OVERHEATING

Modern fuel has ethanol in it to increase the octane rateing. Ethanol burns slower and in a more controlled way so when mixed in with low octane fuel it stops it burning in an explosive way. it increases its octane rateing. it promotes initial igntion in 1 spot in the chamber and a flame front out in all directions.

in the old days that higher octane rateing was achieved with a different set of additives and to be honest a potentially longer and more expensive refining process to make a fuel of the same octane

so just on that l front you have a fuel that burns in a subtly different way from the stuff the engine was designed around
hence you need to make sure you tune carb and ignition curve to match what you now have to use.

to complicate matters the ethanol mix fuel is also considred to be, "oxygenated" ethanol is made mainly from carbon and hydrogen like much of the rest of the oil derived stuff in the fuel but ethanol also includes an OH Ethanol:- 2 Carbons 5 hydrogens and an OH molecule

so not only are you sucking in oxygen at approx 20% from the air you are carryinging a bit more in the fuel ethanol is 35% Oxygen and ethanol is denser than the base its mixed with. it just messes with everything we depended on for a carburettor set up

a smoother more controlled burn coupled with a greater level of oxygen in the mix will conspire in different ways and at different RPMs and load, to give lean or rich situations at a time where some nice fresh 1968 petrol would not.

you end up having to make a change or a compromise in respect to jetting and ignition timing
more initial
same or less total
potentially a faster advance curve and no guarantee that the vacuum advance orginally applied is now correct

choice
run no vacumm advance and modify the mechanical curve so you can run something like 15 BTDC initial and 34 total just of mechanical advance

Run the vacuum advance, have less than ideal initial mechanical advance, and the attendent blacking of the tail pipe when running at idle, but be happy that the vacuum will kick in and deal with issue as soon as the throttle plate passes the ported vaccum port no more than 34 total mechanical

or plug the vacuum advance into none ported vaccum which will pull in vacuum advance at idle and on cruise which might just do the trick. i.e run it like the pre "emissions" days.

either way a bit of work will result in good performance, cooler running and better ecconomy on the modern fuel than you will be getting if the tune is still as per the factory manual for the 196? fuel.

ethanol fuel seems to be more prone to vapour lock issues, it has an impact on the vapour production of the fuel it is mixed with in a totally none linear way, heat makes matters worse at some ratios used in fuel. so running a penolic insulator under the carb on a standard manifold may help stop heat soak into the carb body.

blocking off ther heat crossover may help but i'd be less inclined to do that in a cool damp environment on a smaller motor with a standard carb . The venturi effect of the carb will keep the manifold really quite cool when the throttle is open, and the heat cross over keeps the area that the accelerater pump jets aim at hot when the throttle is closed so that the first pump shot is effectivly vapourised (wet, but no puddle) as you pull away from an idling standstill on a cold rainy day... this is of greater benefit on a manual car with a smaller motor in places where some of the year it rains snows and is cold...

big motor, auto and torque converter, don't worry you will be spinning it up to whatever rpm makes the wheels turn anyway, not trying to move 1.5 metric tonnes in the 1300 rpm area with the motor connected mechancially to the wheels via a clutch..

60s 4 cylinder spots car with a manual shifter. this is important
hulking great v8 on an auto, much less so.
any kind of "performance" motor well whatever is necessary to acheive your aims.

someone will be along in a minute to tell me i'm wrong.. :)

Dave