Is your weather nice and warm?
Garage getting alittle toasty???
Not been that windy? Garage pretty air tight???
Before tearing your cars apart bear in mind that as the cars warm up, so does the petrol. As older cars have vented tanks direct to the atmosphere rather than through charcoal filters and traps as per modern "smogged" cars. THus as the pertol evaporates that's what your smelling.
On the odd sunny warm days we get in Scotland
(ha, ha) my garage has that certain smell.
Malc