Yes you have to be careful Roger but remember the upper explosive limit of petrol is something like 7.6% which means you have to get a lot of air into the tank (92.4%) before it will combust. Petrol evaporates so easily that all the time you have got some in liquid form in the tank, bearing in mind how few openings there are, its almost impossible to get enough air in to be dangerous. The danger comes when the tank is emptied and left open...