I finally got some time to work on the bike. I cut the chain off earlier so I couldn't walk my bike and pump it out. I did a modified version of this by initially taking the plug out and giving it a swift kick. It resulted in a face full of fuel (luckily I was wearing safety glasses). I put the plug back in and kept kicking. I would remove the expansion chamber and drain the fuel out, refit and keep kicking until there wasn't any fuel on the chamber.

The replacement carbs now seal and the bike runs with a mean crackle whilst there is still unburnt fuel in the chamber. I have to get my son used to the sound of the NSR, he git scared by it all.