[Synchro909]

I am of the opinion that a lot of our carburettor issues these days can be traced back to the differences in fuel. Petrol (or gas as you guys call it) is a mixture of several hydrocarbons. There is a very wide range of them cracked out of crude oil. Back when these cars and their carburettors were designed, fuel chemists chose a group of hydrocarbons with similar boiling points etc so that hot or cold, any brand, they all performed pretty much the same. As demand for fuel rose, more and more hydrocarbons were included in the mix. Some had higher boiling points, some had lower. Then there are the additives for higher compression engines to maintain a high enough octane rating and "burn clean" additives. The demand these days is so high that fuel as we know, doesn't last long before it spoils and if left to evaporate, it leaves behind an oily residue. The more volatile hydrocarbons contribute to such problems as vapour lock, the lower ones to incomplete vaporisation and subsequent sluggish (lean) running. Fuel in those days more closely resembled aviation fuel of today than what we buy for our cars. Even though a Model A does not need anywhere near the high octane rating of Avgas, if the carburettor on your car is good, you will not believe how much better the car behaves on it. Sure Avgas is expensive but if you understood what I have written above, you will understand why and understand what a burden running a car in those days was for your average working Joe.
Computer controlled engines with variable cam shaft timing, fuel injection etc etc have made today's fuels acceptable in a modern engine but it is not so good in an old one.