Mostly reinforcing what others have said. I looked into this technology pretty deeply some years ago. And then last year did some experimentation with a 50 cc motor converted to alcohol.
Alcohol has about 1/2 the energy contant as gasoline. It is able to produce the same work output as gasoline. But, it consumes twice as much fuel per volume. Mileage, of course, suffers.
It is less volatile than gasoline, with a higher flash point temperature. One of the reasons why it is mandated in some racing events.
Because of this property, engines can run with a higher compression ratio, with less concern about preignition and detonation. This often yields much higher horsepower levels due to the higher compression ratio.
Alcohol actually burns cooler. So, an air cooled engine conversion without a compression ratio change will burn cooler. However, one of the attractive features of a water cooled motor is its stable operating temp, in that all the metal parts heat and expand to fit well together at a temp controlled by a thermostat. Air cooled motors have a higher expansion variability range due to external temp variations and power output settings. The point is during engine break in the pistons and cylinders, for example expand and wear in for that temperature "fit". When the temperature changes more wear occurs while the metal finds a new relationship.
Converting an engine worn for fit at temperatures befitting gasoline heat out put, will incur more wear( or very sloppy fit) while operating on alcohol. In other words, a new engine run only on alcohol out to last longer than a new engine running on gasoline due to the reduced metal expansion effects.
PJ made a good point about the cooling fins. Hot engines are more efficient at fuel atomization than cooler ones. Simple conversions of air cooled motors are not as efficient unless they are run at a higher temperature as what the standard cooling fins allow. The inefficient combustion temps make fuel ratios richer than they should be and make spark plug heat range selection "interesting".
Alcohol absorbs moisture from the air. This also decreases its energy content by volume. Long term storage of a pure source is the issue here. Home distilleries aren't known for their product purity or preservation. Controlled water injection or infusion can be of benefit under high power settings. However, water in alcohol also develops alkalies, which are corrosive to metals. Note that the SOHC4 fuel tank and carbs are vented to the atmosphere where a ready source of water is always available.
Exhaust systems don't get as hot. So, condensate doesn't expell as quickly or easily. Exhaust systems corrode more rapidly. (Alkali in the exhaust, too, I think)
Lastly, the seals. Yes, seals impervious to gasoline aren't necessarily impervious to alcohol. But, don't worry, you'll be replacing these soon anyway, with the governments mandated gasohol requirements.
Stimulates the economy when people are forced to repair or replace with new equipment. What could be better?
Cheers,
