So, I've been trying to tune my new Holley 4160 (600 cfm marine carburetor model 80551) on 5.7 block. Had to increase main jets from #63 to #71 and switch to black (strongest) vacuum secondaries spring. Still have hesitation around 3500 rpm before the surge at WOT (as secondaries open). I don't have a lot of experience with carb tuning, tried reading plugs with little success. Fearing lean condition and potential damage to the engine I bought a broadband AFR gauge to help with tuning. Installed the broadband O2 sensor in a spacer under port riser. The gauge worked flawlessly and I got readings from the entire RPM range (while moving, of cause). I am getting 12.5-13 AFR all the way from idle to about 3500rpm where the hesitation starts. When the hesitation starts, the AFR readings get sporadic jumping all over the place from 11 to 16 and sometimes showing blank. Once RPM goes over 4000 I feel the boost from the engine and AFR readings stabilize around 13 again. Does anybody here have any experience with tuning marine carburetors using AFR gauge? Can you help interpreting these results?