Scuttle fuel tank repairer request.

[Duncan Grimmond]

If carbon monoxide were the only ingredient of exhaust gas it might well be flammable but without oxygen it would not burn inside a tank. Fortunately there is also carbon dioxide (non flammable) nitrogen dioxide (non flammable) water vapour (non flammable) and sundry other traces of non flammable by-products in exhaust gas.
The candle trick sounds like sheer folly to me, probably devised to scare the sh * t out of an apprentice...