AFAIK (and accdg to the article posted previously), 10% ethanol is the most that carb engines can survive without any problems. above 10%, there will be problems because:

1. alcohol can dissolve/remove gum formations in the fuel system which asounds good at initial look until large particles of gum blocks your fuel line or carbs.

2. stock rubber fittings (oil seals etc) are not made for ethanol and may result to leaks.

3. and if i am not mistaken, ethanol is corrosive as compared to gasoline. so fuel lines which can corrode can cause problems with higher concentrations of ethanol in fuel.