Yep, I believe it to be the ethanol. If a carb had been sitting for years you would see some of the same thing or close to it. At that time I would blame it on water from condensation. To help with this problem I use one ounce of Stabil red with every tank and I have opened the carbs after a year of use and the float bowls are fine (WCFB's) a friend uses a Mercury outboard fuel additive with very good results too.
I found a sonic cleaner using Pinesol helps with the removal of the "stuff" with old tooth brushes and brass brushes. I've heard vinger works too but both need to be rinsed well and blown air. Good luck.

56 Vic, B'Ville 200 MPH Club Member, So Cal.

A friend is rebuilding the Holley 4160 carb on my 1956 Ford with a 292 y-block. A greenish/yellow powdery residue completely covers the interior of both the primary and secondary fuel float bowls. Does anyone know what this residue could possibly be, how it forms and how to get rid of it?  I only use Shell V-Power with a 93 Octane rating in my 56. On some sites, they say it is aluminum oxide corrosion caused by ethanol or gas additives and/or water and is almost impossible to clean out. Any help would be greatly appreciated.

Thanks,
Danny