Z650 running on 3 cylinders - could this be anything other than carbs ?

Sean, are you getting the black 'specs' every time you drain the float bowls? Is there any in the tank petcock bowl? Is it possible to identify them using a powerful magnifying glass? Do you buy your fuel from the same garage every time you fill up? After cleaning does the bike then run fine again? Quite a mystery this one, am very tempted to agree with others thats it may be an ignition or electrical fault. Have you always used the stabiliser treatment? Have you tried not using it?