is it possible to make my own petrol tank out of carbon fibre ?

Sakke's suggestion of a tank sealer sounds very interesting - I've never encountered this but it sounds like it could be a good solution. I can understand that it would be a good idea to have a 'coating' rather than just a suitable resin for lamination because this would create a barrier between the fibres in the reinforcement and the fuel which is what you'd want/need to prevent the fibres from absorbing fuel.

they made fibre glass tanks in the 50s and 60s the main concern then was in accident damage.

Consider Kevlar; stringent regulations in Germany at the moment are requiring Kevlar protection around fuel tanks in many of their motorsport classes. You get a much higher elongation to break property and also impact/abrasion resistance all of which would be help a lot regards the strength of the tank from a safety point of view.

Matt Statham
Easy Composites / Carbon Mods - Technical Sales