I think ethanol gas boils easier than what the engines were intended to run on back in the day too...it also makes a carb jetted for old leaded gas run leaner,having less BTU content--a book I have on how to make alcohol fuels has a whole chapter dedicated to carb mods,how to open up the jets so it'll run right on alcohol vs gas,make a gas "primer" device to aid cold starts,as alcohol does not vaporize well below freezing,etc......the leaner mixture that results from using a old carb designed for old leaded gas with ethanol laced fuel,might make the engine run hotter,and add to the potential for vapor lock..
I know small engines dont like to run well on it unless you open up the jets a bit,and it makes them run hotter than they did on leaded gas..
In my area where its often damp,foggy and cool,carb icing was more of a problem than anything else on my GM's..the carbs would actually frost up on the exterior when it was below 50 degrees under those conditions,and it took a several mile ride to heat up the carb enough to "de-frost" it..
During the warm up period the driveability sucked--the engine would die at idle unless I held the gas pedal "open" and power braked it at every stop sign (try that with icy roads !)--sometimes ice would keep the butterfly in the carb from closing too,so the gas pedal would "stick"..
.--and it would stutter and blubber under acceleration like it was over choked--then once the engine hit a certain temprature,it would suddenly smooth out and run perfect,almost like a switch was turned on..as the carb de-frosted..
Several of my GM trucks and my '69 GTO all acted that way on those damp cool mornings on my way to work..I'd stop at a drive thru to get a coffee about 5 miles away,and I'd have to fight to keep them from stalling at idle--then by the time I reached the window where I'd get the coffee,the engine would suddenly smooth right out..
Worst offender for that was an '86 305 I put an Edelbrock and aluminum intake on--the thing looked like the freezer compartment in my fridge on a damp cool morning after a cold start,for a good 15 minutes sometimes..in order to make it driveable,I ended up bolting the carb to the intake with just a thin flat gasket,ditched the spacer,and I hooked manifold vacuum to the EFE heat riser valve to increase heat to the base of the carb...it decreased the amount of time it took to warm up,but still it was a pain to drive the first couple of miles...
In most other parts of the USA,you might have to "delete" all these things that could increase the carb bowl's temperature to prevent vapor lock--up here there isn't too many hot summer days that would be ideal for vapor lock..but some older vehicles that never had symptoms of it now do,with the ethanol laced gas...
I think a lot of it is not closely monitored,its supposed to be 10% ethanol,but I bet if samples were taken more often,finding some sold with much more than that is not uncommon..I've bought some that my mowers and tractors barely ran on,it--dumped it and went to another gas station and tried that,they started up and ran much better...
