Experiments were carried out in the 70's using Hydrogen as a breathing gas by Comex. They found that if Hydrogen was mixed with oygen at a percentage above 4% the risk of a spontaneous explosion even without an ignition source was extremely high. The outcome of these experiments was the adoption of helium instead despite the scarcity and cost of it.
Petrol is only flammable in vapour form and is pretty safe even at 21% oxygen concentration unless ignited. It does have a low point of igntion but won't spontaneously explode! The experiments found that there was even a significant risk of a diver breathing hydox spontaneously exploding and if a gas can be ignited inside a wet human then I am not keen in carting it around in a fuel tank beside a battery and a warm outboard
AFAIK fuel/air bombs are "fueled" by a variety of methods-kerosene, ethylene oxide or explosive slurry, not petrol.
Anybody know for sure?