£25 each from jhp i beleive

How can it go bang from the fuel/air mix being too lean? The fuel doesn't provide the lubrication to the engine like a 2 stroke would so surely if anything all that will happen is the engine will stall???

Someone will probably explain it much better than me! I'm no engine expert but I do know that a very lean mixture leads to higher combustion chamber temperatures which can lead to piston etc damage. A melted or holed piston would cost a little more than getting the fueling sorted with the correct chip!