It's the cylinder base shims or one of the lower studs. I stripped it down and cleaned it off, and started it with no shroud (that's kinda fun!) and found that the leak originates on the lower part, in the center, either at or near the #4 stud. It's not the oil cooler.. that's clean.
I pulled the stud out, hoping to get some sealer on it and fix it, but no dice. Have to pull the head and jugs off that side and have a closer look.
Then I will re-torque both sides, adjust valves again, and see how we do.
Seems hard to get anything going with the two separate carburetors and no linkage yet. The 1/2 side seems to have a time popping through the carb barrels if I run it up, so I am gonna make sure the idle circuits are clean, and start messing with jets if all else smooths out.
Seems real touchy right now, but MAN, there is so much damn muffler pressure it's crazy. Really pumps it out. Sounds like it's gonna be mean.
Just sucks to have to go back and re-work that base seal. I hope everything is coolio with it, just re-seal and not re-work.