Cb125 tdc 1 cylinder smoking

[rustbucket]

Hi

Recently bent a inlet valve on my 125tdc due to a screw from the choke butterfly valve coming loose and getting stuck between the valve and the valve seat.
Fortunately engine was just on tick over .
Fitted new valve and stem seal now noticed engine smoking badly on that side.
Before I strip the top end down again, has anybody got any ideas what it could be.
There was no obvious damage to the piston or bores.
Ian
____________________
If its got more than 2 wheels its boring.

[davebike]

Did you replace the Valve guide seal ??
I seen poor quality pattern ones in Gasket kits cause this
____________________
Dave
NC750Xdct + others at work !
[email protected]

[Teflon-Mike]

cracked valve guide
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
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[misscrabstick]

If the valve was held open and struck the piston top then the top piston ring may have been trapped in it's groove because the alloy of the piston has been squashed a bit by the collision.

[rustbucket]

Thanks for the replies

The valve seal was replaced with the inlet valve.

I have this afternoon just took the engine out the bike and took the head and barrel off.
I notice that there was oil on the top part of the exhaust port which continued into the exhaust pipe. ?? could this be the valve seal but didn't touch that valve.
Unable to see any damage to the valve stem but are cracks in them hard to find?
Check the pistons and again unable to see any damage to them or the piston rings.
Should point out that when it smokes its after a short delay after starting and when it does smoke its like having a stroker.
____________________
If its got more than 2 wheels its boring.

[Teflon-Mike]

Only thing that can bend a valve, really, is if it gets hit by the piston before it's shut. It's a Hemi-Head, the val;ves at an included angle of almost 90Deg, ie 45Deg to the piston, so it smacks the valve edge on, which is why they bend... penny on a stick... penny wont bend... stick will.. and it'll bend along all the exposed stem stuck out the guide.. BUT being shoved back in the guide, as its being bent, by the piston; hence very good chance that if the piston hit it hard enough, it'll bend the valve and 'damage' the guide. guide's are a soft iron, ie brittle, so they will tend to crack, but they can get a groove worn in them. If the crack is small, then it acts like the groove in an ink-pen nib to wick oil from the rocker gallery into the combustion chamber. Bad cracks are easy to spot.. minor ones, not.

Other things that it could be, and possibly more likely....

Cylinder head gasket / torque... its a low torque head, and there are three 'hidden' M6 screws, which just LOVE to 'strip'.... very easy to over torque one of these, so could be simply the head-gaskets not doing its job & oil's coming in from one of the stud holes that act as drain galleries. In similar vein... head could be warped.

Smoking after 'delay' after start up? Well, suggests that the oil's being sucked into the cylinder on the induction stroke, then, rather than 'dripping' when left... that would hint towards Head-Gasket-Failure, rather than a stem-seal... when stem seals fail, oil in the rocker gallery 'drips' down the guide into the ports.

Oil in the exhaust? the valve you didn't touch? So that's got an old stem-seal on it.. a hard one, most like, not doing it's job?

COULD it be.... that bent valve has made you look... and that has been happening for an age? and in fact all four stem-seals were kerfuckkered and dripping.... now, you have disturbed the head, and you are adding to that with a head-gasket issue?
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[Papa Lazarou]

Did you grind the new valve in as per spec? Just checking.

[rustbucket]

Thanks Mike for the reply.

I ordered a new gasket kit from Dss the other day.I am now thinking it would be worth ordering 2 new valve guides for the LH cylinder along with the 'o' rings. I fond the new part numbers for the inlet and exhaust valve stems from Cms and will order them soon. Just wondering how do I go about changing the valve stems and should I change the exhaust valve and springs?
Ian
____________________
If its got more than 2 wheels its boring.

[rustbucket]

[Teflon-Mike]

valve guides are not service replaceable - you replace the whole head - Chinese Copy-Parts are available on e-bay for about £80ISTR

A good Machine-Shop / Engine reccon place can often mill out worn or damaged guides and replace with either stock items, if available, or make from stock, but is expensive, so not an ecconomical repair on these engines when new or used heads are cheap and easy to come by, and M/C shop would have a night-mare at the job, cos of the tiny size and the awkward angle to 'jig'.

Do all four valve stem seals... they all sit in the same hot oil, they all age at the same rate, and they all wear at the same rate, the valves dont turn different speeds like gears, do they? So if one needs doing, they ALL probably need doing.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[Papa Lazarou]

I've replaced valve guies in the past, tho I must admit that they were on old British bikes (mUCH easier to work on).

Teflon-Mike is probably quite right to suggest replacing the head, particularly as parts are cheap.

[davebike]

Mike

Guides are listed as a part and in stock

Inlet 12021149405 £9.19
EXHAUST 12023465405 £10.82

BOTH EACH AND + VAT

Me I get another head as it is a major job the change guides
____________________
Dave
NC750Xdct + others at work !
[email protected]

[rustbucket]

Thanks for the advice I shall look for another head.
____________________
If its got more than 2 wheels its boring.