My M20, my favourite riding bike of several I own, started smoking from the exhaust, just a little at first but it gradually got really bad.
So...
I've had the barrel off and checked the bore ... fine, can still see honing marks
Rings and gaps fine and gaps between 8-10 thou
Check oil is returning to tank, yes and bubbles in it as you'd expect
Hand pumped oil though pipes to make sure they are clear, yes
Took pump off to bench check it pumps, yes (and all inside looks like new and turns easily)
Checked valve in timing cover; put it on side on bench with oil in oilway to see if ball is leaking. A little after tapping to reseat- so keep an eye on wet sumping.
Put bike back together and start it up, after a couple of minutes running she starts smoking!
I'm at my wits end so any suggestions would be very gratefully recieved
Thanks in advance
Dean
Hi Dean,
I assume you mean white smoke? Black or dark smoke is from a too-rich mixture. White is, usually, from over oiling.
You seem to have checked all the important things, except for the valve stems. Worn stems can let by a lot of oil vapor, and from there into the combustion chamber and exhaust.
It might be worth a look, perhaps?
Otherwise perhaps you need to fit new piston rings?
Good luck!
Allan
email (option): allanmatchless@yahoo.com
Is your crankcase breather functioning correctly?...Ian
Thanks for your swift and helpful advice guys.
Yes its white smoke so oil.
New rings fitted. Second ring and oil ring didn't have any markings to indicate which way up to fit. Top ring didn't have any markings either but was slightly chamfered on both outer corners so looked like it could be put in either way up.
Ian, the breather is interesting because after posting I took it off to check and it seemed stuck when I blew through it. I stripped it and oil was sticking the tufnol(?) flapper piece closed. Cleaned it out and engine doesn't seem to smoke as much although still more than it did previously (ie not at all)
I'll check the valve guides.....perhaps my jaunt up the motorway without oil did for them!
Check the valves, seats and guides...Also, remember that you'll need to do a few miles to bed in new rings so there is no 'blow by'....Ian
Thanks Ian
will do
OK so I've got the head off and valves out.
The top of the piston was covered in a thick film of oil (not from assembly)
Valve seats look good with regular contact ring and no signs of damage
I checked valve stems/guides for wear and could just detect a small amount of sideways play with the valve held open as far as it would in service.
There was no sign of oil on the valve stems. So it doesn't look like the oil is coming via the guides.
Not sure what to do next. I may get the barrel off and re-check the rings or, as Ian suggests, give them time to bed in.
Still feelng a bit lost though.......
Usually new rings would require the bore to be cross hatch honed. Ron
email (option): ronpier@talk21.com
Hi Ron. Yes, cross hatching still there from last rebore!
A few things come to mind:-
what is the gap between the ends of the piston rings, the hand book should give a tolarance, both high and low
Are the piston ring gaps set 120 degrees to each other
Are the piston ring lands worn allowing the piston rings to "flutter"
A blocked oil filter would reduce the oil returning to the oil tank
Worn valve guides or worn valve stems would allow oil into the combustion chamber.As the piston decends in the barrel it creates a partial vacum
e possibly sucking oil down worn valve guides into the combustion chambe
How many miles ago were the new piston rings put in?
Was the cylinder rebored? If it was are you sure you have the correct size piston rings. I have seen cylinders rebored and then standard size rings used. This can be difficult to see to the eye as all looks OK but it will smoke a lot.
Are you using modern rings or old Hepolite ones?
Was the piston and cylinder coated with a lot of oil on assembly? This can make the rings never seat. I put the piston in the cylinder dry. No oil at all.
Don't worry about the ring gaps being 120 degrees apart as they will start rotating the instant the motor is started.
Now on the the oil return pump, start the motor and get it smoking as much as possible. Shut it off and drain the sump. How much oil drains out?
Make sure the breather is actually working with the motor running.
It happened to me a while ago. I had cleaned the petrol tank and washed it the last time with I thought petrol. It appeared to be diesel... I could not see my neighbours anymore. It costed me a lot of time to get it free of diesel again.
Well I seem to have fixed the problem. But I never found the smoking gun!
I checked everything people suggested including, as a last resort, removing and checking the oil pump (perfect condition)
I checked for wet sumping....120mm after a week so as I had a Bri-tie external valve I fitted that. But I has cleaned the timing chest valve and clouted the ball as advised and that didn't seem to make a difference.
Having given up smoking, her new hobby is dropping embarrasing amount of clutch oil. After stripping I think that was due to me overfilling and with too light an oil :grimacing:
Anyway, thanks to all of you who took the time to offer such helpful suggestions and advice