Problem with brand new Honda Izy, dies after starting?

Hi Guys,

I just bought a new Honda Izy 415 4-stroke petrol mower. I checked the oil and filled it with brand new Shell unleaded. It starts perfectly and runs for a bout 2 or 3 minutes before it starts banging and popping and then stops.

I checked the plug and it was extremely sooty so I cleaned it with a brass bristled spark plug brush. Also checked the airfilter and it was pristine, I don't think the mower has been stored or tipped over the wrong way...

The weird thing is, if I turn the petrol cut-off to 'off', it will start again instantly and run perfectly for 20 minutes until it runs out of fuel and shuts down .

Any advice appreciated .