GAV at 3/4 is a little rich. I run 1/4 turn open. Maybe try no more than half?
Repop jets for the carb are MOST misleading.
The literature has information of carburetor jet sizes and their effect on GAV. See
http://www.fordgarage.com/pages/jetflowrates.htm for Vince Falter's take on jet variations. Also scan down that page for more info on Zenith Carbs than you can absorb at one sitting.
Vince Falter has words on advance too. The Model A has about 2x the normal required advance capability. Might have been made this way to accommodate varying skill levels in Timing.
http://www.fordgarage.com/pages/bdistributoradvance.htm
Quote:
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The Model A ignition uses a zero crankshaft degree (TDC) initial timing point so that the engine can be easily started by hand crank, however it should never be run under load in this "retarded" position. The advance lever should always be advanced 1/3 to 1/2 travel (13 to 20 crankshaft degrees BTDC) on the steering column quadrant after starting the engine, and should generally not be run more than 3/4 advanced on the column under any normal circumstances.
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More discussion on that page comparing the Model B timing with Model A timing ranges.
Joe K