ESPEN,
        Yes, I would check the valve clearances (ex/in, 0.35mm/0.30mm). If the
engine has valve problems, it would NOT idle smooth and if they we set
tight the engine would backfire out the carb. I would say that it's more
fuel and/or spark related. Do as the others suggested and observe what
the engine timing is doing with a timing light. You might have bad
gasoline? It could be your spark plugs? The engine will run if there are
three things: Compression, Fuel and Spark.
Keep us posted on your findings,
jan servaites
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