smoke on overrun I agree points to valve seals,guides,as suggested
That will be oil being sucked past the valve seal (s),the valve guide(s) to head fit or porus guides
You could do the seals again,they can be done without removing the head,a typical tool is this one
http://www.ebay.co.uk/itm/271069691844? ... 1423.l2649
Do a web search if you want to go that way,have seen it as low as £50 ish not on ePrey
Fill the cylinder combustion chamber with rope or similar and raise the piston to squeeze up to valves hence stopping them dropping unretrievably into the piston area.
Will be doing mine that way likely later this year when done working in August
this time just try this one,remove the spring from the seal and unscrew it,its a piece of coil spring wire,joined together then twist it past a couple more turns then screw it together,on the new seals you just fitted.
Practice on a larger seal if it not makes sense.
What in effect you are doing/achieving is making the spring tighter which tightens the rubber on the shaft,in this case the valve stem a couple more thou'.
Done that trick on many shafts weeping a little,learnt it from a guy donkeys older in one of the places I worked,when a groove worn in shaft.(Worked a treat for me)
Since you done seals anyways,next is rings,as telling if guides to head fit is causing the problem I suggest is more unlikely and difficult to pinpoint to..
Short cut could be borrow a known good head,still smoking more than likely to be a ring problem.
If on a tight budget reuse the head,rocker gasket as it's only a few miles and a leak there will be of no consequence for this exercise..
Or buy a base gasket and do the rings anyway,with a hone chucked in for good measure,if you decide a head off jobby.
Trouble then how is the cam chain and guides looking,the list can get longer...
Can do a hone yourself with the correct size/diameter purchased and an electric drill with variable speed on slowish(not at 9million rpm),you just need to glaze bust the shine to help new rings bed in and seal,aim for crosshatch scratch marks
Move drill in a fastish up down movement about 6-8 inches per 20 seconds at about 300 odd rpm
I just checked honing cylinders and this basically ok if a clean up,but if scratches past nail feel,then looking at an oversize rebore and pistons and rings.
Go here for self honing
http://www.dansmc.com/hone_cyl.htm
And likely your oil bother problem found
If you thinking oil breather blocked did you try removing it completely,an easy option to negate that thought.
Didn't the z1000 have cast iron guides as std have the idea they were porus to help lubrication from where I picked that one up from not remember,when you did the valve guides did you manage to view inspect valve stems,any scores there will affect seal,and could be a sign of a slight stem to guide seizure which will affect guide to head seal.
A compression and a leak down test may help guide better in the direction you need to go.
Make sure you paper and pen every detail to do each cylinder under same conditions
When you noticed the condition of smoking on overrun had you done any work on the engine within months 500 miles or so..
Changed oil to a different grade(thinner as believed to be better)
In the Uk with 20 degree normalish temps,20/50 is I believe good,as a 3-5 psi,high flow system,needs some viscosity to be pushed around.Thinner to be the same means a need for closer tolerances on the pump,since it's not changed (unless you changed tolerances) flow I suggest will be a lower flow and will escape easier to combustion chamber
That's neither to say likely a lot here will be using the new oils,semi or synthetic 10w somat or thinner with no difference,except costlier oil changes,although supposed to last tons of miles more..
Just some idea's to chuck atcha,i have similar bother but it's mostly all the time,runs 100 mile a pint,but since I rarely home to ride,no bother at moment.
Plus since it's a 1260 motor I think a reply said,yep it's an oil burner aswell..and I know the oil is pretty newish without having to regular change.
Excuse the long reply,i do trouble shooting and diagnostics as a living for the last 15yrs,mainly cnc stuff but anything in a steel structural factory including engines so able to offer many possible idea's.
Others here that has had a similar problem will likely be better suited to point you..but your choice and budget at end of day.
I generally aim my customers to cover all possibilities,regardless of cost,but then what I work on is a minimum of a $500/1000 per hour down time...another ball game agreed.
These are just my points of view,do your research,there is many members a lot of far more knowledgeable in this vein of view,plus tons via the web..
Read it all,
A piece of music digitised (sampled)will be more accurate sounding at 5,644,800 Hz than the same piece at 8,000 Hz
But it will never be true analogue like lp's etc,why the trend to go back to analogue music equipment.
And you CAN hear the difference..open your ears both of them...
