In manual says it's 1 l,it's that true?

At this point you might as well just drain everything out of the entire bike , take every bolt that holds oil (oil tank, frame bottom, motor drains everything, catch it all , measure it all . Tell us how much came out . Only way your issue can even begin to be resolved and be done with it.

I'll do that this week,now i'm not home. Thanks for everything!

crankcase oil: 800+ ml overnight

Is bad or not?

800ml is just about 1 quart , when I change my oils I get close to that , maybe a bit less out of all of mine . there is 1/2 to 2/3 quart in the engine anyways that the oil pump can't suck up . For it to smoke from too much oil you would have had to drain out the whole amount of oil in the bike, 1600-1800ml , IIRC on your model. It shows the check valve is leaking some, not very much though , and not your smoking issue , especially if it smokes like that after just sitting a few hours .

Now you can get into what needs to be fixed , unfortunately. You said there is no smoke once the motor is warmed up right ? Just on cold startup for a minute or 2 ?

agree with JJ, not that much to justify heavy smoking after a few hours.... it could be better of course... 800ml overnight, say 8 hours, means an empty oil tank after 2 days/48 hours, so worth fixing the valve in any case.

if you want to easily make sure thats its not an oil in the cases problem, drain the cases oil, close the plug and start the motor. if smoking problem remains, then as JJ said, problem is elsewhere.

Yes..now i have 1200miles done since the engine was rebuild. Maybe the rings are not yet break in?

The rings were done breaking in long ago.

Hei guys.I descover something weird. Today,i drained all oil from crankcase and surprise..no smoke.
I do the same thing tomorrow morning,but with oil in crankase..i'll be back with news.

Open the the front or rear cover for adjusting valves and check that you actually get oil up there when starting up, maybe take some time when you drained system. You can also fill some extra oil in the top end and check if you get smoke.

I turned it on now and,smoke. (with oil in crankase) So i have no ideea what's happened here..

It looks to me like you have found the problem. You drained the oil from the crankcase and no smoke, after sitting overnight it smoked since oil drained in the crankcase. However, since you only drained 800ml from the crankcase, that is not excessive.

Replacing the check valve is easily done by removing the clutch cover. Check the parts diagram under oil pump on Partzilla: OEM Motorcycle Parts, ATV Parts, Marine Parts to see the check valve and seat. items 22 and 23 on the 1986 XT600 USA model. They are all the same. Before tearing down, order the check ball and seat plus a clutch cover gasket. USE OEM gaskets.

You probably didn't have any/much oil getting to your topend after draining . The oil pump was emptied mostly so it'll take a bit to reprime. Like XTrock said take one valve check cover of and see if there is oil in there when it's running. You did make sure you now have the correct amount in there after draining 800ml out ?

news: i did a short ride,i left the bike aprox 1 hour,i drained the oil from crankase, this time aprox 1,200 litres. Start the engine,smoke! WTF? I'm desperate:(:(

I put back these 800 ml. So,in this morning i drained all the oil from crankase,800+ ml,start the engine,no smoke,i put back these 800 ml. At afternoon i did a short ride,i left the bike 1 hour,then i start it again,smoke...i let the bike for another 1H and then i drained again the oil..aporx 1200 ml..i start the engine without these 1200 ml and..smoke.