Fast idle

[427TJ]

Last summer I began burning a 50/50 mix of 100 octane low-lead aviation gasoline and 92 octane pump supreme in my stock, unrestored, 105,000-mile '67 Camaro 396/325. The car runs great on this brew, however, it began idling fast after about two or three tanks over a two-month period of pleasure driving. I back the idle adjustment (Q-jet) all the way off and the idle remains at 1300 or so. The car has the stock Th-400 trans and stock Chilton's manual-specs tune up. My four year-old son thinks it's neat when I put it in reverse to back out of the garage and the tires chirp but I'd like to get it back down to 700-800 rpm.

How come it's idling fast?

Thanks in advance.

Bill