Another non-A&P -

Of the two O-300s overhauled among family and friends both had lean
operation problems. The first one others thought might be a little
carb ice (it would smooth out when carb heat was added, a sure sign of
lean operation) until it disassembled itself at about 50 hrs. The
cause was never found.

The second one (a 172H) appeared lean on first annual after OH. We
drilled out the carb jet ever so slightly. I now know that was both
illegal and gauche. But it ran well then to TBO for whatever reason.

I also had lean indications with my 172M (Lycoming) after the one piece
venturii mod - especially on autofuel in the winter.

You might want to make up a test fitting epoxying a 6 inch length of
1/8 inch dia clear plastic hose into a 1/8 inch pipe plug to screw into
the 1/8 inch pipe port in your carb bowl. Using that rig on the
ground, you can find out just where the fuel level stabilizes in the
bowl.

Make sure there are no intake leaks (the gaskets on each cyl adapter
tend to suck in) & that all baffles are in place AND rigid enough that
air pressure doesn't force them away from the fins.

Is the baffle to direct cooling air around the crankcase properly
spaced too? We added a couple of small rubber foot grommets to space
that baffle out from the crankcase. Previously it rubbed on the
casting.

Might your Cyl number 5 thermocouple be miscalibrated?

You might consider the use of the autofuel STC to minimize the lead
contamination. High oil temps and oil that quicly becomes black are a
symptom of piston ring blowby. Lead of course will make it turn gray.