Briggs 31C707, 17.5HP. Amazon carb hunts constantly; applying choke reduces surging. Original Briggs carb runs briefly, then bogs, surges and blows black smoke. Closing fuel valve temporarily improves it. OEM carb also floods overnight. Fresh gas, good fuel flow, governor linkage free, solenoid gets 12.75V and clicks. No fuel pump. Suspect OEM float needle/seat and aftermarket lean mixture. What should I test next?