it sounds like its running weak ,a backfire is often caused by a weak mixture and if you say it runs for a while and cuts out then thats more evidence of weak running,get the carb off it and take the jets out and make sure they are free from any dirt or debris and make sure the fuel is fresh and the fuel tap is working properly as it does have a vaccum tap and they do play up also make sure the fuel cap is breathing ok.
adam bike doctor(of milton keynes motorcycle centre)
February 2011
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