This is caused by the evaporation of the fuel. As any liquid evaporates, the side effect is cooling. (This is the principle on which refrigerators work)
I can't speak for SU's, but on some non crossflow engines, the loss of the hotbox, i,e from the addition of tube headers, can cause a flat spot. That is when there is no heating to the inlet manifold, the engine develops a nasty stumble, just off idle. Six cylinder chrysler valiants are a bastard for this. And it's a real pain in the jacksie. You go to make a right turn, , with not much of a gap in traffic, boot it, and it just goes BUHHH......vroom. Buy which time the small gap has gotten quite small indeed.

Anyway, I'm assuming your car doesn't see a lot of road use. And at any rate, I can't speak with any authority with reguards to if an SU will suffer from the dilema.