I've dealt with this issue for many, many years. What has worked for me to fight percolation woes, and I fill her up with 93 octane 10% ethanol, is to 1) Use a Turkey Pan with a thick aluminum base for heat insulation; 2) Do
NOT set your floats as per the 60 year old instructions of putting the fuel level at the base of the sight holes; 3) Set your floats instead to as high as they can go without dribbling fuel out of the boosters when you shut the engine off HOT and look down the top of the carburetor to see if there is a puddle on top of the butterflies; 4) Once you have your floats set to a point that doesn't have the gas percolating out when you shut it down hot, begin your classic tuning steps with an emphasis on the
primary butterfly being set just above the bottom of the transition slot and the idle RPMs being controlled by the opening of the secondary (if you have a Holley 3310 replace the secondary set screw that is hidden under the base with an easy to adjust Allen head screw). Now, if you have really crappy gas, then you might have to live with it. But try this first before you do.
