whats causing misfire on volvo penta 250 4cyl engine?

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it was running fine and just started missing and losing power,i noticed the rev counter started jumping from one side of the scale to the other so thought it must be an electrical misfire,i have changed the plugs,leads,distributer cap,points and rotor arm,to no change! Even when sat out of gear if you rev it past 2000rpm the rev counter jumps all over the place and the engine starts missing,it will tick over all day long.I havnt changed the condenser as it is located in a crap place and i would need to remove the whole distributer to change it,could it be this at fault?? I have also cleaned the carb jets and fitted new fuel filter although it was new anyway,just to satisfy myself that it isnt a fuel problem.just the other week i had the plugs out and they were a lovely light brown colour,now they are very sooty and black with unburned fuel.In fact there is quite a whiff of fuel from the exhaust when you rev it as the engine isnt burning it correctly.Any ideas will be welcome.