5 mm? That's like a foot in metric!!
START with a NEW plug. Gap that dog 0.020-0.025 and go from there.
At this point I also suggest things such as:
Drain the tank and remove that absolutely stupid in tank screen.
Make a FRESH batch of premix (go BUY FRESH low octane fuel from a busy station). Get a good oil and mix it 30:1.
Remove the cylinder head, wipe down the sleeve and head, replace head gasket and retorque.
Check the needle taper.
Get a new plug wire.
Replace the intake gasket.