Does the motor use oil ? It looks more like oil fouled than fuel fouled. You are fully correct, if it was over rich in gas, it would actually be opposite in hard starting issue, easier to start cold, but hard to start once hot. I had one messed up carb that made the bike miss and run real bad, any hard throttle and black smoke blew out the exhaust, still the plug never fouled, it takes a lot of gas to do that, enough to be very bad running at all times that you would have noticed.

You need to check the spark condition, is it a sharp, big blue spark or tiny weak orange. Then a compression check and leak down test to see where any comp loss is from. Valve clearance should also be verified correct if any concerns about previous work.

I suspect weak spark topped with some oil burning. Ignition coil may be weak or a poor grounding of the motor. Can you get an Iradium plug to try? I use Denso IX24B , splug fouling issues and weak spark is where iradium plugs are a big help , otherwise not a real benifit for the cost.


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It does use some oil. It has since purchase (used). Maybe 2-3dl on an 800km trip. Fuel consumption seems unchanged for 20 000km+ but I havent done any serious testing.

If its not black from gas I guess it has to be oil (the fouling doesnt seem "oily" though). However I dont look forward to all the job and money (in relation to the total value of the bike) that I probably have to put in to test compression, find problem and then fix cylinder if its broken. Feeling a little disheartened to be honest Leaving it at a shop for fixing would probably cost as much as I could sell it for.

I will check the spark again, but Im pretty sure its ok.

Thanks a lot for your help!

EDIT:
btw since I got home it started puffing out white smoke when I start it. It goes away after maybe 15 seconds of enginge running. I heard it could be ok and climate has been cold, but ofc it could be related to this (oil seeping into cylinder while bike rests?)

That would be a sign of a bad valve seal, oil leaks down into cylinder when cooling down. The smoke should be blueish, white smoke is water and unless your dumping gas in the pipe it shouldn't be that. Excess burning fuel is black smoke. I'd say it is from that seal, just enough oil is fouling the plug, but I wouldn't doubt if the sparks a bit weak also to foul that easily plus the hard starting, I've seen a plug that was a ball of sludge yet still the motor ran (smoked like hell though).

An Iradium should cure it as it is for now to get by.

Pull the plug and spin it over to have a quick look at the spark.

I will def check the plug spark and try iridium plug. Im planning on buying some other spare parts so I might throw in a valve seal or two just in case.

I have not noticed any blue smoke but I havent specifically looked for it either.

Thanks again!

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

 

Have you checked if the bike is overfilled with oil? That and worn valve stem seals cause white-ish/light blue smoke on startup.

Kicking a bike over is pure technique, I have a frined that can start just about every bike in 2 kicks or less. He has always been riding kickers, so he has it down to an artform.

Thanks for you reply!

When cold the oil is below inspection window. When warm it just just at the top (i.e. moving a lot, maybe a little much but only very slightly). Before I have had too little oil also. Same smoke from what I can remember.

About kicking... yes its a technique. But since sometimes I try for 30 minutes until my leg/ankle hurts (cold/bad spark/whatever) and sometimes it starts on first try (hot), doesnt it seem reasonable that its more related to the bike than me?