Problem of mixture or cylinder?

[Manifesto1984]

After restoration of my R26 (new cylinder, piston and valves) i bought and a used carb (1/26/55) in very good condition. It wears 120 main jet and 35 idle jet.
At the begining the operation of motorcycle was good.
One day while setting the new carb the engine started to warm a lot after 6-10 minutes and started to smell something like burnt plastic. I'm scared and stopped the operation of the engine.

After two days I noticed sometimes white and sometimes black smoke from the exhaust, mostly like white smoke. The problem is, when i go a trip in the middle or full throttle there is not smoke from exhaust but after 10-15 kilometers i have problem with the spark, when i unscrew it, is too black like coal (but there isn't something like oil) and it's not working. Now after 15 kilometer i must to replace the spark with a new one 

So, i decided to buy other one carb and i bought a Bing 1/26/111 (it wears 140 main jet and 35 idle jet). When i set up this carb and start the operation of engine i saw much more white smoke from exhaust than 1/26/55.

Now i don't know what to do. I'm very disappointment and confuse. There is a problem with cylinder or with gas mixture? Which of these carbs to use and how to adjust?

Please help! 

[Manifesto1984]

[Kurt in S.A.]

The /55 carb should be correct for the bike...the /111 carb is actually for an R60/5.  I wouldn't know how to adjust that carb for your R26.

Running the bike for that long at idle with no air flow isn't good for the engine.  If you're making adjustments to the carb, you should have a fan blowing air onto the engine to keep it from overheating.  Hopefully, nothing has been damaged.

I don't know what the burning plastic was, but it appears something got pretty hot...maybe something of the wiring harness has melted.  You should check around to see what it might be.

As for the smoke, what are the other settings of the /55 carb?  You mention the 120 main and 35 idle jet.  Where was the jet needle set?  The Bing manual says it should be in the 3rd clip position...I believe there are 4 holes in the needle and the numbering starts at the blunt end.  The 3rd position means that the needle is pulled far out of the slide for a richer running situation.

I would relook at the carb and make sure it is set up correctly.  Possibly recheck your valve clearances as well.  Be sure that the head torque is set correctly.  If you are still in the break-in stage of the engine, things may be seating in and you need to reset back to spec.