Hi Pete,
WELCOME, I am sure that somewhere on here is the answer to your problem, is the "bobbin" the ignition coil?, if so is it cracked or damaged anywhere? are the power leads, including the plug lead unchafed and clean and properly connected? is the float bowl drained and refilled with clean petrol? is the air cleaner free flowing and not blocked, is the timing set correctly? is the plug the correct grade? is the cut-out switch ON every time? are there any cracks in the inlet rubber? is there enough battery when starting to both spin the engine and fire the plug under compression?
A few bits to be going on with, someone with more specific SRX 600 knowledge than me will be along soon.
Furry