Mayor technical mystery

[Luuk]

I have a technical problem which I can’t seen to figure out. I hope someone here has the answer.

The bike:

A KLE 500, build in 2001. Which has run about 50.000 kilometres. Every year I change the oil, re lube the filter, check and adjust the valve clearance and put in new sparks.
About a year ago I have driven it from Holland to Cape Town without any problem.

The problem:

The bike has been standing idle for two months (outside but under a roof) and when I to it for a ride it felt as if it lacked about 10 to 20 percent of its power. I put that down to the fact it had been standing outside so long and maybe some moisture had gotten in. I finished my ride and didn’t really think about it anymore. Two weeks ago I went for another ride. Again it felt as if it lackt about 10/20 percent of its power and every now and then it just didn’t want to speed up. Not every time I increased speed but every now and then it just seem to lose about 40/50 percent power. It stuttered just as if it was running out of fuel (an yes I did check the fuel tank, it had enough left)

So I thought because it behaved as it it was running out of fuel maybe the fuel filter was clogged up. So I replaced it with a new one, drained al the (two month old) fuel, because maybe it was deteriorated. Checked the spark plug cables, they were ok. Filled up with new fuel. Put some valvoline carburettor cleaning stuff in the fuel. I an a bit sceptic about stuff like that but my dealer insisted it would word. And went for a ride.

The problem had seemed to have gone away. The bike worked fine. I tried it on the highway. Doing 80 tot 120 sprints in high and law gear. I thought if it was I fuel quality or quantity problem it would show op then. But the bike worked fine, so I went home again. But than when I was doing 50 on a country road it again lost about 50 percent of its power. I stopped it, killed the engine, started again and it worked fine again.

So what the *&^% is the problem? I have no idea. I cant, reproduce it, it seems to come and go. Next week I’ll put in new sparks but I don’t really think that is the solution. The fault finding page of the manual didn’t help my either.

Anyone any idea?