Hurray, my bike is running almost properly... after few months, the problem occured to be water in the fuel.
Basically I was not using my bike so often, but after spilling about 300ml of isopropanol and about half tank of ride, bike is running well.
Still had some minor stutters, but finally after around 200 miles done during weekend it's gone.
Conclusions (propably for most of you it's obvious). If you want to make sure if your fuel is bad, just fill glass of petrol straight from carbs or from tank and see if there is water inside.
If you want to replace fuel with new one, remember to drop the petrol from carbs, then from tank, then fill new fuel and drain (from carbs again) about 100ml of fresh fuel. This is to remove the old fuel from fuel filter and rubber hoses.
Of course it does not mean that everything it's alright:
- still looking for the tank, unfortunately missed the ebay auction, there was brand new (used only 4 weeks) tank for 80 quid,
- still bike runs on idle on 3 cylinders, and stutters between 1000-2000RPM. I reckon that it may be problem with idle mixture screw on 3rd cylinder (it's colder than the others), also I don't have tool to adjust that screw without removing carb (it's in really bad place).
All the best.
