Update. Today I went for the year's first longer ride. Total about 2-3hours.

Bike started after ~10 kicks with the "new" plug that had only been run twice shortly.

As usual nowadays white smoke. I was outside and it was a little windy, but I could only see white (i.e. not blue) smoke.

After running the bike for a while I stopped for ~20 min. After that started first kick.

Rode some more. Stopped for ~2h. It started after ~3-4 kicks.

Rode home and checked spark plug again (first time I check without having problems).

See spark plug in attached photo (slightly white on tip. Some black soot near threads.

The smoking only occured right after first start of the day, after resting a week.

Amateur diagnosis:

Hard to start + a little white spark plug = Slightly lean mix (I did lean it out a little a while ago).

Last 4 spark plugs were fouled pretty quick and they were very black (see prev photo). Also white (not really blue) smoke when I start after ~1week rest). I guess this could be oil burning. But the black is really more like soot than oily. Not especially wet, although I could quite easily get it off with my thumb.

Would anyone make a different diagnosis? Maybe the plug doesnt look so bad..? It could ofc just be bad valve clearances or leaking valves/piston ring. Or even weak spark. I checked spark on plug but didnt see. Its a little tricky to do alone though, so probably just me doing a bad test.

I ordered iridium spark plug Monday this week but it didnt arrive yet... I also bought shims to test valves. Have not invested in a compression tester... Might do that if other things wont solve.

Attached Images