A very smokey '73 CB125S

[Mikep]

Hi guys,

I have searched and read a topic similar to my problem but I hope you dont mind me posing my own questions.

I will try to keep it short. I have just finished a full resto of my '73 CB125S including a full engine re-build. Firstly whillst trying to set the timing and adjust the carb the engine started spewing out smoke. It was ok at first but over a period of a few days it has got worse to the point that there are oil spots on the floor by the exhaust, a drip of oil on the end of the exhaust and the oil, having been full, is now on the minimum mark When I originally stripped the engine I found light scoring in the bore and on the piston skirt; I took this into an engine specialist who re-honed the bore and polished the skirt, he told me that the rings were fine and could be used again. Could this be my problem and really should I have had a slight re-bore and fitted oversize pistons?
I checked the compression today and initially got a reading of about 60psi, squirted in a bit of oil and the reading did go up to about 70/80psi, the Honda manual says it should be 170psi :o( I have now removed and stripped the engine this morning and the scoring seem to have appeared again on the skirts and the bore has some very faint vertical lines. I have measured what I can with my vernier calipers andit seems that the bottom ring is slightly out of limits:o? Also when removing the head the head gasket(which was an OE part) it was ripped apart, should it have 'welded' itself like this, I expected to be able to remove it in one piece and also the oil seal round one of the cylinder studs seems to be intact, shouldn't this have got squashed?

My piston and bore seem to be within limits, should I just fit new rings or oversize and/or could my problem have been the head gasket, I did torque the nuts correctly.

Sorry for rambling, any help is greatly appreciated.

Mike

[Fizzer Thou]

I bought a basket case Honda XL250SA from a friend.I did the same as you have done where I measured the piston skirt to bore clearance and it was within limits.I then wanted to measure the end gap of the piston rings within the bore. They dropped straight through without hanging in place. So I bought some new standard bore rings,bust glazed the bore,checked the end-gap within the bore and assembled the engine.There were new valve guide seals fitted while I had the valves and valve springs out to replace them.

When it came to road testing the newly rebuilt/refurbished bike,there was a large cloud of white smoke following me around.At first I thought that this could not be coming from my bike,as the engine had been fully checked over and compression was excellent.But it was the bore that was too worn.

I bought a first oversize piston kit,had the barrel bored to suit and rebuilt the engine.Job done.It made a very big differance,to such an extent that my friend wanted to buy the bike back from me.
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[Mikep]

To be honest I think this is what I am going to do. Save any messing around and just fit an oversize piston.

[Ed Case]

Did you put new valve-stem oil seals on ?.

[Ariel Badger]

There are three ways that oil can get into the combustion chamber..... Past the rings, down the valve guides or through the carb from a breather.
Your bore is just about in tolerance as are the rings. If you have a breather feeding back into the inlet tract investigate otherwise take the valves out and look for wear and play.
It is also possible that you are getting a build up of pressure in the cases because of a blocked breather.
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[Mikep]

Thanks for the help, I took my barrel and piston in to a reputable engineering shop and they confirm that the rings were worn, partly due to extended exposure to over fueling, and that I should have been advised the first time round to change them. The upshot is that the bore is at the extreme limit so a re-bore is in order and it should be ready by wednesday

[philoldsmobil...]

Glad to hear you got it sorted, but the first 'specialist' should know that you never re use rings after a hone..

you wouldn't re use big ends or mains after polishing a crank, never in a million years..

[Mikep]

I agree. My inital thoughts were that the rings should be changed but as he was the 'expert' and measuered and checked everything I took his advice to re-use them. I didn't want to spend money if I didnt have to but in hindsight as the engine was in pieces I should have changed all servicable parts. You live and learn.