Replace the plug.
The non-resistor version, BP6ES will be fine. I also had a spark with old plug (BPR6ES, which contains a resistor to prevent radio interfrence!) but fuel change and plug clean had no effect.
As soon as I replaced the plug it ran; with plenty of black smoke for a few mins but now runs OK on all throttle positions including idle. Something it never did before.
Tom Wilson
September 2011