vapour tends to dissipate rapidly, oil less so and has obvious oily stink.
If its oil you may have cracked a ring on assembly, easily done.
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bikers smell of wee

Ollie125 · Derestricted Danger Joined: 22 Oct 2015 Karma :

Hi, thanks for the reply, its is certainly oil, although the rings definitely werent cracked on assembly and indeed the bike was totally fine for the first couple of miles of the run in.

WD Forte · World Chat Champion Joined: 17 Jun 2010 Karma :

If it ran clean for the first 15 miles I doubt its burning leftover oil from assembly
that leaves:
Oil from below - rings/bore
Oil from above - valve guides/seals
Oil from behind - air filter/breather system
As its only one cylinder that tends to exclude fuel contamination

I'll still have 50p each way on a cracked ring
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bikers smell of wee

Vincent · Banned Joined: 16 Oct 2006 Karma :

Leaky head gasket?
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Space Is Deep

Ollie125 · Derestricted Danger Joined: 22 Oct 2015 Karma :

I am going to perform a compression test (just need to borrow the equipment, so should happen in the next few days). There was oil leaking from the the front of the engine which I initially thought was coming from the exhaust joint but on closer inspection I`m not sure anymore- could things have worked loose? Although I did torque the head down correctly to begin with.

Vincent · Banned Joined: 16 Oct 2006 Karma :

It's a good idea to re-torque head bolts/nuts after startup and after 50 miles or so. More so with two strokes but it can't harm to do the same for a four stroke.
Did you use an OEM or pattern head gasket? The quality of some pattern head gaskets leaves a lot to be desired.
Were the surfaces nice and clean? No gouges or corrosion?
____________________
Space Is Deep

Ollie125 · Derestricted Danger Joined: 22 Oct 2015 Karma :

Hi, I replaced the gaskets with ones from David Silver https://www.davidsilverspares.co.uk/CB125TDC-SUPER-DREAM-1982/part_28059/ , all the faces were totally cleaned up before reaasembly.

Ollie125 · Derestricted Danger Joined: 22 Oct 2015 Karma :

After doing a compression test I can safely conclude that the rings are absolutely fine (phew!) . I thought that the oil may have been escaping from the oil ways but this is not the case. Anyway I took the engine out and apart again, the replacement gasket really isnt great in terms of fit. I couldnt find any cracks in the head itself although I`m going to ultrasonically clean it to be sure. The oil was escaping at a very high rate though so I`m a little concerned that I`m going to have to replace the valve guides etc (although they seemed fine and were creating a vacuum) but this will cost several hundred pounds. Or do you think a proper head gasket will solve it all?
Thanks for looking