one O2 senser on each bank is just an average of AFRs. He could have one cyc rich and one lean and so on. This is also an indication of the plugs showing rich and lean conditions.

I understand this is a carb engine but will add for whomever

efi engine fuel injectors flow rates and spray patterns play in the mix if the set of 8 plus if they are not balanced as close to possible across the board. This really holds true for used sets of fuel injectors and in many cases are commonly missed when one rebuilds their efi engine(s). That will cause rich and lean conditions alone. Just posting added info

That does not invalidate the O2 readings. Cars have had one O2 sensor per bank for years and give vital information, much more so than reading a plug. True balanced injectors are beneficial, but in this case we aren't saying one cylinder is at 17 and the others at 11.5. That just doesn't happen even with a carb. I will take the AFR readings over plugs any day assuming all else is working properly.

I believe both are valuable tools. AFR is great for tuning carb and also when permanently mounted dash gauge is a quick glance reference is to weather anything is changing in the combustion chamber. Like a warning indicator.
The AFR will tell you nothing as to detonation and heat in cylinders, this requires Plug reading (or pyrometers).
So, in my mind the two work together to give a more informed analysis of the motor health and tune.

For guys like me who dont have a mechanical uncle or anyone in the area to assist - very interesting thread. FWIW

Thanks everyone for the feedback and advice.

I thoroughly scoped each cylinder and all pistons looks good, no damage, cylinder walls look good too, I also did a compression and leak-down test and compression looks good and leak-down shows each cylinder within 2% of each other.

I reinstalled new Autolite 3924's and my plan of attack is to lower timing to 30deg (from 34deg) and take the boat for some runs to record the afr's. Then re-jet accordingly to get th afr's down etc. Then hopefully do some plug reading runs.

Thanks again

I dont like the aluminum specs on the plugs, would try to get the AFR's down in the cruise range, also might look at going colder on the plugs

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Don't change more than one variable at a time. Change jets and fatten it up before you take to the water. 34° shouldn't be the death of a NA sbc engine, richening it up should ward off detonation and get your AFR's in line. That much aluminum on spark plugs would really concern me. This isn't a 2 stroke snowmobile where you can lean it down till you start seeing aluminum on the plugs, pull the jug, swap out the piston for $75, put back together add a + size jet and be on your way.